WO1985001579A1

WO1985001579A1 - Water test kit and components therefor

Info

Publication number: WO1985001579A1
Application number: PCT/GB1984/000342
Authority: WO
Inventors: Barry John Lloyd; Martin Snook
Original assignee: University Of Surrey
Priority date: 1983-10-05
Filing date: 1984-10-05
Publication date: 1985-04-11
Also published as: GB8326659D0; AU3500884A; EP0158662A1

Abstract

Portable water test kit (1) comprising in unitary and modular form: means (5, 6) for testing a water sample to determine feacal coliform count thereof, means (10) for determining the turbidity of the said sample and means (9) capable of determining the residual chlorine level of the said sample. The novel use of unitary and modular form is vital in adapting the various tests to a single sample under an enormous variety of possible environmental conditions.

Description

WATER TEST KIT AND COMPONENTS THEREFOR

This invention relates to a critical parameter portable water test kit and also includes a new form of turbidity meter. Microbiological contamination is the greatest cause of human disease attributable to water supply in the vast majority of countries outside Europe and North America. The new "WHO guidelines for Drinking Water Quality" formulated for provincial and rural water supply surveillance (Vol. Ill 1983) have therefore reiterated the importance of the only practical method of establishing the safety of supplies, namely by routine bacteriological testing together with chlorine residual testing and checks on turbidity reduction through treatment plants. No unit kit is currently available which combines the ability to carry out bacteriological testing with chemical and physical tests and which can be used in the field. Coimnercially available water test kits are inappropriate for widespread use in the great majority of developing countries - such kits are very expensive, require regular servicing, chemicals which have to be imported and technical training for their effective use. However, by combining a variety of conventional technologies, together with recent developments in reliable electronic instrumentation and careful test selection, it has now been possible to develop a kit which will enable reliable routine monitoring of even the most remote rural water supplies to be done at very low cost. Furthermore, the kit is very adaptable in that the same basic kit as that proposed for rural surveillance could be used in provincial water treatment works and health centre laboratories, supplemented only by supporting facilities. These would include media preparation, including autoclaving and washing equipment. Out of 30-40 parameters listed in the International Standards for Drinking Water (WHO 1971) a limited number are critical for the day to day quality control of water treatment and supply. The majority are chemical characteristics which vary little over protracted periods of months or even years. Although all parameters should be examined when planning new water supply projects, it is generally impractical and uneconomic for most provincial and rural water organisations to consider the routine analysis of other than essential tests. Careful consideration must, therefore, be given to their selection. High priorities include:-

1) Reliability, sensitivity and value.

2) Simplicity in execution and interpretation.

3) Speed in execution and obtaining the result. 4) Low cost.

The following selection of tests is critical for water quality:-

1) Faecal coliform (E.coli) count.

2) Turbidity. 3) Chlorine residual.

4) Conductivity.

5) pH.

1) Faecal coliform determinations provide the most sensitive and universally applicable test of hygienic quality. A field kit should enable the processing of the sample on site and a quantitative result within 14 hours. The four other tests listed should be capable of being effected on site in about 2-3 minutes each and together will give a good indication as to whether a bad faecal coliform result may be expected. Thus, for example, a water which has low turbidity (<0.5 NTU) together with a rfieasurable free chlorine residual (0.1-0.5 mgl^-1) is most unlikely to contain faecal coliforms or enteric pathogens. 2) Turbidity in water is caused by the presence of suspended particles ranging from visible to microscopic size (<1.0 μm). It can be measured very accurately over a wide range, so that water, which to the naked eye may appear clear, still has measurable turbidity when tested by electrophotometric light scattering sensors. This can be executed immediately following sampling in the field and this is particularly important when storage of the sample may allow particle aggregation to lower turbidity values in transit to a laboratory. High turbidity is commonly associated with high microbial contamination whereas high clarity is typically associated with low microbial contamination.

3) Chlorine residual determination is obviously essential wherever disinfection is practised. Disinfection should be applied wherever possible to water of low turbidity in order to ensure that treated water retains a measurable free residual of hypochlorous acid. Detection of free residual chlorine is an excellent indicator of the hygienic safety of water.

The first three tests are therefore absolutely essential and complement each other.

4) Conductivity measurements may be substituted for chloride analysis. The chloride ion is one of the major inorganic anions which is increased in sewage over that in fresh water. In coastal areas intrusion of sea water into drinking water may also be readily detected by changes in conductivity. The WHO permissible limit for chloride is 600 mgl^-1, whereas the desirable limit is 200 mgl^-1. Excess salt in drinking water may cause renal problems. Irrigation water must have a conductivity of <100 ms cm^-1 to prevent salinity damage to crops.

5) pH is worth monitoring for several reasons. Low pH waters are corrosive and produce a health risk where lead piping is employed. High pH waters require higher doses of chlorine for effective disinfection. pH regulation, to near neutral, is also needed to minimise and optimise the amount of the commonly used coagulant in water treatment. The acceptable pH range recommended by WHO is 7.0 to 8.5.

No water test kit currently available combines facilities for measuring the three critical parameters noted above in a single unit portable format, i.e. faecal coliform determination, turbidity and a means for determining the level of a preselected solute in the sample, e.g. chlorine residual measurement.

Thus, the present invention provides a portable water test kit comprising in unitary and modular form: means for testing a water sample to determine the faecal coliform count thereof, means for determining the turbidity of tho .said sample and means for determining the level of a preselected solute in said sample. The means for determining the level of a preselected solute in the sample must be capable of determining the residual chlorine level, but it may be desired to determine the level of other solutes, such as the following: Amine ,

Ammonia / Ammonium ion,

Bromine / Bromine ion / Hypobromous ion,

Iodine / Iodide ion / Hyproiodous ion, Fluorine / Fluoride ion / Hypofluorous ion,

Copper / Cupric ion / Cuprous ion.

Cyanide ion.

Hardness ions,

Hydrazine, Hydrogen peroxide / Peroxide ions,

Iron / Ferric ion / Ferrous ion.

Nitrate ion.

Nitrite ion.

Ozone / Ozonides, Sulphate ion / Sulphite ion.

Zinc ion.

Phosphate ion, pH,

Cadmium ion. Calcium ion. Arsenic ion, Lead ion. Magnesium ion. Manganese ion.

Potassium ion. Selenium ion, Silver ion,

Sodium ion or Zinc ion. The use a modular construction in the present kit allows the addition of supplementary testing equipment e.g. pH, conductivity and temperature probes. These may be further added to or replaced by dissolved oxygen, NO₃ and other established probes as particular demand dictates.

