WO1992008956A1

WO1992008956A1 - Apparatus for measuring physical properties of fluids

Info

Publication number: WO1992008956A1
Application number: PCT/CH1991/000227
Authority: WO
Inventors: Pierre Favre
Original assignee: Cableries Et Trefileries De Cossonay S.A.
Priority date: 1990-11-13
Filing date: 1991-11-05
Publication date: 1992-05-29
Also published as: CH683800A5; JPH05503361A; EP0510143A1; CA2073688A1

Abstract

The apparatus is comprised of two sensors of the thermal type (11a, 11b) made of materials resisting without any alteration to a temperature of 650 °C. Probes (15a, 15b) are controlled by a microprocessor (16) to come periodically and momentarily in contact with said sensors in order to calculate (through microprocessor (16)) the thermal time constant of said sensors presenting a deposition layer of environmental pollutants. The microprocessor (16) compares the value of said time constant with an order value that commands the closure of a shunt switch (17) of a resistance (18) which, when short-circuited, determines a reinforced heating of the sensors to bring them to a temperature of incineration of organic materials originating from the pollution of the ambient medium and deposited on said sensors. Thus, the sensors are maintained in the cleanliness conditions of their surface, thereby providing for the accuracy of information supplied by said sensors outside cleaning periods.

Description

Apparatus for measuring physical properties of fluids

A large number of sensors are known (for example: humidity sensors, temperature sensors, thermal effect sensors such as those used in anemometry).

However, when these sensors are used in a

atmosphere (the atmosphere in general) loaded with elements

pollutants (hydrocarbons and fumes for example) this

pollution causes deposits of pollutants

on the surface of the sensors, which has the effect of adversely influencing the measurements that one wishes to perform.

The invention aims to provide a solution for eliminating this harmful effect of ambient pollution

on the accuracy of the measurements made by means of sensors intended to be used in a polluted environment.

The object of the present invention is consistent

to claim 1.

In what follows, the invention is considered mainly - but not exclusively - in the case of thermal anemometric sensors.

The performance of thermal anemometers described in numerous patents: US 4,206,638 (1980) - US

4,279,147 (1981) - US 4,793,182 (1988) - US 4,794,795

(1989) - US 4,936,144 (1990) - US 4,920,793 (1990) - and in reference works such as "Resistance

Temperature Transducers "by Virgil A. Sandbord, Colorado

State University and "Kitzdraht-und Hitzfilmanemometrie" by Dr.-Ing. Herbert Strickert, are more or less altered by the presence of contamination agents. This is due to the fact that the heat exchange is linked to the properties

physical properties of the layer present at the sensor interface

resistive and air.

So thermal conductivity, specific heat, thickness, roughness and other properties, again

modify the heat exchange law between the sensor

and the air. The thermal anemometric sensor with heating

direct or indirect is based on the principle of a heat exchange between a heated element which is cooled

by forced convection due to the moving fluid.

By far the most common direct anemometers, the elimination of

contamination agents deposited on the sensor can

by modifying the heating of the sensor

in such a way that the temperature of the sensor

reaches the temperature of 600 ° C.

It is possible without great difficulty to control this overheating by the very increase of the current circulating in the sensor, but it is clear that the sensor must be made of materials supporting without alteration

temperatures of the order of 600 to 650 ° C without the

performance, in particular the stability of the resistance and the temperature coefficient thereof, are not impaired.

The invention is applicable without technological difficulties to the case of the omnidirectional thermal anemometric sensor. In the case of the directional anemometric sensor, it is more difficult. However, for example,

the split-film sensor described in the Djorup No patent

4,794,795 US is perfectly suitable for use in the context of the present invention.

The accompanying drawing shows, by way of example, an embodiment of the device according to the invention, in the particular non-limiting case of the directional anemometric sensor with heat loss described in

Patent CH 638,618 (and in the corresponding patents of other countries, including in particular the US patent 4,279,147).

Fig. 1 is a perspective view of the sensor according to this example.

Fig. 2 is a cross-sectional view of the sensor according to FIG. 1, on a larger scale.

Fig. 3 is an electrical diagram of the device.

The sensor, which is of the type shown in the

Fig. 5 of US Patent 4,794,795, is formed of a body hollow 10 made of insulating ceramic material on which are fixed two parallel semi-cylindrical detectors,

11a, 11b, which are resistive thermal detectors. A protective layer 12, made of fused silica or aluminum oxide for example, covers the detectors.

The detectors are made of a metal whose electrical resistance and temperature coefficient are not altered under the effect of heating to 600-650 ° C.

Platinum is an example.

13 and 14 show rigid support members of the sensor.

All other technical information relating to such a sensor can be found in the aforementioned patent CH

638.618, so we will not repeat them here.

The example device according to the invention comprises one or more detectors according to FIG. 1 and 2 and its electrical diagram is shown in fig. 3. This diagram derives directly from that according to fig. 3 of the cited patent CH 638.618, with however an important and new addition, namely means ensuring the automatic self-cleaning of the detectors 11a, 11b, to ensure the constancy of exact measurements by the automatic elimination of the deposits of polluting materials on the sensor.

