DE19619133A1 - Sensor for determining the thermal conductivity and / or the temperature of non-flowing, liquid or gaseous substances and method for exciting the sensor - Google Patents

Abstract

A sensor is disclosed for determining the thermal conductivity and/or temperature of liquid, gaseous or viscous substances, for example engine oil. A process for driving the sensor is also disclosed. The sensor has a supporting body (1) and a measurement winding (7) of a type that reduces self-induction and inductivity. In order to create an appropriate sensor for determining the thermal conductivity and/or temperature of substances, the resistance wire (8) is uniformly arranged in an embedding space (9) provided for the measurement winding and having everywhere the same heat storage capacity, on a supporting body (1) of which each part has at the most the thermal capacity of a pair of adjacent resistance wire layers, so that the caloric medium temperature of the sensor may be sensed by means of the resistance wire (8). A predetermined cavity (10) for the substance to be examined is provided next to the sensor. The sensor is at first driven with a constant, low electric supply which is then changed to test the transmission behaviour of the sensor.

Description

The invention relates to an electrical resistance sensor for the Determination of the thermal conductivity and / or the temperature of non-flowing, liquid or gaseous substances according to the generic term of Claim 1 and a method for bringing electrical Auxiliary energy in the sensor (excitation of the sensor) according to the generic term of Claim 12.

Gases, liquids or non-flowable substances can due to their different thermal conductivity including the fabric temperature be examined or analyzed if either a binary mixture is present or if only one variable component is the thermal conductivity of the Mixture significantly influenced. The condition of the fabrics can also often vary their temperature or their change are determined.

It is known to determine the proportion of solid, liquid or gaseous substances of a mixture or stratification of these substances with the help of an electrical resistance wire, as in DE 31 22 642 C2 is described. The known method is characterized in that for the mixing or layering the number n of the different Substances determined and in the individual substances the electrical Resistance wire embedded and in each case by means of predetermined n-1 different amounts of electrical energy for a short time within Milliseconds up to a few seconds at most up to a maximum below Evaporation or ignition temperature of the mixture or Layering existing substance with the least evaporation or  Ignition temperature is heated while doing calibration measurements of the Feed times for the supply of the predetermined amounts of energy be made, and that with one in the to be examined Mixtures or layers of embedded resistance wire of the same type mechanical and electrical data as well as when feeding the same predetermined amounts of energy as in the calibration measurements Feed times measured again and from the values of these measurements and the calibration measurements the individual substances of the mixture or Stratification can be determined. The known analysis method is based on the thermal conductivity of the substances, but the actual parameter of the investigating substance, d. H. the thermal conductivity, can not from the Measured variables of the measuring method, in this case from the feed times, can be determined selectively. Since the effects of influence and interference effects such as e.g. B. the effect of the unstable temperature of the substance to be examined cannot be eliminated, the measuring method meets the higher ones Precision requirements are not.

DE 41 35 617 A1 describes a device for determining a Heat transfer coefficient of substances described for their assessment, the device being a measuring probe for converting the thermal conductivity into has an amplitude signal. The measuring probe consists of a housing heat-insulating material and from a slidable and lockable therein stored, elongated temperature sensor. In its non-working position the front measuring end of the temperature sensor is inside the Housing, while in the measuring position just outside the Housing is located in contact with the substance to be examined come. To do this, a temperature measurement is first carried out if the extended temperature sensor with the substance to be examined is in contact. Then takes place on the basis of mathematical relationships including the constant values of the measuring probe Determination of the thermal conductivity of the substance to be examined. This Measuring probe is preferably used when assessing the cervical mucus the woman. It has been found that with this known measuring probe for the determination of the thermal conductivity of the substance to be examined none sufficient selectivity when converting the thermal conductivity into a Amplitude signal can be achieved, and therefore the application of the Measuring probe for mixtures with small changes in the Thermal conductivity not possible.  

For examining or analyzing gases it is known Use thermal conductivity meters (Dr. T. Pfeifer, Dr. P. Profos: Handbook of industrial measurement technology, R. Oldenburg Munich-Vienna, 6. Edition 1994, pp. 913-923; J. Hengstenberg, B. Strum, O. Winkler: measuring, Taxes, rules in chemical engineering, Springer-Verlag Berlin, Heidelberg, New York, Third Edition, Volume II, pp. 94-112). Big meaning have analyzers for determining the gas content in a gas mixture e.g. B. the CO2 content. Carbon dioxide has one in relation to air lower thermal conductivity and the CO2 content therefore affects the Thermal conductivity clearly. Measuring the thermal conductivity of a Gas mixture takes place in cylindrical, thermostated measuring chambers, in which heated platinum measuring wires are stretched out. The measuring wire takes the higher the temperature, the lower the thermal conductivity of the surrounding gas. The resulting change in resistance of the Measuring wire is evaluated. The application of this Thermal conductivity meter requires a sample and needs one very great care in the temperature control of the measuring chambers and the Elimination of external temperature effects and keeping the constant Measuring current and thus a relatively large amount of equipment.