The kit in the present invention is designed to provide a rapid indication of water quality on site and to obviate the need to transfer water samples to a central laboratory. The only retrospective result obtained is the bacteriological test for faecal coliforms, which is set up at the same time as the turbidity and chlorine residual results are obtained.

The design of the present kit thus enables routine data to be obtained for defining the quality of water, whether in a rural or hostile environment or not, with the maximum convenience and ease for the operator at minimum cost. For the use of a single water test sample to be possible, a unitary construction is in practice vital. It is highly preferred that the turbidity meter used in the present kit avoids conventional galvanometric devices which, although accurate, are extremely sensitive to damage and are not adapted for, for example, tropical conditions. Thus, the invention includes both: a turbidity meter comprising means for receiving a water sample, optoelectronic means for developing a signal representative of turbidity in the sample, and solid state processing means responsive to the signal to display the turbidity of the sample;and the use of such a meter in the present kit

A further disadvantage of existing turbidity meters is that the use of photomultiplier devices may be necessary to detect the very small turbidity levels which may be present in water which is apparently "clean" to the naked eye but which nonetheless contains bacterial contamination. Thus, the principle of conventional turbidity meters is to shine a source of light onto the sample and to measure scattered light using a light detector positioned at 90° to the light source/sample light transmission path. It will be appreciated that with water which is "clean" to the naked eye the amount of scattered light thus measured may be very small. The present invention also avoids this difficulty by providing a wholly new concept in turbidity meters. Thus the invention includes a turbidity meter comprising means for receiving a water sample, a light detecting means for generating a signal in response to light falling thereon, high intensity light generating means positioned to direct light towards the sample so as to provide a non-linear (i.e. indirect) light generating means/sample/detecting means light path, low intensity light generating means positioned to direct light towards the sample so as to provide a substantially linear (i.e. direct) light generating means/sample/detecting means light path, timing means for activating separately in sequence the low intensity generating means and both high and low intensity generating means simultaneously, solid state processing means for comparing a signal from the detector corresponding to total light received from the high intensity generating means and the low intensity generating means with a signal from the detector corresponding to light received from the low intensity generating means only so as to develop a signal representative of the turbidity of the sample, and means for displaying the turbidity of the sample in response to the signal representative thereof. It will be appreciated that, in effect, the transmitted light is used as a reference for adjusting the measuring range of the instrument. It is highly desirable that the kit of the present invention include such a turbidity meter. In one embodiment of the invention, the necessary modules for faecal coliform examination are contained within the base of a box used to house the rodular kit in unitary form and the interior of the lid of the box contains an absorpsiometer for chlorine residual measurements and a turbidity meter (or a combined chlorine residual and turbidity meter - see later).

in the faecal coliform section of the present test kit there is preferably an incubator, a combined field sterilisable stainless steel sample jug and filtration assembly complete with hand operated vacuum flask.

The sterile filtration assembly may be integral and mounted within the sample jug in transit and, following the processing of a water sample, both jug and filtration assembly may be simultaneously resterilised by igniting 1-2 ml methanol in the sample jug.

The incubator for incubation of samples obtained by filtration should be designed to operate independently of any support base laboratory. One way of achieving this is by the use of lead/acid or Ni/Cd batteries which operate for at least one incubation cycle (possibly with two or more incubators). Alternatively, the incubator may be powered by separate 12 v batteries. Of course, the incubator can also be designed to be powered by conventional mains voltage supply if this is available. It is an option that the kit of the present invention incorporate solar photocells to provide power to operate the incubator.

The basic material from which the incubator oven is made can be aluminium or an aluminium alloy or copper.

These materials are chosen as the most suitable in a small incubator structure designed to hold, e.g. up to about seventeen petri dishes at a temperature of 44.5°C ± 0.5°C from 2/3 hours after switch-on for a period of approximately 14 hours. In general, the ambient temperature encountered when using the present kit will be from 15ºC to 35ºC. It is desirable for the incubator to reach the working temperature of 44°C from an ambient temperature of, say, 20°C in approximately 2 hours. It is obviously also desirable that the materials used in the construction of the incubator should be hygienic and not absorb water to the extent that the growth of bacteria is encouraged in such materials. The seventeen petri dish capacity is preferred for a kit of the present invention for use in provincial towns and rural areas, where seventeen membrane filters in separate petri dishes having thereon bacteriological samples is the expected load per day.

The incubator oven is surrounded by an insulating material to provide the overall incubator structure. The material for formation of the incubator oven is chosen on the basis of high conductance (to ensure that all sections of the oven are at the same temperature) and low thermal capacity (to ensure reaching the desired temperature in a fairly short period of time using minimum power). A preferred insulator material is polyurethane beads or foam (preferably with a surrounding layer of reflective metal foil) which has an exceptionally low conductivity.

The preferred structure for the incubator oven in the incubator of the present test kit is a wall surrounding a hollow interior in the shape of a right cylinder. An aperture may be provided in the base of the oven for a semi-conductor temperature sensor connected to a control circuit. The shape of the ovenblock is also preferably cylindrical. The exterior of the oven block may be provided with grooves to accomodate incubator winding. The diameter of the hollow interior of the oven may be, for example, 63 mm to accomodate commercially available petri dishes. Although a separate chlorine residual meter and turbidity meter may be included in the present kit, it is perfectly possible for a single meter to be used which is capable, upon appropriate switching, to give alternately turbidity and chlorine residual readings for any given sample. However, the combined instrument is best understood by first considering the use of separate chlorine residual and turbidity meters.

It will be appreciated that in the use of the present invention chlorine residual values are determined. The instrument for so doing may however be used, as already indicated, for other solute detection. In the following description reference is made for convenience to chlorine residual measurements only, but the invention is not limited to this use.

It must be appreciated that conventional chlorine residual meters give a more or less inaccurate reading where the water sample being examined has more than a tiny minimum turbidity level. This results from scattering of light as a result of the turbidity and a consequent false high positive reading for absorbance. It has been discovered that the use, as a "control", of the absorbance reading generated by a red light source deals with this problem by enabling substraction of the "red" signal from the "green" signal. The signal corresponding to the period when the red light source is activated is attributable to turbidity and not free chlorine. It is highly preferred that the means for generating alternate red and green light be a single light emitting diode since this obviously is convenient in terms of structural simplicity. Turning now to the absorptiometer (chlorine residual meter) module which may be used in the present invention, conventional chlorine residual meters operate on the basis of a simple spectrophotometer assessing the absorbance of green light by the sample. The present invention includes an entirely new concept in chlorine residual meters and such a meter is included in the present invention. Thus, the invention also provides a chlorine residual meter comprising means for receiving a water sample, means, including timing means, for generating alternate green and red light positioned so as to direct such light onto the sample, means for detecting light transmitted by the sample and for producing a signal corresponding thereto, means for comparing the signal produced by the detecting means during passage of green light through the sample with the signal produced by the detecting means during passage of red light through the sample so as to produce a signal representative of the free chlorine content of the sample, and means for displaying the free chlorine content of the sample, and means for displaying the free chlorine content of the sample in response to the signal representative thereof. It is, of course, highly preferred that such a chlorine residual meter be used in the test kit of the present invention.

