GB2082780A - Electro-physical Measuring with Variable Resistance Transducers - Google Patents

Abstract

By using a dual slope integrator for determining the ratio of a variable transducer resistance 1 (e.g. a temperature dependent resistor or diode) to a single reference resistor 3 connected in series therewith, the disadvantages of known measuring bridge circuits where numerous essential resistances etc lead to errors according to fluctuating ambient temperature, ageing effects and production tolerances are avoided insofar as only tolerance values of the single resistor 3 come into the matter. The series connected resistors are fed a controlled current via lines 63, 64 and the ratio of the voltages across them is determined by operating switches 101 and 103 in turn whereby to charge and discharge capacitor 27 of integrator 7 at rates determined respectively by the voltages across resistor 1 and 3, charge being for a predetermined number of clock pulses; during the discharge and until the capacitor falls to a reference voltage, comparator 9 enables clock pulses to be counted. For measuring temperature differences, different transducers are connected in turn in series with the same resistor 3. Non-linear charges of resistance with temperature are compensated by inverter 10 and associated elements 43-5 and 110. <IMAGE>

Description

SPECIFICATION Electrical Calorimetry The invention relates to effecting electrical calorimetry. Generally, this is done by comparing voltages proportional to temperatures or temperature difference with reference voltage in an analogue-to-digital converter. Known devices measure heat loss from a flowing medium, in general water, passed through a heat exchanger device, for example a hot water radiator.

Temperatures probes are provided both upstream and downstream of the heat exchanger device in the feed and outlet conduits, which probes may be temperature sensitive electrical precision resistances for measurement purposes. In addition a volume or mass measuring device is provided. The two precision resistances for measurement purposes take up values corresponding to the temperatures, and the volume or mass measuring device supplies pulses according to predetermined units of throughflowing volume or mass. Each pulse applied to an analogue-to-digital converter results in a measure of the heat quantity.

Such a device, using two precision resistance measuring devices arranged in a bridge connection to provide a differential voltage at the input of an analogue-to-digital converter and further digital logic circuitry for further evaluation, is known, for example from DE-OS 2,816,61 1. In such a bridge connected measuring device, there are also provided at least two further ohmic resistances in addition to the two precision resistance measuring devices constituting temperature probes, and a further potentiometer for development of reference voltage. Moreover, at least four resistances are necessary in a sumand-difference amplifier. The errors and drifts of all these resistances, as well as the offsets of the sum-and-difference amplifier, an integrator and a comparator constitute sources of errors in the measurement results.In particular, we have found that long-term drift can cause an error of quite a considerable size.

Similar considerations are applicable for the temperature measuring circuit according to DE OS 2,801,938 which also has a complete measuring bridge with all its sources of error.

DE-OS 2,710,782 shows a temperature gauge with two linear themistor networks, which, as for the aforementioned measuring bridges, give rise to unavoidable sources of error.

The invention is based on the problem of providing a method and means for electrical calorimetry whereby the measurement accuracy, particularly its consistency is optimised through minimising the sources of error.

The invention provides that voltages proportional to temperature or temperature differences as well as a reference voltage are produced as voltage drops resulting from controlled current in a series connection of at least one precision resistance measuring device or equivalent and always the same single reference resistor or equivalent.

Because of the presence of only one single reference resistor, or equivalent, in series connection with the precision resistance measuring device or devices the sources of error are attributable to those of this single resistor or equivalent. Further sources of error from further resistors and their circuits are eliminated.

Accordingly, optimum conditions apply to analog parts of the overall circuitry. At least one reference magnitude is, of course, essential.

A device for carrying out this process for calorimetry is equipped with temperature probes one provided upstream of and the other downstream of a heat removing device in the feed and outlet conduits for the heat medium.

Temperature sensitive electrical precision resistance measuring devices are suitable, and require to be associated in turn and serially with only a single reference resistor.

Alternatively, provision is made to derive a voltage proportional to a temperature difference.

Capacitors may be associated with the precision resistance measuring devices, which, in a first phase, are connected in parallel to these precision resistance measuring devices and, in a second phase, separated from the precision resistance measuring devices. The capacitors may be connected seriaily, and on one side are subjected to a reference potential.