With liquid lubricants, such as lubricating oils, in particular Motor oils, there is a risk of aging and / or contamination of the substances with the disadvantage that these lubricants after a long period of use become unusable and must be replaced. The exchange of the In practice, lubricant mainly takes place after fixed operating times. There the quality of the oil used, depending on the degree of use, Machine condition, type of oil, degree of refining u. a. after fixed operating times is different, the optimal operating time of the oil can usually not be set universally. They are accurate but relatively expensive time-consuming laboratory tests of the lubricants in question known, e.g. B. according to DIN 51 551 (the coke residue as a measure of the Aging condition of an oil). Known simpler methods use e.g. B. dielectric constant (U.S. Pat. No. 4,733,556) or impedance (Impedance) of the oil (U.S. Pat. No. 5,200,027), but it is difficult to do so Characteristics of the affected oil in any case with its lubrication quality correlate and therefore they can provide the necessary selectivity in Do not differentiate between a new and a used oil secure all operating conditions.  

Electrical resistance sensors are known which are used to determine the Temperature of substances are provided. Because the dynamic parameters of such a sensor are variable, they can be delayed sensors in their dynamic transmission behavior describe only approximately and therefore its exact, dynamic Correction that eliminates the dynamic errors of the sensor and thus a Guaranteed increased accuracy of the temperature measurement, practically not possible.

The invention has for its object a heat sensitive for Determination of the thermal conductivity and / or the temperature of flowable, liquid or gaseous substances suitable sensor create and excite him so that from the output signal of the sensor a determination of the thermal conductivity and / or the temperature of the substance becomes possible, and that the heat-specific and electromagnetic Interference effects that overlay the actual measured variable when it occurs, in their impact can be largely eliminated. It will possible to exactly the thermal conductivity and / or the temperature of substances determine in order to change the state of matter or the Determine substance properties or concentrations precisely, the Furthermore, the sensor is simply constructed and in place, without sampling and sample preparation, d. H. without sampling errors, uncomplicated in the Application is.

The following are to be understood as heat-specific interference effects:

• a. the unstable temperature of the substance to be examined.
• b. the movement of the substance to be examined or its gas or Liquid fraction when determining thermal conductivity.

The following are to be understood as electromagnetic interference effects:

• a. the electromagnetic interference from the environment.
• b. the electrical interference voltages on the part of the electrical excitation signal (of the electrical feed signal).

The object is achieved by the features of Claims 1 and 12 solved. Preferred embodiments of the invention are specified in the subclaims.  

The invention is therefore based on the idea of an interference-free examination the transmission behavior of a sensor proposed by the invention, which is sensitive to heat thanks to its design and is dynamic Transmission behavior behaves like a linear 1st order measuring element for which principle-related determination of the thermal conductivity and / or the temperature of the to touch him directly, to be examined, to extract from the Output signal of the sensor to get information about how intensely the determined in the measuring winding of the sensor determined electrothermal power on the fabric with a changing Thermal conductivity is transferred, the interference effects on the The result of the investigation has a negligible influence.

For a better understanding of the context of the present invention to explain that:

• a. Under such a heat-sensitive, linear sensor of the 1st order Sensor is to be understood, its output variable, in this case the electrical one Resistance of the measuring winding, the thermal energy stored in the sensor is proportional, d. H. the heat sensitivity of the sensor is constant, and also the transmission behavior of the sensor through a 1st order differential equation, d. H. with the help of only one dynamic Parameter (the so-called time constant T) can be described exactly can.
• b. Under the most interference-free investigation of the transmission behavior of a such a sensor is understood to be an experimental determination of the functional Relationship between a certain input signal, in this Case the electrical excitation signal of the sensor, as the cause and the Output signal from the sensor, in this case the change in resistance of the Measuring winding, as an effect, the output signal being determined by an influencing variable, in this case affects the thermal conductivity of the substance to be examined becomes. The interference effects are determined by the determination and Construction measures suppressed.
• c. Under the principle-related determination of thermal conductivity and / or Temperature is understood to be the basis for such a determination thermal conductivity the first law of thermodynamics (energy balance) and Fourier's law of heat conduction (kinetic approach), and that it was the determination of the caloric, dynamically corrected Temperature of the sensor.

With the invention, the heat-specific and electromagnetic Interference effects that occur during the transformation of the thermal conductivity and / or Temperature of the substance to be examined can occur in their Eliminate impact to a large extent and therefore reliable statements can be made about the finest changes in the state of matter or properties or concentrations of liquid, gaseous or non-flowing substances be made. The solution according to the invention can be used universally and allows an exact determination of the thermal conductivity of the examined Carry out fabric under practical conditions with a relatively little equipment. The relevant investigations can be made on the spot such that a Sampling is not necessary. This means that there are also measurement errors switched off due to sampling and sample preparation surrender. The solution according to the invention also serves, with the same sensor, a measurement of the current fabric temperature, either as Reference temperature for the thermal conductivity should be taken, or after eliminating dynamic errors as a measure or a Influencing variable in various fields of application of measurement and Control technology can be used.