The present invention also includes a novel turbidity meter. The instrument preferably includes a source of. low intensity red light which may be referred to as "ref red" and a source of high intensity red light which may be referred to as "high bright red".

It will be appreciated that in the quality assessment of water, high turbidity almost always indicates that action must be taken. The problem arises when the turbidity level is so low that the water appears substantially "clean" to the naked eye. Conventional turbidity meters utilize a non-linear light path between a source of bright light, the sample and a light detector. The problem is that in such a "clean" sample relatively little light is scattered and the light detector reading is therefore very low. Furthermore, high intensity light sources such as tungsten filaments require a considerable amount of power to activate them. In the arrangement of the present invention, in contrast, a source of low intensity light is positioned in a substantially linear pathway including the sample and the light detector. The alternation of a combined pulse of light from both sources of light with a pulse of light merely from the low intensity source in the linear optical pathway more readily enables the instrument to be adjusted to detect low amounts of scattered light and hence to measure low turbidity levels. It is immaterial whether it is the combined signal from both light sources which is pulsed first followed by the low intensity light source or whether the low intensity light source is pulsed first followed by both light sources. However, the use of a low intensity light source as a reference in the linear optical pathway of transmitted light markedly improves the operation of the turbidity meter. The preferred arrangement of turbidity meter in this invention differs from conventional turbidity meters in that in place of a conventional high intensity light source, such as tungsten filaments, a high intensity red light source is used. A suitable such source is the H-500 HiSuper Bright LED produced by STC Meridian of

Harlow, Essex, UK. The use of such a high intensity red light source in a turbidity meter is of itself novel and is included within the present invention.

In the case of a combined meter as mentioned above, a single set of controls is required, a single sample receiving cavity and a single display. Thus, although in some circumstances it may be desired to have separate meters, the invention also clearly envisages both a combined turbidity/chlorine residual meter as described and the use of such a meter in the present water test kit. Thus, the present invention provides a meter capable of determining turbidity and chlorine residue levels in a water sample, which meter comprises means for receiving a water sample, a light detecting means for generating a signal in response to light falling thereon, high intensity light generating means positioned to direct light towards the sample so as to provide a non-linear (i.e. indirect) light generating means/sample/detecting means light path, means, including timing means, for generating alternate red and green light positioned to direct such light towards the sample so as to provide a substantially linear (i.e. direct) light generating means/sample/detecting means light path, timing means for activating separately in sequence the means for generating alternate red and green light to generate red light andsaid red lightgenerating means simultaneously with the high intensity light generating means, solid state processing means for comparing the signal produced by the detecting means during passage of green light through the sample with the signal produced by the detecting means during passage of red light through the sample so as to produce a signal representative of the free chlorine content of the sample, solid state processing means for comparing a signal from the detector corresponding to the total of light received from the high intensity generating means and red transmitted light with a signal from the detector corresponding to red transmitted light only so as to develop a signal representative of the turbidity of the sample, switching means for setting the mode of the meter at turbidity signal generation or at free chlorine content signal generation as desired, and means for displaying either the turbidity of the sample in response to a signal representative thereof or the free chlorine content of the sample in response to a signal representative thereof in response to the setting of the switching means.

As used throughout hereinabove in relation to turbidity meters the term "non-linear light path" includes any geometry where scattered light (but not transmitted light) is receivable. Usually the pathway light source to sample to light detector is substantially a right angle. There is, however, no limitation to this arrangement and, for example, an angular path of 45° or 60° would be operable if desired.

It will also be appreciated that the combination of outputs from the red/low intensity and high intensity light sources in any turbidity meter described herein must be chosen by the skilled man so as not to "swamp" the receiving capacity of the detector used. The external structure of the turbidity meter of the present invention (and, indeed, of the new chlorine residual meter of this invention) may be chosen to suit the particular kit in accordance with the invention or the desired alternative use of the meter.

It will be recalled that the possibility of including separate conductivity, pH and temperature probes in the kit of the present invention is envisaged. It is highly preferred that such probes be incorporated as part of a combination module having the three functions.

The present invention also provides an entirely new calibration means for instruments such as turbidity meters and chlorine residual meters. It will be appreciated that, particularly in the field there are problems in calibrating instruments of this type particularly if the turbidity measurements are expected to be low. Thus, significantly more extraneous material may be deposited on the outside of a cuvette by repeated handling thereof than may be accounted for by turbidity in a sample being measured. The present invention obviates this by using a simple calibration means. The present kit may include such a means.

Thus, the present invention includes a solid calibration means comprising a column of transparent solid material of uniform cross section having a centrallypositioned aperture along a part only of the Icngitudinal axis of the column sealed from the exterior of the column. Preferably, the column has a circular or cylindrical cross section and it may be made of perspex. It is also preferred that that section of the column which does not contain the centrally positioned aperture has at least one face thereof tinted with a colour which corresponds to a standard free chlorine concentration in a water sample.

It is also possible to combine a suitable plastics material (e.g. perspex) and conventional formazine standards for NTU values in a suitable solvent (e.g. chloroform) to generate a "cloudly" plastics material. A column cast from such a material may be used as an alternative standard.

The invention embraces, of course, the use of standards such as described in standardising an instrument not necessarily a chlorine residue meter or a turbidity meter, which is designed to accomodate a cuvette of sample therein.

Returning now to the kit of the present invention overall, it is highly preferred that the results of measurements such as conductivity, pH, temperature, chlorine residue amount and turbidity be displayed in digital manner (e.g. LCD). However, the invention is not, of course, limited to this. The use of LCD format is particularly important in apparatus for use in the field having regard to the need to provide maximum protection against shock in the apparatus overall. In the present kit the nature of the carrying case is not particularly significant but it is preferred to use a metal, e.g. aluminium, case. Individual modules may be encased in plastics material.

The arrangement of modules within the carrying case in the kit of the present invention is largely to suit individual convenience.