A capacitor can also be associated with the reference resistor.

With an integration resistor in an alalog-todigital converter, there may be associated a selectively connectable parallel resistor.

Entire circuitry can furthermore be so constructed that the reference potential is produced as required for each measuring step.

That may be done through the same circuit elements so that offsets, drifts, etc are effectively neutralised as their values should remain sufficiently constant during only short measuring cycles.

The invention is applicable with the same advantages when, in place of two precision resistance measuring devices, only one precision resistance measuring device is provided in a special calorimetric device as in a temperature measuring device. Also, in place of the special provision resistance measuring device for measuring temperature, one or more precision resistance measuring devices can be provided for investigating any other parameter so that the method and means hereof are open to other fields of use.

In developments, correction of the measurement results can be made according to the density and enthalpy of the measuring medium as well as for the characteristics of the precision resistance measuring device or devices.

This occurs, for example, by way of return connection from the output of the integrator of the converter circuit to its input through a compensator member controlled from suitable circuitry, usually digital. In place of a multi amplifier, a threshold voltage circuit can be provided. Accordingly, any tendency to oscillations of the amplifier connection provided in the basic circuit can be avoided without it being essential to use a capacitor with a relatively large charging time.

Moreover, in a further development of circuitry hereof, there can be provided in the measurement input an additional difference amplifier with regulated resistances, in order to be able to reverse the direction of integration and therewith to be able to shorten substantially the measuring time.

In developments, correction of the measurement results can be made according to the density and enthalpy of the measuring medium as well as on the characteristics of the precision resistance measuring device or devices.

This occurs, for example, by way of a return connection from the output of the integrator of the converter circuit to its input through a compensator member controlled from suitable circuitry, usually digital. In place of a multiamplifier, a threshold voltage circuit can be provided. Accordingly, any tendency to osciilations of the amplifier connection provided in the basic circuit can be avoided without it being essential to use a capacitor with a relatively large charging time.

Moreover, in a further development of circuitry hereof, there can be provided in the measurement input an additional difference amplifier with regulated resistances, in order to be able to reverse the direction of integration and therewith to be able to shorten substantially the measuring time.

Practical implementation of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a basic temperature measuring device; Figure 2 shows voltage variation against time at the output of the circuit of Figure 1; Figure 3 is a circuit diagram of a preferred calorimetric device; Figure 4 shows voltage variation against time at the output of the circuit of Figure 3; Figure 5 is a circuit diagram of a further preferred calorimetric device; Figure 6 shows a voltage-time diagram for the circuit of Figure 5; Figure 7 shows another preferred circuit diagram; Figure 8 shows a further circuit example in which offsets are neutralised; Figures 9a and 9b show the associated voltage-time diagrams; and Figure 10 shows a further circuit inlcuding storage capacitors.

In the circuit of the temperature measuring device of Figure 1, a voltage (U5) is applied to a potentiometer formed by a precision resistance measuring device 1 and a reference resistor 3.

The potential at point 61 between the two resistances 1 and 3 is used as null reference potential. An amplifier 5 acting as a voltage follower is connected to the reference potential point 61. At the output of the amplifier 5 on the voltage rail 62 the potential null prevails. With an amplifier 5 free from offsets the potential at 61=Null'. The potential +U5 is equal to the potential, designated "Reference voltage" of a dual-slope arrangement viewed in the customary manner. The potential UB is then equal to the potential designated measurement potential viewed in the customary manner. The further analog parts shown in Figure 1 correspond to a dual-slope standard circuit.

A null amplifier 8 follows, in known-manner, an integrator 7 as also resistors 47 and 48 and a comparator 9. Further known digital circuit means can be connected to the comparator 9 as shown in the further circuit diagrams. Two switches 101 and 103 are shown in lines 63 and 64 for the operating voltage, which switches feed integrator input resistor 41 with the measuring voltage -U5 or the reference voltage +UB. A switch 11 3 is provided between the reference potential point 61 and feed line 65 for the integrator input resistor 41 operative relative to integrator capacitor 27. The null-amplifier 8 is connected to integrator 7 via resistor 46, amplifier 8 also being connected via resistors 48 and 42 and switch 114 to the input of the integrator 7. A capacitor 49 is located between resistor 42 and the voltage rail 62.The comparator 9 is connected to the output of the amplifier 8.