We will briefly recall the part of the diagram according to fig. 3 which is taken from the aforementioned Swiss patent, in order to facilitate understanding of the invention.

The circuit according to fig. 3 provides speed and direction signals as a single compound output signal. The pair of detectors 11a and 11b is connected to two of the four arms of a bridge.

Wheatstone, formed of resistors 23 and 24 used to balance the bridge when the fluid around the sensor is at rest or at zero speed. The excitation of the bridge in Figure 3 is provided by connections 25 and 26 and the balance of the bridge is measured between points 27 and 28 then amplified by a differential amplifier 28, which thus provides a signal 30 which is a measure of the degree of imbalance of the determining bridge The direction. The signal 30 indicates the imbalance by taking a polarity either positive or negative, when one or the other of the detectors 11a and 11b of the pair is touched at higher speed by the current of the fluid. At the current sheltered detector the current speed

will appear weaker because of the screen represented by the detector directly exposed to current. The size of the resulting differential output signal 30 is a direct measure of speed. Resistors 31 and

33 are the amplifier input resistors

29 and the resistors 32 and 34 are the resistors

feedback. The differential amplification factor is determined by the resistance ratio

feedback 32 and 34 with respect to the input resistors 31 and 33 respectively. The typical amplification factor is 20 to 25 for a maximum current of, for example, 20 m / sec. The bridge formed by resistors 23 and

24, with the pair of detectors 11a and 11b can be

considered electrically as a single resistor

which in turn becomes a branch of a second bridge

of Kheatstone formed by a power resistance

35 in series with the first Wheatstone bridge, determining the direction, and by the resistors 36 and 37 used to balance the second bridge at an operating point defined by the value of the resistors 36 and 37. Each resistance 36 and 37 can be modified during the design of the bridge circuit or a potentiometer or a variable resistance can be used for one or the other of these resistances.

It is preferable not to take potentiometers for the two resistances. This allows the operator to choose the operating point, the level of

power and sensitivity of the instrument. The amplifier 38 is differential and has an amplification factor with a high output current which is connected in feedback to the bridge at point 39. The input of the amplifier 38 is connected between the dots

26 and 40 of the bridge and you have to pay attention to the phase. to make sure you get a feedback, not a positive one.

The detectors 11a and 11b with the resistors 23 and 24 seem to form a unique resistance for the amplifier 38 which changes with each variation.

of its constituent parts. Detectors 11a and

11b are in fact resistors with a non-zero temperature coefficient, are subject to their own heating and, when the film is platinum, have a high positive temperature coefficient. This fact makes it possible to choose the values of the resistances 36 and 37. in such a way that the values of balance resistance of the bridge for condition of balance of the bridge are satisfied when the total series-parallel resistance of the bridge for the direction, interpreted as a resistance

unique equivalent, with power resistance

35, the two together balance resistances 36 and 37 because the same resistance ratios are established on both sides of the bridge. The active side includes resistor 35 with the bridge for direction, formed of resistors 11a and 11b with resistors 23 and 24. The side of

bridge reference, controlled by feedback, includes resistors 36 and 37.

When detectors 11a and 11b are cold or

out of service, their resistance is lower than during operation and by controlling their operating value by adjusting the resistance ratio

values, the values of the heated resistances

necessary to balance the bridge can be chosen, all this being controlled by the feedback

through amplifier 38 to the bridge at point 39.

The feedback acts so as to automatically regulate the current through all of the combined bridges until the resistances of the detectors 11a and 11b reach the values balancing the bridge. A little tension

offset must be present at the output of amplifier 38 when the circuit is switched on for the first time and detectors 11a and 11b are at room temperature, so that the first bridge current, which is established following the offset voltage, is sufficient to

a small error signal appears between the dots

26 and 40, thus allowing the circuit to establish the operating conditions itself. The above operating mode has been described as a constant temperature (constant resistance) method for

the operation of a hot film or wire anemometer

hot.

The resistor 37 may be a temperature-sensitive resistor placed so as to take the temperature of the ambient fluid. If the temperature coefficient

the resistance of damper 37 is chosen appropriately, the level of operation of the bridge can be

automatically adjusted so as to follow the ambient temperature, the detectors 11a and 11b thus working

with a constant temperature difference from the ambient temperature. This operating mode

provides constant sensitivity for fluid speed, regardless of ambient temperature.

The surface temperature of the detectors 11a

and 11b resulting will be of the order of 125 to 135 ° C.

The output 30 is bipolar and indicates which detector 11a or 11b faces the direction of the current. As a result of the cooling, the facing detector will have a lower resistance than the current-protected detector, a detector which is less cooled and therefore has a higher resistance, while the total of their resistors put in series remains

constant. The size of the outlet 30 is not linear with respect to the incident speed of the fluid and it

indicates the amount of heat lost in the fluid stream.