In general, in the use of the kit of the present invention, the sequence of tests employed is defined according to the source and type of water under investigation. Thus, for example:

1) a piped supply which is know to be chlorinated will be tested for chlorine residual first;

2) the second test should be for turbidity;

3) depending on the results of 1) and 2), the operator will proceed to processing the sample for faecal bacteriological contents; a) if chlorine residual is below .05 mg 1^-1 then bacteriological testing is essential; b) if turbidity value is higher than say 5NTU and chlorine residual is below 0.1mg then bacteriological testing must proceed; c) if turbidity is below 1NTU and chlorine residual is above .05 mg 1^-1 , then bacteriological testing need not be undertaken. In order to facilitate understanding of the present invention, the invention will now be further described and illustrated (without limiting the same) by reference to the accompanying drawings which illustrate preferred embodiments.

In the drawings:-

Figure 1 is an exploded view of a water test kit in accordance with the present invention;

Figure 2 is an exploded view of an incubator unit suitable for use in the kit shown in Figure 1;

Figure 3 shows a combined temperature, pH and conductivity meter for use in kit shown in Figure 1; Figure 4 illustrates a filtration assembly and sampling device for use in the kit of Figure 1; Figure 5 is a diagram of a temperature controller for the incubator of the water test kit of the invention;

Figure 6 is a diagram of the circuit for conductivity measurement which may be employed in the present invention; Figure 7 illustrates a circuit suitable with modifications for either a residual chlorine meter, a turbidity meter or a combined meter in accordance with the invention;

Figure 8 is the circuit diagram for a turbidity meter which may be used in the present invention; Figure 9 shows in diagramatic form the physical arrangement of the components in the sample receiving area of a turbidity meter in accordance with the invention; Figure 10 shows a perspex standard which may be used in the operation of a water test kit in accordance with the present invention; and

Figure 11 is a flow chart illustrating in general terms how a water test kit in accordance with the present invention may be used.

A water test kit in accordance with the invention is generally shown in exploded form in Figure 1. Figure 1 shows a portable box (1) comprising a lid (2) and a base (3) containing the essential components and generally weighing 25 lb (approximately 11 kg) in all. The lid (2) bears a carrier handle (not shown) and the end walls of the base may each carry centrally located attachment points (not shown) for an alternative carrying device, namely a strap. The box (1) is hinged (44) at the back and provided with locks (4) at the front. When the lid (2) is opened the components shown in Figure 1 are revealed. The base (3) contains all the components necessary for sanitary bacteriology. It can be seen that in the left hand compartment of the base (3) is located a filtration assembly (5) and sample jug and sample recovery cable. A petri dish incubator module (6) is located in the central part of the base (3), and as with all components in this modular construction, it may be removed from the box (1). On the right hand of the base (3) is a narrower compartment (7) in which may be located all essential consumable items. in the lid (2) is located an array of three electronic meters each mounted in a similar housing. This row of instruments, from left to right are a conductivity, pH and temperature meter and associated electrodes (8), a chlorine residual meter (9) and a turbidity meter (10). Returning to the bacteriological components in the base (3), the construction of the filtration assembly (5) and sampling device is specifically illustrated in

Figure 4. A filtration tube (13) made of stainless steel is calibrated (not shown) on the inside of the tube with a line at the 100 ml and the 50 ml mark in order to facilitate the processing of these volumes of sample. A polycarbonate collar (14) slides over the filtration tube, so that they may together be located in a filter support base (15). The support base (15), which is dur-aluminium, may have a number, e.g. three, of internal stubs (20) which locate notches in the polycarbonate collar (14). When the filtration tube (13) and collar (14) are located in the support base (15) in the fully engaged position, the flange face of the filtration tube is in contact with an upper 'o' ring (16b). This 'o' ring surrounds a permeable bronze screen (16a) allowing the free passage of water through an outlet in the centre of the base (15). The bronze screen (16a) is supported on a lower 'o' ring (16c) which has a smaller internal diameter than the upper 'o' ring (16b). This arrangement enables a standard commercially available, 47 mm bacteriological membrane filter to be introduced onto the bronze screen (16a) and then locked into position in a water tight seal between the filtration tube flange and the 'o' ring and bronze screen (16a). The filter support housing base (15) is provided with another 'o' ring (15a) on the outer circumference of the lower half, which provides an airtight seal between the support (15) and a stainless steel vacuum flask (17) into which the support base (15) is located. At the middle of the support base (15) is a rim (21) which is proud of both upper filter tube location and the lower vacuum flask location lines. This rim (21) bears a hole (22) which is continuous with the underside of the filter support and provides the location point for a rubber bulb vacuum pump (18) which permits a water sample to be drawn through the membrane filter from the filtration tube (13) into the vacuum flask (17). The water sample to be tested is collected in a previously sterilised stainless steel sample cup (19) of similar dimensions to the vacuum flask (17). Up to 3 metres of cord may be provided with the sample cup (19) and an attachment point may be provided on the cup (19) so that samples may be recovered from difficult locations e.g the depths of covered reservoirs or points remote from the sampler in rivers, reservoirs and lakes. The design of the filtration assembly (5) allows for the simultaneous sterilization in the field of both the filtration tube (13) and its associated parts as well as the sample cup (19). This can be done by virtue of the fact that the sample cup (19) may be inverted over the filtration tube (13) and collar (14), and located on the filtration support (15) up to the rim (21) and the vacuum pump location point (22).

Consumables which are provided for use with the bacteriological testing equipment include commercially available cellulose cleaning tissues, membrane filters, cellulose pads, methanol and selective isolate medium. These may be stored in a waterproof box in the consumables storage compartment (7) in the base (3). In addition, forceps, for manipulating sterile membranes and agas lighter for igniting methanol and sterilising the forceps, may also be located in the storage compartment (7) along with data report sheets.

Considering now the incubator (Figure 2), this comprises one or two ovens (23) which are accessed by means of removing UPUC plug(s) (24) in the front facia (33). On removal of each plug (24) a stack of up to 17 aluminium petri dishes may be

removed from the cylindrical aluminium chamber (25) of each oven (23) by means of a metal carrier. Thus, in transit, the oven(s) (23) provide (s) the storage space for 17 or 34 petri dishes which may be precharged with cellulose pads and selective isolation culture medium. The incubator oven block (23) may be provided with grooves (not shown) to accommodate a heating element, which may be surrounded by an insulating material polyurethane foam or styrene beads). The preferred structure for the oven has been indicated earlier. Figure 2 shows an exploded view of the incubator (6) and it will be noted that it incorporates a commercially available battery charger (26) in the right-hand floor space of a case (27). A removable side panel (28) permits the provision and incorporation of a conventional 12 volt D.C. lead-acid battery (29). Above the battery (29) is mounted the controller circuit board (30), but this is suspended on pillars (31) mounted below the facia (33). On the left-hand side of the facia (33) are oven access holes and plugs (24). On the right of the facia (33) LEDs may be used to show when power is on, and toggle switches control power on/off, power to each incubator and a temperature of 37°C or 44°C for each incubator (23). The incubator unit (6) includes as a housing an aluminium case (27) and the entire unit may be removed by a fold down metal handle (34). External power may be supplied to the unit (6) from a mains 110/220 volt supply by means of a cable (not shown) plugged into the unit in a socket which may be located in the upper part of the left side wall. Alternatively power may be supplied to the unit from a 12 volt D. C. battery source through coaxial sockets which may be located in the lower right-hand side of the facia panel (33).