Inverting amplifier 10 has its input over resistor 43 from the output of the integrator 7 and is returned to the input of the integrator 7 through a switch 110 and resistor 45.

As initiation for measurement purposes, the switches 113 and 114 are closed in order to set the output voltage for the beginning of the measurement to Null'. When this is done the switch 101 is closed and there will be a rising integrator output according to the measurement voltage over a time interval of period t=NT, wherein N is the number of recorded pulses at intervals T, as determined by an impulse generator (see 12 in Figure 3) and a measurement period counter (see 1 3 in Figure 3). After expiry of these N recorded pulses the measurement period counter 1 3 gives a control signal to control logic 11, whereupon the switch 101 opens again and switch 103 is closed. Then, the integrator output declines according to the reference voltage.

When, after a time t2, corresponding to n2 recorded pulses from the pulse generator, the integrator output goes through null, the comparator 9 switches off further recording of pulses via the control logic 11. As the reference resistor 3 is so sized that it is always smaller than the value of the measuring resistor 1 n2 must always be greater than N. When the reference pulses (that reduce the integrator output) reach N, the measuring period counter 13 gives an output to the control logic 11, which, from that instant, directs the pulses to be recorded to the results counter 1 6. If the relationship 6R ST of the measurement resistor is linear, then the result is proportional to the temperature.Since, however the relationship 6R ST is unlikely to be linear, in practice, say being diminished by closure of platinum contacts of switch 11 0. However, the latter forces through the resistor 45 a current to summation point 66 for the integrator 7 that is opposed to the current through the integrator input resistor 41. By correct dimensioning and relative proporting of the resistors 41, 43, 44 and 45, a good approximation to a linear relationship can be obtained between temperature and impulse results.The linearisation can be improved for higher voltages by incorporation of further resistors in place of the resistor 45 by means of corresponding switches depending on If a heat calorimetry is to be undertaken instead of a simple temperature measurement, then two resistors are provided, namely, 1 in the return side and 2 in the forward side. Again, even with two precision resistance measuring devices 1 and 2, only one reference resistor 3 is necessary. Instead of the switch 101 a switch group 101,102,111,112 is employed. In addition, a quantity recorder contact 52 is provided, which, via resistor 51, applies voltage to the control logic 11.

Associated with the control logic 11 are the impulse generator 12, measurement time counter 13, an accumulator 14, a comparator 15, the result counter 16, and an indicator 1 7. Gates 53, 54 and 55 are provided between these circuit parts.

After null-corrections, see Figure 4 as already described with reference to Figure 2, a rise of integrator output for the measurement voltage controlled by measurement resistor 1 takes place through N pulses followed by a decline of integrator output according to the reference voltage (to null) during, for example, n, impulses.

The member n, is stored in the accumulator 14.

After changing to the measurement resistor 2 (and a fresh null-correction) a rise of integrator output again takes place through N pulses followed by decline to null, over a different number, say n2, of pulses. Since the upstream temperature is always greater than the downstream temperature, n2 is always greater than n,. In the second decline of the integrator output through the stored value n1, the measurement time counter 13, store 14 and comparator-5 act on the control logic 11 in such a way that the excess pulses are recorded in the results counter 16, which has associated therewith the indicator 1 7. Because of the nonlinearity of the measurement resistances and the enthalpy and density of the heat carrier a correction of this result n2-n1 can be made according to temperature.For this purpose in the passage of n2 through n, the switch 110 closes, which also in this case forces a current through the resistor 45 into the summation point 66 for the integrator 7. Thus an almost completely linear relationship can be obtained between the actual heat quantity and the recorded results.

The null-correction can lead to certain difficulties in the circuit according to Figure 3.

because of the tendency to oscillation of the amplifier connection 7 and 8, or because of relatively large charging time of the capacitor 49 required in the suppression of the tendency to oscillation. In a further development, these difficulties are avoidable by the circuit of Figure 5.