The sensor constructed and excited according to the invention for the determination the thermal conductivity and / or the temperature becomes quasi-continuous with a suitable measuring circuit for monitoring the state of different fabrics used. An economically particularly important one The field of application is the monitoring of a lubricating oil, in particular the motor oil of a motor vehicle, in connection with a measuring device based on microcomputer. Through our own Aging processes in the lubricating oil, e.g. B. by the oxidation taking place in the oil and by mechanically chopping the oil molecule chains, or by Penetration of dirt particles into the oil occurs in the lubricating oil Oxidation, polymerization and other foreign products that are in their inner Structure have significantly shorter molecular chains and thus over have poorer lubricating properties than the new oil itself. One speaks of an increased external degree of freedom of the molecules, which is an ongoing Weakening of force between molecules, d. H. a Deterioration in the viscosity behavior of the oil. It was through ongoing oil sampling found that some determined physical and chemical properties of a lubricating oil in the course of its  Change the load, including the viscosity of the oil (lubrication Engineering, August 1994, pp. 605-611). Since the thermal conductivity of Liquids are essentially determined only by the intermolecular forces is (VDI Warmth Atlas, 7th edition 1994, p. Da 31), influences the "Foreign matter content" noticeably affects the thermal conductivity of the oil. Hence the Change in the thermal conductivity of an oil with the change in its Lubrication quality can be correlated and it is possible through precise determination the pure thermal conductivity to conclude the aging stage of the oil. It It is also known that once an oil has reached a certain stage of aging has passed, the further aging progresses very quickly, which the oil can quickly become unusable. It can then be displayed on the Meter read that the condition of the engine oil is rapidly changing deteriorated so that the oil should be replaced soon.

The size and shape of the free space 10 should generally, depending on the fluidity of the material to be examined, be adjusted in such a way that the toughness of the material to be examined practically prevents its convection. If the subject of the invention is used for the determination of the pure thermal conductivity of low-viscosity or gaseous substances, a mechanical device 15 is to be provided around the sensor, which encloses the sensor in such a way that the specific space 10 comprising the sensor in the form of at least one Cavity is formed, inside of which the friction suppresses the natural convection of the material to be examined, heated by the sensor, and strongly inhibits possible movements.

In order to be able to reduce the heat capacities outside of the embedding space 9 of the sensor, the heat capacity of the carrier body 1 should be considerably smaller compared to the total heat capacity of the sensor, ie the volume of the carrier body 1 in relation to the total volume of the sensor should be very small.

The negative effect of deviations of the sensor from the ideal sensor of the 1st order due to the construction can be reduced by the fact that the materials of the sensor components (carrier body 1 , protective varnish, insulation cover of the resistance wire and the resistance wire 8 itself) have almost the same thermal conductivities, i.e. the temperature compensation takes place in these Components off quickly.

The recording of the entire temperature field of the sensor through the resistance wire 8 can be better ensured if there are no temperature differences on the cross section of this resistance wire, ie if the wire of the resistance wire 8 and the insulating coating are very thin.

The electromagnetic Störeinflußeffekte can be reduced in that the measuring coil 7 is two-wire, that is wound with a double resistance wire 8 with same wires on the carrier body 1 and the two half-windings 7 thus formed, 7b opposite to a Wheatstone bridge in its impact electrically arranged are, the starting points 11 of the wires of this double resistance wire 8 are on the same bridge diagonal.

A very advantageous electrical independence of the output signal Uy from fluctuations in the supply voltage Uo can be achieved in that the amplitude signal Ul is divided by the supply voltage signal Uo with the aid of the multiplier 28 arranged in the negative feedback branch of the amplifier circuit 27 .

The heat-specific and electromagnetic interference effects are in the 2nd process step - thanks to an easy to implement, the requirements of Correlation measurement technique corresponding, pseudo-random pulse-shaped Supply voltage signal for the sensor - also suppressed correlatively and thus is the immunity of the measurement, in connection with the structural Features of claims 1 to 11, significantly increased.

Due to the short duration of the pseudo-random supply voltage signal in the second process step of the excitation, the heat that is transmitted by the sensor in the direction of the negative temperature gradient in the substance to be examined is only able to spread up to a maximum within the specific free space 10, possibly the cavity .

The invention is based on two embodiments and two process examples in connection with the attached drawings explained in more detail. It shows:

Fig. 1 is a partial cross-section through an embodiment of a sensor S1,

Fig. 2 is an overview diagram of one embodiment of a sensor S2, with a surrounding him mechanical means,

Fig. 3 is a repeaterless measuring circuit, as applied for example in the sensor of FIGS. 1 and 2,

Fig. 4 is a linear feed and measurement means for the electrical supply of the repeaterless measuring circuit and processing of the measuring signals as well as a computational device,

Fig. 5 1. Example of a time course of the supply voltage Uo of the amplifier-less measuring circuit and the output signal Uy.