Turning again to the lid (2) of the kit it will be recalled that the electronic meters are presented as an array of three instruments (Figure 1). The combination pH, conductivity temperature meter (8) is preferably located on the left of the lid (2) for reasons which will be explained shortly. The meter (8) incorporates an LCD (see Figure 3) display (35) in the uppermost part of the meter case, control buttons (36 to 42) on the right-hand side of the case and electrode insert sockets (43) in the lower part of the meter case. Location of insert sockets (43) adjacent to the hinge (44) of the equipment box (1) permits electrodes (45 to 47) to remain connected to their respective insert sockets (43) during transit. Hence tha electrode leads may trail into the base (3) of the box (1) where the electrodes themselves may be located alongside the bacteriological filtration assembly (5). Figure 1 shows the three electrodes (45 to 47) and cables alongside the meter (8), but as stated it is preferable for convenience and speed of operation that the electrodes (45 to 47) are located as just described. There are seven control buttons on the right-hand side of the instrument. The upper four (numbered 36 to 39) are pressed in order to select appropriate increasing concentration ranges of conductivity measured in us/cm. Below these is located a temperature push button (40) and below this a pH button (41). The bottom button in the series (42) has to be held in order to activate the circuitry for each test. The electrodes (45 to 47) are conventional and are commercially available from companies such as The Kent Group. Each is generally provided with up to 50 cm of cable. The layout of the main internal components meter (8) is indicated in Figure 3b.

The second of the three electronic meters, the chlorine meter (9), may be located centrally in the lid (2) (see Figure 1 ). This meter has an identical liquid crystal display (48) (for the free and combined chlorine levels) as found in the other two meters. Below the LCD (48) is a cuvette reception hole (49) designed to accept a circular test tube or standard 1cm diameter square cross-section cuvettes. Once the sample cuvette is in place the sample may be covered by a separate circular lid (not shown) which excludes extraneous light. The lid may also be fitted with a cylindrical magnet which activates the circuitry by means of a Reed switch within the instrument. On/off indication (50) is provided on the face of the meter (9) adjacent to the cuvette reception hole (49) and the circuitry is activated when the magnet is located in the on position. On the right of the instrument are located two calibration controls (not shown). The upper may be used to set a standard in the range 2-3 mg/litre of chlorine residual. An artificial plastic coloured standard housed in a cuvette may be used for this purpose and is generally included in the kit. The lower calibration control is used to set zero/blank sample. The particular advantages of the LED providing alternating "red" and "green" light flashes has already been described. The actual light source is located two thirds of the way down the depth of the cuvette hole. The light sensor is located opposite the LED i.e. at 180° in line. All three of the electronic instruments may be powered by small 9 volt batteries located behind a removable panel in the back of the respective instrument. These can be readily and quickly removed for replacement by removing the meters (8, 9, 10) from the lid (2) of the box (1).

Consumables required for the chlorine test are DPD tablets or solution which produce the requisite colour in chlorinated waters and these are commercially available from Wilkinson and Simpson. These may be stored in the compartment (7).

The electronic meter (10) for measuring turbidity is located on the right of the chlorine meter (9) in the lid (2) of the box (1) (Figure 1). Its face layout is similar to the chlorine meter (9), thus the turbidimeter also has an LCD (51) and a centrally located cuvette holder (52) and the circuitry may be similarly activated by an identical magnet incorporated into the cuvette cover. The principal differences between the turbidimeter (10) and chlorine meter (9) are as follows. First in the turbidimeter (10) there are two "on" positions activated by the magnet, one of which is for low range turbidities (0-20 NTU), whilst the other position in "on" mode is for the high range (20-200 NTU). Secondly, meter (10) incorporates two light sources (LEDs ) , the second being set horizontally at right angles to the first. This provides the additional light source to provide measurable readings at the higher turbidity range (see later). Thirdly, the turbidity meter (10) is provided with two calibration knobs (not shown) located at the right-hand side of the instrument which are used to set standards at high and low turbidity levels. Fourthly, the turbidity meter may be provided with two novel, synthetic (perspex) turbidity standards (such as that shown in Figure 10). These may be substituted in the place of cuvettes containing conventional formazine standards in the cuvette sample hole (52).

Referring now to Figure 9, the physical arrangement of the components around sample cuvette chamber (58) is shown. Thus, a cuvette (59) is positioned in chamber (58) with a high intensity light source (60) arranged at right angles to the receiving direction of a conventional detector (61). Source (60) is a conventional, commercially available component. A low intensity light source (62), also conventional and commercially available, is aligned with the main receiving direction of detector (61). The advantages of this arrangement have been described previously and will not be repeated here.

If reference is now made to Figure 10 of the accompanying drawings it will be seen that a solid calibration standard of the type noted earlier is illustrated. The column, designated (53), consists of two parts, a first and upper part (as seen in Figure 10) having a centrally positioned aperture (54) therein which terminates at a central point (55), and a second and lower part (as seen in Figure 10) designated (56) in which no aperture is present. Lower section (56) may have recessed into a surface thereof a tinted portion which corresponds to a standard chlorine concentration (not shown). Aperture (54) is closed from the exterior by a lid (57) although any closing means may be used. The reason for closing aperture (54) from the exterior is, of course, the avoidance of the accumulation of dirt in aperture (54).

The use of the standard illustrated in Figure 10 is simplicity itself. Lower section (56) of the standard may be inserted in a chlorine residual meter (such as meter 9) and the meter adjusted to the standard provided by the tinted face. The opposite (upper) section of the standard may then be inserted into a turbidity meter and the presence of aperture (54) will result in light scattering which enables the provision of a standard turbidity measurement to be recorded and used for the instrument.

The flow chart of Figure 11 is self-explanatory and such a chart may, if desired, be included in the kit (in compartment (7)

Some of the electrical circuits provided in the kit may be, if provided, in well known form. These include a digital thermometer and a circuit for a pH meter. Thus these, which may for example be as described in "Radio Spares" Data Sheet No. 4490 will not be described, nor will the (optional) battery charger, which may have any suitable form. However, for the sake of completeness, preferred forms of the other circuits are described below with reference to the respective Figures.