Amplifiers 29 and 38 can be operational amplifiers powered by positive and negative sources. The 15-volt power supply makes it possible to obtain at least a signal amplitude of 10 volts at output 30- when two or more bridged circuits similar to those of FIG. 3, with a network of two or more sensors are used, correct connection of the

mass and power is required to avoid unwanted interaction between the sensors and the resulting errors. Amplifiers 29 and 38 can also be of the type using a single supply potential such as 15 or 20 volts. In this case the + input of amplifier 29 can be shifted in the positive direction, the adjustment to zero for zero speed

being shifted at the same time to a positive determined value at output 30.

The cited Swiss patent and its equivalent from the United States contain examples of numerical values of the resistances indicated in FIG. 3.

The diagram of the device according to the example of the

present invention differs from FIG. 3 of patent

CH 638.618 with the following.

Lines 19 in dashed lines in FIG. 3 indicate functional reasons.

Each of the sensors 11a, 11b is associated with a probe 15a, 15b, controlled by a microprocessor 16

to be applied to these probes in order to determine, using the microprocessor 16, the time constant

of each of these probes and to trigger

a self-cleaning operation by incineration of these sensors, if this time constant is equal to or greater than a set value. This triggering is controlled by the microprocessor, which causes the closure of a switch 17 which then shunts a resistor 18 which is in series with the resistor 37. When the resistors 18 and 37 are both in circuit, they ensure

that this is the normal heating current of the sensors

11a, 11b which reaches these, while when

resistance 18 is short-circuited by the switch

17 the current received by these sensors is much more

intense and brings these sensors to a temperature of approximately 600 ° C for a few moments, which ensures self-cleaning by incineration of the pollution deposits on these sensors and, consequently, restores them provide exact indications of the air speed to be measured with the device.

The measurement of the establishment time (index response) of the current flowing in the sensor is a measure of the thermal response time and is therefore a measure of the degree of pollution of the sensitive element.

In a variant of the example of the apparatus which has just been described, the control of the switch 17 to trigger a self-cleaning phase by incineration could be ensured periodically by a timer adjusted as a function of the pollution of the medium. ambient, when the latter is substantially constant. Thus the timer would replace the probes 15a, 15b and the microprocessor 16.

In another variant, corresponding to the cases of constant and slight pollution of the ambient environment, the control of the self-cleaning phases could be carried out simply by means of a switch 17 actuated by hand at prescribed intervals depending on the degree of pollution.

Although the example described corresponds to the case of one type of thermal sensor, the invention is not limited to this example. The invention extends to all types of thermal detectors, with direct or indirect heating and to other sensors such as, for example, temperature or humidity sensors.

Cleaning overheating can be achieved by indirect heating.

In the described case of direct heating, it is planned to shunt before switching on the cleaning current, the electrical components which could be altered by this cleaning current.

We understand from the description of FIG. 3 that the frequency of the self-cleaning periods is defined by measuring the thermal time constant of the detectors, since the contamination of these detectors has the effect of increasing this constant.

Claims

claims

1. Apparatus for measuring the physical properties of fluids, comprising at least one sensor (fig. 2)

of materials withstanding a temperature of 650 ° C without alteration, characterized in that it comprises a device (15a, 15b, 16-19) for self-cleaning by incineration

products of pollution of the ambient fluid deposited on the sensor (s) and modifying the physical properties of the resistive sensor / fluid interface in which the device is located,

and in that this self-cleaning device comprises a heating means (15a, 15b) of the sensor (s)

(11a, 11b) to temporarily bring them to a temperature ensuring rapid incineration of said pollution products deposited on them and control means

electric (16, 17, 18, 19) of said heating means

(15a, 15b) depending on the state of pollution of the

surface of the sensor (s).

2. Apparatus according to claim 1 and wherein the sensor (s) (11a, 11b) are of the thermal type, characterized in that said electrical control means of said heating means for cleaning comprise an additional electrical resistance (18) arranged in series with a member (15a, 15b) for electric heating of the sensor, intended to heat the sensor to its temperature for measuring physical properties

ambient fluid, and further comprise a switch (17) for shunting this additional resistance (18) during the phases of this cleaning by incineration, to then produce the additional heating of the sensor necessary for this cleaning by incineration.

3. Apparatus according to claims 1 and 2, characterized in that the electrical control means of said heating means for cleaning comprises a

adjustable timer for periodically actuating said switch (17) bypassing said additional resistance

(18), with a frequency adjusted according to the state of pollution ambient fluid, to automatically ensure self-cleaning in good time.

4. Apparatus according to claims 1 and 2, characterized in that said electrical control means comprise a probe (15a, 15b) intended to be applied periodically and automatically on the resistive sensor (11a, 11b) and to cooperate simultaneously with a circuit (16-19) arranged to measure the thermal time constant of this sensor (11a, 11b) coated with a layer of pollution products deposited on it, and to control the activation of cleaning by incineration of this layer , when the measured value of this time constant reaches or exceeds a predetermined setpoint.

5. Apparatus according to claims 1 and 2, characterized in that the electrical control means of said additional heating means, include manual control of said switch (17) for shunting this additional resistance (18).