Fig. 6 2. Example of a time course of the supply voltage Uo of the amplifier-less measuring circuit and the output signal Uy.

Fig. 7 test results of the sensor S1 in connection with the ground.

Fig. 8 test results of the sensor S2 in connection with an engine oil.

Embodiment S1

The present example describes an embodiment of the invention, which Determination of the thermal conductivity and / or the temperature of non-flowing substances.

The exemplary embodiment S1 shown in FIG. 1 has a carrier body, generally designated 1 , which is in the form of a very small, coil-shaped component which does not separate very much in the sensor body, with a thin, inner metal rod 2 and with two thin metal end walls 3 and 4 is trained. The metal rod 2 has a diameter of approximately 0.5 mm and the metal end walls 3 , 4 are approximately 0.3 mm thick. The carrier body is equipped with a holder 5 , which is provided for fastening the sensor in a measuring insert 6 . The total volume of the carrier body 1 is very small in relation to the total volume of the sensor. The heat capacity of the carrier body 1 is considerably smaller than the total heat capacity of the sensor (approx. 2%), so that the heat stored in the carrier body 1 is relatively small and the part of the heat stored outside the embedding space 9 is as small as possible.

A measuring winding 7 is arranged on the carrier body 1 , the measuring winding consisting of a double resistance wire 8 formed from two identical, copper, parallel, mutually insulated resistance wires. Each wire of this double resistance wire 8 has a diameter of approximately 0.1 mm and an insulation thickness of 5 μm. The double resistance wire 8 of the measuring winding 7 is spatially uniform and tightly arranged in the embedding space 9 and impregnated with a metal-based, very good temperature-conductive protective lacquer, so that a homogeneous winding coil results, which can be seen in FIG. 1. The outer layer of the protective lacquer is very thin in order to reduce the heat capacities outside the embedding space 9 of the measuring winding, which was also the case when the carrier body 1 was downsized. For this reason too, the sensor must not be equipped with a protective fitting. All construction materials of the sensor (carrier body 1 , protective varnish and the resistance wire 8 ) have similar thermal conductivities and heat storage capabilities, ie that the sensor is thermally identical and the temperature field of the sensor corresponds to the distribution of heat in it. Each wire of the double resistance wire 8 can absorb practically the entire temperature field of the sensor and thus its change in resistance is proportional to the total change in the heat stored in all parts of the sensor.

Since the sensing winding on the carrier body 1 7 consists of a double resistance wire with same wires are located on the carrier body 1 two identical half-windings 7 a and 7 b, each with the initial and Endanschlußstelle 11 and 12. FIG. The points 11 and 12 are connected to inner lead wires 13 of the measuring insert 6 , which in turn are connected to the terminals of the measuring insert 6 . The measuring insert 6 itself is similar to a measuring insert for resistance thermometers according to DIN 43 762 and consists of a flexible jacket tube 13 a with four inner lines 13 , a flange and a base with the connecting terminals.

The exemplary embodiment S1 shown in FIG. 1 represents a basic form of the sensor and is only suitable for determining the thermal conductivity and / or the temperature of non-flowable or viscous substances which cannot flow by themselves. B. the soil that is to be examined for agricultural use or a mucus, especially the cervical mucus, whose changes in state during the course of the woman's cycle or in a woman's disease should be determined for medical reasons.

Embodiment S2

The present example creates S1 on the basis of the exemplary embodiment another embodiment of the invention, which the determination of Thermal conductivity and / or the temperature of thin and allows gaseous substances and, for example, when examining the Aging level of a liquid mixture is applicable.

The exemplary embodiment S2 shown in FIG. 2 consists of a basic form of the sensor S1 from FIG. 1, generally designated 14, and a mechanical device, which surrounds it, generally designated 15. This mechanical device 15 is designed in the form of a cylinder 17 and creates an incompletely closed space (cavity) around the sensor 14 , in this case an annular gap-shaped chamber 16 that is partially open from below and completely open from above. The lower end wall 18 has inflow openings 20 , which make it possible for part of the liquid or gaseous substance to be examined to flow into and out of the chamber 16 . This ensures that the same state is present in chamber 16 and in the room behind the chamber. As can also be seen from FIG. 2, the sensor 14 is arranged at a small distance (approx. 5 mm) from the cylinder 17 . In this way, there is no sufficiently large flow space around the sensor for the natural convection of the material to be examined, which is heated by the sensor 14 , and thus the movement of this material is strongly suppressed by the friction. The free space 10 , in this case the chamber 16 , is, however, sufficiently large to be able to absorb all the heat which is transmitted by the sensor in the direction of the negative temperature gradient in the thin liquid or gaseous substance during the second process step of excitation. The provision of the mechanical device 15 also has the advantage that no interfering external currents can act on the sensor, ie the partially open chamber 16 represents a calming space for the substance to be examined.