Figure 5 is a diagram of a temperature controller for the incubator. This has a positive rail 400 at a regulated 8 volts positive, an earthed rail 401 and another, unregulated, rail 402 at a nominal 12 volts positive. A set point for the controller is adjustable by means of a variable resistor 403 which is connected to the rails 400 and 401 via protective resistors 404 and 405 respectively. The tap of the resistor 403 is connected to the non-inverting input of a comparator 406. A temperature sensor 407 is connected to the inverting input of the comparator by way of an RC smoothing circuit 408. The amplified error between the sensed temperature and the set point is fed to the base of a power transistor 409 of which the base is connected to the positive rail 402 by way of a resistor 410 and of which the collector drives the incubator's heater winding 411. A light-emitting diode 413, in series with a resistor 412 indicates when the transistor 409 is conductive and providing current through winding 411.

Figure 6 is a simplified diagram of a conductivity measuring circuit which includes terminals 414 and 415 between which is located the liquid sample. The sample thus constitutes an input resistance to a difference amplifier 416 which has a feedback circuit 417 comprising a set of switchable resistors to provide different ranges of measurement. An oscillator 418 provides a low frequency (e.g. 1KH_z) square wave between a reference input of the amplifier and the input resistance, the amplitude of the square wave being adjustable by means of a variable resistor 419. The output of the amplifier 416 is converted to a direct current signal by a r.m.s. to d.c. converter 420 and the direct current signal is sensed by a voltmeter 421. The circuit is easy to calibrate and can easily (by subtracting the input square wave to the amplifier from the output of the amplifier) be made to give a direct reading of conductivity if desired.

Figure 7 illustrates in schematic form a circuit which is suitable with minor modifications for the residual chlorine meter, the turbidity meter or a combined meter. The same circuit can be used for the chlorine meter and the turbidity meter because each meter effectively consists of two sources of light which illuminate, preferably alternately, a photocell by way of a sample or standard. In the circuit shown each light source is energised in turn and a corresponding output signal from the photocell is developed; after suffient time for the output signal to settle, it is sampled and held; a signal corresponding to the difference between the held signals is displayed. Thus the circuit is generally useful when an optical measurement of a sample must be compared with a reference measurement. Power for the circuit is provided by a battery 430; the voltage of a positive rail 431 is set by a voltage regulator 432 controlled by a timer 433. When a series switch 434 is closed, the regulator 432 provides energisation of the rail 431 for a time, such as 10 to 30 seconds, set by the timer 433. Energisation of the rail initiates operation of a closed chain of monostables 441, 442, 443 and 444. The ON periods of the monostables 441 and 443 are relatively long, those of monostables 442 and 444 are relatively short. The monostables control two transistor switches 435 and 436, one for each of the light-emitting diodes 437 and 438, two analogue switches 439 and 440, in synchronism with the switches 435 and 436, and two further analogue switches 445 and 446 which feed the output of an amplifier 447 to the non-inverting inputs of amplifiers 448 and 449 respectively. A variable resistor 450 is connected to receive the difference of the outputs of the amplifiers 448 and 449 and serves to calibrate a meter 451.

During the ON periods of monostables 441 and 442, switch 435 is conductive to energise the light source constituted by diode 437, which illuminates photocell 452 by way of the sample (or standard) 453. The photocell provides an input for the regulated amplifier 447, which is energised from the power rail 431 by way of switch 439. When monostable 441 times out, monostable 442 turns on, maintaining switches 439 and 435 conductive but also rendering switch 445 conductive, whereby the output of amplifier 447 is sampled and held on a capacitor 454 coupled to the input amplifier 448.

When monostable 442 times out, monostables 443 and 444 turn on successively to provide a similar sequence at the end of which switch 446 feeds to capacitor 455 the output of amplifier 447, which now represents the light, as modified by the sample or standard, from diode 438. The signals held on the capacitors 454 and 455 are amplified by the amplifiers 448 and 449 respectively and the meter 451 responds to the difference between those signals. If the circuit shown in Figure 7 is for a chlorine residual meter the diodes 437 and 438 may provide "red" and "green" light respectively. Different pairs of wavelengths may be selected, using different light sources and filters if necessary, if a different absorpsiometer is employed, instead of a chlorine residual meter, for example for the quantitative detection of such ions as nitrate, cadmium, phosphate, magnesium, lead, iron or fluoride. Figure 8 is a diagram of a circuit suitable for a turbidity meter. The only electrical differences between this circuit and that for the chlorine residual meter are (i) the use of a "low-intensity reference red" diode 437 and a "high intensity red" diode 438a; (ii) the slight alteration of the switching circuit so that both diodes are on simultaneously and then only one diode is on; it is preferable to provide energisation by the low intensity alone and then energisation of both diodes or vice versa. It is a simple matter to provide a single circuit with switches to convert between the circuits of Figures 7 and 8.

Alternatively, one diode may be capable of emitting red and green light alternately and the other may be a diode capable of emitting high intensity red light. Again, simple switching may be incorporated to enable use of the red/green diode for the chlorine residual measurement and the red part of the red/green diode together with the high intensity diode for the turbidity measurement. All the aforementioned diodes are commercially available.

The following description gives general guidance as to the use of a kit as described.

Thus, the water quality officer first undertakes a simple visual inspection of the site and water supply facilities and records details of location, source type and time of sampling on a Water Quality Daily Report Sheet pad. The sample is taken using the pre-sterilised sample cup (19) which has been transported inverted on the filtration assembly (5) within the base (3) of the water testing kit. Where necessary, e.g. in a well, the sample cup (19) is lowered into the water to be sampled using an attached chain. Otherwise it is held in the hand during collection of the water sample, care being exercised not to introduce contamination from extraneous sources into the sample cup.

A sterile membrane filter (not shown) is placed on the sintered bronze disc (16a) in the filter support or base (15) and the filter funnel assembly consisting of tube (13) and collar (14), is clamped onto the filter base (15). The funnel is filled with water from the sample cup (19) up to a precalibrated mark which represents 100 ml. Using a suction device (18), which connects directly to the filter support (15), a vacuum is drawn within the vacuum vessel (17) and the water sample passes through the membrane filter. All bacteria, including those which will be cultured on the selective medium are retained within the membrane.