This circuit has the further advantage that the control logic 11 according to Figure 5 can be constructed more simply than for Figure 3. In this circuit, the capacitor 49 is omitted and also the resistors 46, 47 and 48 as well as switches 113 and 114. In their place there are resistors 29, 30, 31,32,33 and a switch 107.

Before the commencement of measurement the switch 107 is closed between the integrator 7 and the integrator capacitor 27 in order to avoid sources of error.

At the commencement of a measurement, which is begun by a pulse through the quantity counter contact 52 and resistor 51 at the control logic 11, the measurement resistor 1 is connected by the switch 101 at the supply line 64 for operating voltage UB. The switch 107 is opened and switch 111 closed. A fall of integrator output occurs according to downstream measurement voltage.When the output voltage at the output 67 of the integrator 7 reaches the voltage of the summation point 66, after the time tMR (MR=measurement voltage in the return or downstream side) and which depends on the sizes of the resistors 29, 30, 31 at threshold voltage switch 4 and the voltages on the rail 62 and feed line 63, the threshold voltage switch 4 changes its output from the potential of the supply line 64 to the potential of the feed rail 63, and yields an impulse to the control logic 11.

Accordingly, the switch 101 is opened and switch 103 closed. The reference voltage from reference resistor 3 is connected to the integrator 7 whose output now goes downwards. After the time tREp the voltage at the summation point 66 goes through null, and the comparator 9 changes its output voltage from the potential at the line 64 to the potential at the line 63, and supplies an impulse to the control logic 11. The switch 103 is now again opened and switch 101 closed and the integrator output rises according to return or downstream temperature for N pulses. After those N pulses, the measurement time counter 13 signals the control logic 11 and the switch 101 is opened and switch 103 closed.Accordingly, integrator output falls according to the reference voltage associated with the return or downstream measurement, say over n, pulses up to operation of the comparator 9. These n1 impulses are stored in the accumulator 14. At the same time, switches 101 and 103 are opened and switches 11 2 and 102 closed. Now, the voltage resulting from the upstream temperature is applied to the integrator. When the voltage at the summation point 66 again equals the voltage at the output 67 of the integrator 7, the threshold voltage switch 4 changes its output from the potential of the line 64 to the potential of the line 63, and gives a signal to the control logic 11 which causes switching over of the contact 102 to "Out" and 103 to "In".The integrator output will fall according to the null voltage rail now on the summation point 66 of the integrator 7 to operate the comparator 9 as well as supplying the control logic 11 in the cutting out of switch 103 and cutting in of switch 102. Then, the measurement voltage proportional to the upstream temperature is upwardly integrated through N impulses, after which a signal from the measurement time counter 1 3 to the control logic 11 causes the switch 102 to open and switch 103 to close. The reference voltage associated with the upstream side measurement then controls decline of the integrator output. Since the measurement resistor 2 (upstream) is greater than the measurement resistor 1 (downstream or return), the number of impulses n2 is greater than n,.In a similar manner to before, result counter is controlled by store 14, comparator 1 5 and control logic 11 to store the number n2-n1. The overflow or excess pulses from 1 6 is further counted in the indicator 1 7.

In the described arrangements according to Figure 3 and Figure 5 the integration times 2xN . T+n1T+n2T are quite long. It would be desirable to shorten integration times, especially for battery operation, where only the results counter 1 6 and the receiver part in the control logic 11 controlled from the quantity counter contact 52 need be permanently switched in, the analog part and the remaining digital part 11-1213-14-15 through a switch 11 5 controlled from the control logic 11 in place of connection 11 6 after the closure of the quantity counter contact 52 being switched in only for the measurement time.

This is made possible with a development of the invention shown in Figure 7. The integrator output rises over N pulses for downstream and upstream resistors 1 and 2 have the periods indicated in Figure 4 and Figure 6 because of the "full" values of their resistances. Thus, the time attributable to the common resistor value, say 120 ohms for 120 ohm and 132 ohm resistors (to give a desired 12 ohm difference) is "operative" twice.