Fig. 3 shows an amplifier-less, generally designated 21 measuring circuit for transforming changes in resistance of the double resistance wire 8 of the sensor S1 or S2 into an amplitude signal Ul. In this case, the measuring circuit 21 is constructed in the form of a Wheatstone bridge. The two half-windings 7 a and 7 b, which have arisen due to the two-wire winding design, are arranged in terms of circuitry as the same bridge resistances opposite a Wheatstone bridge, the two starting connection points 11 of half-windings 7 a and 7 b being located on the same diagonal of the Wheatstone Bridge are, as can be seen from Fig. 3. The two other bridge resistors 22 are the same, non-adjustable resistors with a small temperature coefficient and are also electrically arranged to the Wheatstone bridge according to FIG. 3 and mechanically attached to the terminals on the connection base of the measuring insert 6 . The ohm value of each fixed bridge resistor 22 is predetermined such that it is equal to the ohm resistance of a half-winding 7 a, 7 b in the operating temperature of the substance to be examined, ie the Wheatstone bridge is in an optimal state during the excitation of the sensor , almost balanced condition.

If the two half-windings 7 a and 7 b of the two-wire winding configuration according to FIG. 3 are arranged to form a Wheatstone bridge, the current flows in them in the opposite direction when current flows through them and thus produces largely compensating magnetic fields in their effect. In this way, the electromagnetic interferences from the environment cancel each other out in the two half-windings, and the electrical interference voltages generated during the excitation of the sensor by self-induction in the measuring winding 7 are greatly reduced. When the sensor is heated by approx. 10 degrees Celsius from the operating point, that is to say in the heating area during the excitation, the output signal Ul resulting from the bridge circuit is proportional to the change in resistance of the double resistance wire 8 and the electrothermal power produced in the measuring winding 7 gives way - on account of the Resistance change - from the value predetermined by the supply voltage signal Uo no more than 0.5%, which can be easily explained electrically.

In FIG. 4, a common housing 23 is schematically shown, in which a linear feed and measurement means 24 and a computational device 25 are provided. Since the devices can be made very compact together, the housing 23 , which contains its own electrical energy source 26 for the linear feed and measuring device 24 and the computing device 25 , can also be of compact construction, so that the sensor S1 or S2 and the housing 23 can be conveniently transported to the respective measuring location or permanently installed there in order to be able to carry out the desired measurements and calculations in a simple manner.

The linear feed and measuring device 24 contains an amplifier circuit 27 , which is arranged in a series circuit with the amplifier-less measuring circuit 21 and with the computing device 25 and amplifies the amplitude signal Ul that has occurred from the measuring circuit 21 and further inputs its output signal Uy into the computing device 25 . The amplifier circuit 27 in turn has an analog multiplier 28 arranged in its negative feedback branch, which is provided for the multiplication of the output signal Uy by the supply voltage signal Uo. The amplification factor of the amplifier circuit 27 is thus also determined by the supply voltage signal Uo, in such a way that the overall amplification factor of the amplifier-less measuring circuit 21 and the amplifier circuit 27 becomes constant during the excitation of the sensor. This ensures that the output signal Uy is electrically independent of the supply voltage Uo.

Fig. 4 also shows that a power supply circuit 29 is provided in the linear supply and measuring device 24 which is constructed in the form of a controlled supply voltage source for the measuring circuit repeaterless 21st The linear feed and measuring device 24 also has an adjusting element 30 which is controlled on the one hand by the computing device 25 and on the other hand controls the feed circuit 29 . The setting member 30 is constructed as a programmable function generator and is provided for generating a transmission behavior test signal, which will be explained later. The computing device 25 has a conversion circuit 31 , which is responsible for the analog-digital conversion of signals Uy and Uo and for the output and reception of binary control signals for the setting element 30 , and a microcomputer 32 , which is used to execute measurement algorithms, is provided for the sensor-specific measurement signal processing and for the evaluation of the signals Uy and Uo, as well as for the computational determination of the time constant T of the sensor, the thermal conductivity and / or the temperature of the substance to be examined.

If a substance is to be tested, the sensor S1 or S2 can be in the substance at all times or it is held in the substance to be examined, in such a way that the substance will be in the free space 10 . The sensor S1 or S2 is electrically excited in one or more measuring processes in connection with the measuring circuit 21 , the feeding and measuring device 24 and the computing device 25 in two process steps, which is based on the time profile of the supply voltage Uo in FIG. 5 is highlighted or Fig. 6. The first process step of excitation can take any length of time and the aim of such an electrical feed in this process step is the experimental determination of the material temperature, including the sensor-specific measurement signal processing. The 2nd The excitation process step can take place at an arbitrary point in time, but subject to the stability of the substance temperature. This process step may not take longer than the time at which the material can be heated outside the free space 10 . The aim of the second step of the method is first to determine the current time constant T of the sensor immersed in the substance to be investigated, on the basis of the feed and output signals Uo and Uy, and mathematical relationships, including constant values of the sensor. The determined time constant T can then be used for the mathematical determination of the thermal conductivity of the substance to be examined and / or for the dynamic correction of the sensor in the permanent temperature measurement. This may be illustrated by two practical examples.