The funnel assembly (13, 14) is then removed and placed to one side. The membrane is lifted using uncontaminated forceps and placed on a pad previously soaked in selective nutrient broth medium within an aluminium petri dish (supplied and reusable) or a commercially available disposable dish. Sterile pads, nutrient medium and petri dishes are prepared in advance. The petri dish is closed and placed in the incubator module (6) in the middle of the base (3) of the water testing kit box (1) by means of an aluminium carrier. The lid (2) of the incubator is replaced and, if the sample is the last to be processed that day, an incubator (23) is switched on using toggle switches located on the control panel (30) (if using the integral sealed leadacid battery power supply), or plugged directly into a mains electricity source before switching on (120 or 240V), or plugged directly into another external 12V (DC) source. The incubators should be capable of independent operation at either 37°C or 44°C. Thus a range of microorganisms may be detected according to the choice of the operator. They would include the three commonest indices of the hygienic quality of water : faecal coliforms, faecal Streptococci, and total coliforms. All of these may be recoverded and enumerated by the membrane method on standard media within the reusable petri dishes supplied or within commercially available single-use, disposable petri dishes. For example, results of the faecal coliform test are obtained following incubation at 44°C for 12-18 hours during which time the bacteria within the membrane grow to produce visible colonies. The presence of faecal coliform bacteria is denoted by the growth of yellow colonies, the number of yellow colonies constituting the number of these bacteria in the original 100 ml sample capable of growth on the selective medium at 44ºC. The result is quoted on the report sheet as "Faecal Coliforms per 100 ml".

If the water supply is chlorinated, the water quality officer may elect not to perform the faecal coliform test pending the results of chlorine residual and turbidity tests, the results of which may be determined on site. In general terms, provided that the chlorine residual value exceeds 0.1 mg/1 and the turbidity value does not exceed 5 turbidity units (TU), bacteriology may not be necessary.

A chlorine residual test may be performed by introducing a sample into a clean, standard cuvette which is placed within the water-tight recess (49) in the cente of meter (9). The instrument (9) may be precalibrated by means of supplied standards. The introduction of a tablet of DPD 1 reagent stimulates the production of a pink coloured complex, the intensity of which is proportional to the concentration of residual chlorine in the sample, and which is measured by means of photo-absorption. As indicated earlier, an LED may be switched on by means of a magnetic switch which also serves as the light- excluding cover for the cuvette during measurement. The reading is displayed on LCD (48) and is entered on the results sheet. Total or combined chlorine may also be determined by repeating the procedure with different reagents.

A turbidimetric reading may be obtained by the introduction of sample in a clean standard cuvetee within turbidity meter (10).

Meter (10) is also precalibrated for this test. The magnetic switch/cuvette cover is placed in the 0 - 20 TU mode and the reading displayed and recorded on the results sheet in turbidity units (TU). Measurement in this case is based on light scatter due to suspended particles (nephalometry). However, if the reading exceeds the linear range 0 - 20, it is necessary to switch from the nephalometric mode into the light extinction mode (20 - 200) in order to obtain a satisfactory reading in TU.

Samples may be introduced into cuvettes by means of supplied plastic pasteur pipettes (reusable) or by careful pouring of the sample into the cuvette held outside the instrument using a clean paper tissue to avoid leaving grease marks on the outer surface.

Conductivity, temperature and pH are performed using the single instrument (8). Three conventional probes or electrodes (45 to 47) are supplied with the instrument (8) and they may be retained on one of the dividing walls (63) in the base (3) of the water testing kit. The instrument (8) is adjusted into a conductivity mode, and the appropriate probe placed in the water in the sample cup (19). LCD (51) provides a reading of conductivity in μS per cm when the "Press to Read" switch is depressed, and this is recorded on the results sheet. The probe is cleaned (if necessary) and dried off with a clean cloth before replacing in a retaining clip in one of the dividing walls (63) of box base (3). The pH is determined by placing the instrument in the appropriate mode (pH). This test also requires precalibration at base using (ideally) freshly prepared standard solutions (tablets are supplied). There is little drift with the instrument and probe is care fully handled, and recalibration should be infrequent, e.g. monthly. A standard pH probe with its protective cover removed is held in the test sample and a reading obtained on depression of the "Press to Read" switch; this is also recorded on the results pad. The probe is rinsed (if necessary) and the protective cover containing buffer solution (tablets are supplied) is replaced to keep the probe membrane moist and serviceable. The probe is then replaced in a respective retaining clip in a dividing wall of box base (3). Temperature is determined by switching to the appropriate mode on the combination meter (8) and immersing the temperature probe in the water sample. For the sake of accuracy this reading should be made in a freshly taken sample or in the water supply or source itself. After the reading is recorded on the data sheet, the probe should be dried and replaced in its holder. The water quality officer should also record any peculiarities in physical nature, taste or odour of the water sample as well as any salient observations concerning the hygienic state of the water supply, storage, distribution network or sampling point. Based on his/her experience of the local water quality, together with training and knowledge of health criteria, the water quality officer designates the water sample satisfactory or otherwise. This may require an overnight delay until the results of faecal coliform analysis is known. Where remedial action is indicated, the water quality officer notes this fact on the report sheet.

In between sampling for faecal coliform or other bacteriological analysis, it is necessary to resterilise the sample cup (19), filter funnel assembly (13, 14), filter base (15) and forceps. For the sterilisation process to be effective all of these components should be thoroughly dried using a clean cloth or tissue. The filter funnel is loosely clamped in place with the forceps placed inside. Approximately 1 ml of methanol is introduced into the sample cup (19) using a reusable plastic pasteur pipette. The methanol is swirled around, ignited using the petrol lighter (rechargeable and supplied) and allowed to burn for a few seconds before being quickly and firmly inverted onto the filter funnel assembly (13, 14), filter base (15) and vacuum flask (17) . The methanol now burns in the absence of sufficient oxygen and converts to formaldehyde vapour which permeates and thus disinfects the components. The equipment is left assembled and sterile until the next sampling point, with the exception of the vacuum flask (17) which is separated from the filter base (15) and the contents (filtrate) discarded. It should be noted that a number of small tubes (e.g. two) containing cuvettes and standards for the chlorine residual meter (9) and turbidity meter (10) may be enclosed within lid (2) along with the electronic instrumentation.

Although the invention has been particularly described above with reference to the accompanying drawings, modifications and alterations within the spirit and scope of the invention and the broad principle of unitary modular construction which it represents will occur to the skilled man and are included in the overall inventive concept.

Claims

CLAIMS :

1. A portable water test kit comprising in unitary and modular form: means for testing a water sample to determine the faecal coliform count thereof, means for determining the turbidity of the said sample and means capable of determining the residual chlorine level of the said sample.

2. A kit as claimed in claim 1 also comprising means for determining any one or more of pH, temperature, conductivity, dissolved O₂ or NO₃ in the water sample.

3. A kit as claimed in claim 1 or claim 2, wherein the means for determining faecal coliform count includes an incubator provided with a controlled heater, a power source,

a sample container and a filtration assembly together with means for sterilizing the filtration assembly and sample container.