If only the difference from the highest value to be anticipated up to the measurement value is measured (rather than from null up to the measurement value) n,T can be very short.

This is achieved by the circuit of Figure 7. The measurement resistors 1 and 2 and the associated resistors 23 and 24 are associated with a difference amplifier 6, for example using field effect transistors, sequentially through the switches 102 and 105 connected in series with the reference resistor 3. This measurement circuitry is between the supply voltage lines 63, 64. The measurement circuitry is connected in parallel to the potentiometer 21, 22 and likewise is located at the supply voltage. The reference potential point produces the reference voltage null, which is transmitted through the amplifier 5, connected as voltage follower, to the potential rail 62.The difference amplifier 6 has its nonreversing input directly on the potential rail 62 and its reversing input at the potential of line 68 from the connection point between the reference resistance 3 and the controlled resistors 23, 24, one of which is cut in for measurement purposes.

Since the difference amplifier 6 has a high amplification, for example 2 .105 and since the requisite control voltage at its output 69 for the controlled resistances 23, 24 differs only by some 100 mV, the inaccuracy between the voltages at the potential rail 62 and at the line 68 at the reversing input of the difference amplifier 6 is only from fractions of ,uV to a few yV. With the voltages proportional to the temperatures through the resistors 1 or 2 or through the controlled resistances 23, 24 of several mV per OC, there results only inaccuracy in fractions of 10-30C.

The progress of a measurement cycle is similar to that as described with reference to Figures 5 and 6.

If operating temperatures between 200C and 1 O00C are involved, and for example the proportions of the resistances 21,22 are so set out that 1 500C (with 2=1 60 ohm; 24=0 ohm) can still be measured, then one obtains in the above mentioned example only resistance differences of 106-120=40 ohms or 160-132=28 ohms (in contrast with 120 or 132 ohm), so that now the time n, . T amounts only to about T of the time n, . T in the arrangement according to Figures 5 and 6. The potentiometer 21,22 is not critical and causes no measurement error. It need only be required that the relationship R2,/R22 remains stable during a measurement cycle.

Figure 8 shows a simplified arrangement. The analog-digital converter here is a saw-tooth generator. The cycle of a measurement is explained with reference to Figure 8 and Figure 9a.

A quantity signal from switch 52 cuts in the control logic 11 through resistor 51. This includes receiver means for switch commands from the comparator 9, control means for switches 301 to 307 and 31 5 as well as a pulse generator for A/D conversion. Switch 31 5 is on the analog part at .

the voltage +UB. A voltage distribution takes place through the resistances 22 and 23. The partial voltage 61 is supplied to the amplifier 5" connected as a constant current source. The current flows from the output through switch 301, the resistor 1 and reference resistance 3 at UB.

At the output of the amplifier 5 the voltage 62 is adjusted, which serves as reference voltage for the reference integrator. The potential lines concerned are shown in Figure 9a.

The integration of the reference voltage takes place through resistor 41 and capacitor 27.

Output voltage 67 of the integrator 7 attains the magnitude of the voltage 69 (which is connected through switch 302 with the comparator), and the comparator 9 changes from UB to =UB. This signal is supplied to the logic control 11, whereupon switches 301 and 302 open and switches 305 and 306 close. The voltage through the reference resistor 3 does not alter therein, since the current remains constant. The voltage 70 is now supplied through switch 306 to the comparator. At the same time the pulse generator is switched on to 11 and connected with the result counter 1 6. Furthermore through switch 307 and resistor 45 the connection voltage formed by means of the resistances 43 and 44 as well as the amplifier 10 is supplied to the integration summation point 66.A further upwards integration takes place until the voltage 67 is as large as the voltage 70. Comparator 9 again changes its output from UB to +UB and the control logic 11 switches everything off.

The number n, of impulses T, which is supplied to the results counter is proportional to the difference in resistances 1 and 2, divided by the reference resistance 3.

In a modification of the operating sequence described in the first phase switches 303 and 304 of the control logic 11 can be closed and accordingly the resistor 53 be connected in parallel to the resistor 41 and resistor 54 in series with the measurement resistor 1. Resistor 54 reduces the voltage 69 to the voltage 69-A.