Process example 1

It is known that the thermal conductivity of the soil for determining the Moisture content can be used very well. Therefore, the Invention for use in soil to the current moisture content to determine that timely moistening measures are used can and to keep the humidity fluctuations within the narrowest possible limits hold.

If the moisture content in the soil is to be examined, sensor S1 is used. The material temperature is measured in the first process step of excitation. Thus, the amplifier-less measuring circuit 21 is excited with a low supply voltage Uo (so-called initial value) in the amount of 200 mV, which brings a low electrothermal power, ie a low heat flow into the sensor and practically does not influence the temperature of the sensor and the ground. The resulting from the measuring circuit 21 amplitude signal Ul is proportional to the change in resistance of the half-winding 7 a and 7 b and it corresponds to the change in the caloric mean temperature of the sensor. The amplitude signal U1 is forwarded to the microcomputer 32 after the corresponding amplification in the amplifier circuit 27 (output signal Uy) and after the conversion into a digital signal in the conversion circuit 31 . The measurement is carried out until it is established that the measured temperature no longer changes, ie the temperature of the sensor is stable and is the same as the temperature of the ground. This stable material temperature, which is the prerequisite for the second process step of the excitation, can be stored as a reference temperature for the thermal conductivity in the microcomputer 32 or shown on a display. In this first part of the measuring method, the sensor S1 arranged in the amplifier-less measuring circuit 21 and fed with the low electrical power represents a temperature sensor with which the temperature of the substance to be examined can be recorded. With the help of the microcomputer 32 , in the course of each 1st method step of the excitation, for example, the correction of specimen scatter from zero point and slope, non-linearities and possibly also the digital or correlative filtering of the output signal Uy is carried out. When the measuring process is repeated, the dynamic correction of the sensor is also possible in the first process step, on the basis of its current time constant T determined in the second process step of the previous measuring process.

At a freely selectable point in time and after the determination of the thermal stability state of the sensor and the ground, the sensor S1 is excited with a time-changing electrical power supply, the course of which corresponds to the requirements of a transmission behavior test. Since the heat-specific interference effects that could occur during the investigation of the soil moisture are weak, the restriction of the free space 10 by a mechanical device is not necessary (the free space 10 is infinite). For this reason, a simple, easily realizable form of the signal for the electrical supply of the measuring circuit was selected to excite the sensor S1, namely a pulsed change in the supply voltage Uo from the low initial value Uo-200 mV to a considerably larger test value. This form of the signal was also selected for the second method step because, due to the construction of the sensor according to the invention, the resulting output signal Uy, ie the change in the caloric mean temperature of the sensor S1, is equivalent to an exponential weight function (impulse response) of the sensor S1 , which is relatively easy to evaluate, and because the thermal processes inside the sensor and in the substance to be examined are relatively easy to recognize.

The test value of the supply voltage Uo should be predetermined in such a way that the sensor S1 is heated to a temperature during the second step of the method, which on the one hand is sufficiently large for further evaluation, but on the other hand no significant natural convection of the gas or liquid component in the Free space 10 located soil to be investigated can cause. Experiments have shown that heating the sensor by up to 10 degrees Celsius generally causes absolutely negligible natural convection and is at the same time large enough for the evaluation purposes. Thus, when examining the ground, a single voltage pulse is used, in which the supply voltage of the amplifier-less measuring circuit 21 reaches a final value of 10 V and which, after about 0.3 seconds, has dropped to the initial value Uo = 200 mV and has risen to this value remains constant until the end of the second process step, which is shown by curve 1 in FIG. 5. Since the free space 10 is unlimited, the second process step can also take any length of time. Experiments have shown that the recording of the output signal Uy in the time of approximately 5-6 seconds after the pulse-shaped change in the supply voltage Uo is sufficient for the evaluation purposes.

Corresponding to the sudden change in the supply voltage, the electrothermal power that arises in the current-carrying measuring winding of the sensor S1 changes, ie the heat flow that initiates the heat conduction. Since the filaments of the current-carrying measuring winding 7 are evenly and densely distributed in the entire volume of the sensor S1, the thermal resistance in the case of heat conduction within the sensor body is also very low and the resulting heat flow is supplied to the entire sensor body, which has a specific heat capacity, at lightning speed. Since the sensor S1 does not currently extract as much heat energy as is supplied, the internal heat energy changes, ie the caloric mean temperature of the sensor, which can be seen from curve 2 , FIG. 5. Due to the temperature difference between the sensor S1 and the soil to be examined, the heat is transferred from the sensor in the direction of the negative temperature gradient in the soil. The thermal resistance of the soil to be examined, ie its thermal conductivity, directly influences the intensity of the energy transport and thus also the change in the internal heat in the sensor. The experimental coupling of the energy balance of the sensor S1 and the heat conduction of the soil to be examined provides an output signal Uy (curve 2 from FIG. 5) which corresponds to the change in the heat stored in the sensor and the transmission behavior of the sensor with only a single dynamic parameter, the time constant T, can be described directly. The evaluation of the experimentally determined output signal Uy during the second method step and the computational calculation of the time constant T of the sensor S1, which is required for the determination of the thermal conductivity of the soil and / or for the dynamic correction of the sensor S1, can, for. B. done in the manner described and explained in the aforementioned DE 41 35 617 A1.