4. A kit as claimed in claim 3, wherein the power source comprises a nickel/cadmium battery or a solar cell.

5. A kit as claimed in claim 4, wherein the power source comprises a nickel/cadmium battery and is associated with a battery charger.

6. A kit as claimed in any one of claims 1 to 5, wherein the means for determining turbidity and the means for determining residual chlorine are separate instruments with separate result displays.

7. A kit as claimed in any one of claims 1 to 5, wherein the means for determining turbidity and the means for determining residual chlorine are combined in a single instrument having a single result display.

8. A kit as claimed in any one of claims 1 to 6, wherein the means for determining turbidity comprises means for receiving a water sample, a light detecting means for generating a signal in response to light falling thereon, high intensity light generating means positioned to direct light towards the sample so as to provide a indirect light generating means/sample/detecting means light path, low intensity light generating means positioned to direct light towards the sample so as to provide a direct light generating means/sample/ detecting means light path, timing means for activating separately in sequence the low intensity generating means and both high and low intensity generating means simultaneously, solid state processing means for comparing a signal from the detector corresponding to total light received from the high intensity generating means and the low intensity generating means with a signal from the detector corresponding to light received from the low intensity generating means only so as to develop a signal representative of the turbidity of the sample, and means for displaying the turbidity of the sample in response to the signal representative thereof.

9. A kit as claimed in any one of claims 1 to 6 or 8, wherein the means for determining residual chlorine comprises means for receiving a water sample, means, including timing means, for generating alternate green and red light positioned so as to direct such light onto the sample, means for detecting light transmitted by the sample and for producing a signal corresponding thereto, solid state means for comparing the signal produced by the detecting means during passage of green light through the sample with the signal produced by the detecting means during the passage of red light through the sample so as to produce a signal representative of the free chlorine content of the sample, and means for displaying the free chlorine content of the sample in response to the signal representative thereof.

10. A kit as claimed in claim 7, wherein the single instrument determining turbidity and residual chlorine comprises means for generating a signal in response to light falling thereon^high intensity light generating means positioned to direct light towards the sample to as to provide an indirect light generating means/sample/detecting means light path, means, including timing means, for generating alternate red and green light positioned to direct such light towards the sample so as to provide a direct light generating means/sample/ detecting means light path, timing means for activating separately in sequence the means for generating alternate red and green light to generate red light and said red light generating means simultaneously with the high intensity light generating means, solid state processing means for comparing the signal produced by the detecting means during passage of green light through the sample with the signal produced by the detecting means during passage of red light through the sample so as to produce a signal representative of the free chlorine content of the sample, solid state processing means for comparing a signal from the detector corresponding to the total of light received from the high intensity generating means and red transmitted light with a signal from the detector corresponding to red transmitted light only so as to develop a signal representative of the turbidity of the sample, switching means for setting the mode of the meter at turbidity signal generation or at free chlorine content signal generation as desired, and means for displaying either the turbidity of the sample in response to a signal representative thereof or the free chlorine content of the sample in response to a signal representative thereof in response to the setting of the switching means.

11 . A kit as claimed in any one of claims 1 to 10

also including a solid calibration device comprising a column of transparent solid material of uniform cross section having a centrally-positioned aperture along a part only of the longitudinal axis of the column sealed from the exterior of the column.

12. A method of testing for the quality of water which comprises taking a sample of the water and testing the sample for chlorine residual level and turbidity using a kit as claimed in any one of claim 1 to 11 and, if necessary, thereafter testing the sample for faecal coliform count.

13. A turbidity meter comprising means for receiving a water sample, a light detecting means for generating- a signal in response to light falling thereon, high intensity light generating means positioned to direct light towards the sample so as to provide an indirect light generating means/sample/detecting means light path, low intensity light generating means positioned to direct light towards the sample so as to provide a direct light generating means/sample/detecting means light path, timing means for activating separately in sequence the low intensity generating means and both high and low intensity generating means simultaneously, solid state processing means for comparing a signal from the detector corresponding to total light received from the high intensity light generating means and the low intensity generating means with a signal from the detector corresponding to light received from the low intensity generating means only so as to develop a signal representative of the turbidity of the sample, and means for displaying the turbidity of the sample in response to the signal represenative thereof.

14. A chlorine residual meter comprising means for receiving a water sample, means, including timing means, for generating alternate green and red light positioned so as to direct such light onto the sample, means for detecting light transmitted by the sample and for producing a signal corresponding thereto, solid state means for comparing the signal produced by the detecting means during passage of green light through the sample with the signal produced by the detecting means during the passage of red light through the sample so as to produce a signal representative of the free chlorine content of the sample, and means for displaying the free chlorine content of the sample in response to the signal representative thereof.

15. A meter capable of determining either turbidity or chlorine residual values in a sample and comprising means for generating a signal in response to light falling thereon, high intensity light generating means positioned to direct light towards the sample so as to provide an indirect light generating means/sample/detecting means light path, means, including timing means, for generating alternate red and green light positioned to direct such light towards the sample so as to provide a direct light generating means/sample/detecting means light path, timing means for activating separately in sequence the means for generating alternate red and green light to generate red light and

said red light generating means simultaneously with the high intensity light generating means, solid state processing means for comparing the signal produced by the detecting means during passage of green light through the sample with the signal produced by the detecting means during passage of red light through the sample so as to produce a signal representative of the free chlorine content of the sample, solid state processing means for comparing a signal from the detector corresponding to the total of light received from the high intensity generating means and red transmitted light with a signal from the detector corresponding to red transmitted light only so as to develop a signal representative of the turbidity of the sample, switching means for setting the mode of the meter at turbidity signal generation or at free chlorine content signal generation as desired, and means for displaying either the turbidity of the sample in response to a signal representative thereof or the free chlorine content of the sample in response to a signal representative thereof in response to the setting of the switching means.

16. A solid state calibration device comprising a column of transparent solid material of uniform cross section having a centrally-positioned aperture along a part only of the longitudinal axis of the column sealed from the exterior of the column.

17. A kit as claimed in claim 9 or claim 10 or a meter as claimed in claim 14 or claim 15 modified by the use of light generating means capable of producing light of an alternative wavelength other than red and/or green, said alternative wavelength being associated with a predetermined solute the value of which is to be determined.

18. A kit or meter as claimed in claim 17, wherein said alternative wavelength is dependent upon the absorption properties of the solute directly.

19. A kit or meter as claimed in claim 17, wherein said alternative wavelength is dependent upon the absorption properties of the solute after an identifying chemical reaction characteristic of said solute.