Process example 2

If e.g. B. the lubricating oil of an engine is to be examined, the sensor as in embodiment S2 through the oil dipstick of the engine in the oil kept in. The measurements of the thermal conductivity should be at the Operating temperature of the oil, d. H. in the almost balanced state of Wheatstone Bridge take place. The procedure for the 1st step in the Examining an oil is no different than doing it 1. Process step when examining a non-flowing substance.

At a freely selectable point in time and after the thermal stability state of the sensor and the oil has been determined, the sensor S2 is excited with a time-changing electrical supply in the form of a pseudo-random pulse-shaped signal. The thermal processes similar to the processes in the first process example are fanned in the sensor and in the substance to be examined. To excite the sensor S2, an easily realizable electrical supply signal in the form of pseudo-random, rectangular changes in the supply voltage with the amplitude Uo = 10 V, the cycle time Δ = 0.3 s and the period duration NΔ = 5 s (curve 1 , FIG. 6) was selected, where the duration of the second step of the process is equal to the period NΔ. Experiments have shown that, with this electrical feed, the oil does not yet heat up outside the mechanical device 15 and the sensor is momentarily heated by approximately 10 degrees Celsius. Due to the large internal friction in the chamber 16 , no significant natural convection of the oil to be examined is caused inside the mechanical device 15 at this temperature. The effective internal friction is particularly large not only because the sensor-cylinder distance is relatively small and large shear stresses have to be overcome, but also because, due to the transient thermal and hydrodynamic processes in the gap chamber 16 , a constantly changing one Acceleration of the oil particles takes place and therefore, in addition to the shear stresses, the inertia of the oil has to be overcome.

When the sensor S2 is excited with the supply voltage signal Uo in a pseudo-random form, the measurement circuit 21 results in the amplitude signal U1, which after processing in the linear supply and measurement device 24 is sampled as the output signal Uy and converted into a using the conversion circuit 31 digital signal is converted. The supply voltage signal Uo is also scanned in parallel with the aid of the conversion circuit 31 and also converted into a digital signal. The autocorrelation function for the supply voltage signal Uo and the cross-correlation function for the output signal Uy and for the supply voltage signal Uo are calculated in the microcomputer 32 . Since the applied supply voltage signal Uo has a wide power spectrum, the random heat-specific interference effects that occur are uncorrelated with it, and it is therefore possible and useful to use the modern tools of correlation measurement technology in order to eliminate the thermal interference effects when determining the time constant T. Once the two correlation functions have been determined, the weight function (impulse response) of the sensor can be determined with the aid of the microcomputer 32, and the interference effects have no influence on the result. The correlation method is in the book by Wolfgang Wehrmann and others; Correlation technique; Encyclopedia publisher; 1st ed. 1977; Pp. 74-78, described in more detail.

The weight function for the sensor S2 is due to its design Exponential function with a similar course as the weight function in the 1st process example. Therefore their evaluation and the arithmetic The time constant T of the sensor S2 is calculated in the manner as it is is described in this 1st process example.  

Because the heat transfer from the sensor to a quiescent environment where Neglect of the radiation, which results from the pure heat conduction, calculates the thermal conductivity λ to be determined investigating substance from the already calculated time constant T to the known equation:

λ = R / T (1)

where the factor R is a body constant determined by calibration measurements of the sensor is that of the materials and dimensions of the sensor is dependent.

Test results of the sensor S1 in connection with the ground and the sensor S2 in connection with an engine oil are shown in FIG. 7 and FIG. 8, the calibration factor R having been estimated.

Using an existing comparison table with the Thermal conductivity values for the substance to be examined can now it is determined whether the tested substance has changed its stock in such a way that it is still usable, needs to be changed or treated shall be.

Since the sensor S1 or S2 behaves strictly in its dynamic behavior as a first-order measuring element, its dynamic correction can also be carried out with the aid of the computer device 25 , taking into account the time constant T, in the permanent temperature measurement, specifically according to the regulation : the temperature to be measured of the substance to be measured is obtained by adding the temperature differential evaluated with the current time constant T of the sensor to the measured temperature. The invention can thus advantageously be used for permanent, dynamically corrected temperature measurement.

Finally, it should be pointed out that the invention is also suitable to determine the boiling state of liquids. Boiling in one Liquid has been achieved when moving from liquid to gaseous Condition passes. The temperature reached is called boiling temperature and is not only of the type of liquid but also of the external pressure dependent. The determination of the fabric temperature alone is therefore not enough to  to be able to determine the boiling state of the liquid to be examined. When boiling, there is much higher total thermal conductivity for line and Convection of the liquid to be examined is achieved and for the Determination of the boiling state of crucial importance. The after of the invention determined current time constant T des directly with the boiling liquid in contact with the sensor, can be used mathematical determination of the total thermal conductivity of this liquid be used. The total thermal conductivity λ g to be determined is calculated from the time constant T for the above equation (1) and based on the detection of changes in the Overall thermal conductivity can draw conclusions about the boiling state of the liquid to be examined. This is the precise determination of boiling, for example in a distillation process.

Claims (14)

1. Sensor for the determination of the thermal conductivity and / or the temperature of non-flowable, liquid or gaseous substances, consisting of a support body and a measuring winding arranged thereon of a type of winding reducing self-induction and inductance, the measuring winding consisting of a metal, electrically insulated resistance wire and is connected via an amplifier-free measuring circuit to a linear feed and measuring device interacting with a computing device, characterized in that the resistance wire ( 8 ) is uniformly arranged in a homogeneous embedding space ( 9 ) provided for the measuring winding on a, in relation to Sensor, tiny support body ( 1 ) is arranged in such a way that the entire temperature field of the sensor is covered with the resistance wire ( 8 ) and from the delimitation of the sensor to a free space (matched to the fluidity of the substance to be examined) ( 10 ) is provided for the substance to be examined. 2. Sensor according to claim 1 for the determination of the thermal conductivity of low-viscosity or gaseous substances, characterized in that the specific, the sensor-comprising free space ( 10 ) is designed in the form of at least one cavity with the help of a mechanical device ( 15 ) arranged around it . 3. Sensor according to claim 2, characterized in that the mechanical device ( 15 ) consists of a thin-walled open cylinder ( 17 ) with a perforated end wall ( 18 ). 4. Sensor according to claim 1 or 2, characterized in that the carrier body ( 1 ) consists of a coil-shaped metal component and has a heat capacity which is at most about 5% of that of the sensor. 5. Sensor according to claim 1 or 2, characterized in that the thermal conductivity of all structural elements of the sensor is similar to the thermal conductivity of the resistance wire ( 8 ). 6. Sensor according to claim 1 or 2, characterized in that the Resistance wire is a copper wire. 7. Sensor according to claim 1 or 2, characterized in that the resistance wire ( 8 ) is a double resistance wire, and that each wire of the double resistance wire forms a half-winding ( 7 a, 7 b) of the measuring winding ( 7 ). 8. Sensor according to claim 7, characterized in that the wires of the Double resistance wire same, very thin resistance wires with fine Are insulation. 9. amplifier-less measuring circuit ( 21 ) for the sensor according to claim 7, consisting of a Wheatstone bridge, characterized in that two opposite, the measuring winding ( 7 ) forming bridge resistors each from the half-windings ( 7 a, 7 b) of the measuring winding ( 7 ) exist, and that the other two bridge resistors ( 22 ) are identical fixed resistors. 10. amplifier-less measuring circuit ( 21 ) according to claim 9, characterized in that the two starting points ( 11 ) of the half-windings ( 7 a, 7 b) on the same diagonal of the Wheatstone bridge are arranged electrically opposite. 11. Linear feed and measuring device for the sensor according to at least one of claims 1 to 10, consisting of a feed circuit ( 29 ) for electrically feeding the unamplified measuring circuit ( 21 ) and an amplifier circuit ( 27 ) for amplifying the unamplified measuring circuit ( 21 ) arriving amplitude signal Ul, characterized in that an analog multiplier ( 28 ) for multiplying the output signal Uy of the amplifier circuit ( 27 ) by the output signal Uo of the feed circuit ( 29 ) is arranged in the negative feedback branch of the amplifier circuit ( 27 ). 12. A method for exciting a sensor according to at least one of claims 1 to 8, characterized by the following method steps:

• 1) that the electrical supply of the amplifier-less measuring circuit ( 21 ) is kept constant, the constant amplitude of the electrical supply signal (initial value) being predetermined such that the electrothermal power produced in the current-carrying measuring winding ( 7 ) has a low value.
• 2) that the electrical supply of the amplifier-free measuring circuit ( 21 ) is changed significantly for a transmission behavior test, the amplitude of the electrical supply signal and its time course being predetermined such that the sensor, during this process step, due to the electrothermal arising in the current-carrying measuring winding ( 7 ) Power is heated to a maximum below the temperature, which can cause a noticeable natural convection of the gas or liquid portion of the substance to be examined in the free space 10, and that no heating of the material outside the specific free space 10 can take place during this process step.

13. The method according to claim 12 step 2, in particular for viscous and non-flowing substances, characterized in that the electrical supply to the amplifier-less measuring circuit ( 21 ) takes place in the form of a single supply voltage pulse. 14. The method according to claim 12 step 2, in particular for low-viscosity and gaseous substances, characterized in that the electrical supply of the amplifier-less measuring circuit ( 21 ) in the form of rectangular supply voltage pulses with a constant amplitude and in a pseudo-random sequence with a short cycle time.