The comparator changes its output when the voltage 67 at the output of the integrator likewise reaches the value 69-A. That occurs in the time ty, which constitutes only a fraction of tx, since the integration time for the closed switch 303 is shortened by the resistor 53. On the first change of comparator output the control logic 11 again opens the switches 303 and 304. Everything else occurs as already described above and illustrated in Figure 9b.

The circuit according to Figure 10 operates in the following way: Each quantity pulse from the switch 52 is detected, triggered and shaped and supplied as drive pulse to the control logic 11.

This controls the switch 11 5, which connects the positive pole (+UB) of a battery with the operation amplifiers (5, 5', 7 and 9) in the analog part.

From the operation amplifier 5, whose input lies at the operational voltage split through resistors 21,22, a stable reference voltage is developed at the potential rail 62 for the anolog part.

The control logic 11 controls a series of anolog circuits 201-to 215. It contains a pulse generator and a measurement period counter.

A measurement cycle takes place in three phases.

In phase 1, the switches 201 to 207 and 214 and 216 are closed. Switches 201 and 202 effect an automatic null (or zero) balance. Switch 214 shortens the time constant of the integrator 7 for the period of the null-balance. Through the switches 203 to 207 the individual voltages are raised through the resistances 1, 2, 3 at the capacitors 221,222,223.

In phase 2, the voltage proportional to the temperature difference and the associated integration product is formed. The switches 201 to 207 as well as 214 to 216 are opened and now the switches 208 to 210 as well as 215 are closed. The negative pole of the voltage UA of the capacitor 222 nulled via the switch 208. The positive pole of UR is connected through the switch 209 with the positive pole Uv of the capacitor 221. The negative pole of Uv or 221 is connected through switch 210 with the input of the amplifier 5'. The potential at the input of 5' is proportional to the negative difference of the voltages through the resistors 1 and 2 and accordingly the difference between the two resistors.

The amplifier 5' reproduces this difference voltage at its output 1:1 and through the integrator input resistor 41 it is integrated in the integrator capacitor 227. This integration voltage at the end of phase 2 is accordingly proportional to the resistance difference between resistors 1 and 2.

The switches 21 5 and 216 serve for prevention of falsification by the effects of any stray current (e.g. surface leakage current from the offset-voltage stored in the capacitor 225).

In phase 3, integration of the reference voltage takes place. At the same time, correction of the integration current is made by a current dependent on the resistance difference of resistors 1 and 2. The switch 210 is opened and switches 211,212,213 closed. The voltage (UVUR) at the base of the capacitor 221 or at the switch 212 is maintained. Furthermore, the result counter 1 6 is coupled with the pulse generator by the control logic 11. The voltage UREF through the capacitor 223 is supplied via the switch 211 to the input of the amplifier 5' and effects the upwards integration in the capacitor 227.

An additional negative (correction -) current is superimposed on the positive current, which is driven from the resistor 41 into the integration capacitor 227, through the switches 212 and 213 as well as the resistors 245 and 246. Accon;:iingly, the total integration current is the smaller the greater is the resistance difference between resistors 1 and 2 or the greater is the temperature difference.

When, in the integration of the reference and correction currents the voitage through the integration capacitor 227 or at the output of the integrator 7 is null, the comparator 8 changes and gives a stop-signal to the control logic 11. Switch 11 5 separates the voltage +UB again from the anolog part and the pulse generator is cut out.

The number of pulses in phase 3 is thus given in the results counter 1 6.

The control logic 11 steps the indicator 1 7 by one step in each cycle of the results counter 1 6.

For measurement of the temperature or the temperature difference semi-conductor diodes can be used. These replace the measurement resistors 1 and 2.

Claims (12)

Claims

1. Process for electrical calorimetry by comparison of temperature, or temperature difference, proportional voltages with reference voltage in an analog-to-digital converter, wherein the voltages proportional to the temperature, or temperature differences, and reference voltages are developed as voltage drop resulting from controlled current in a series connection of at least one precision electrical resistance measuring device and always the same reference electrical resistance.

2. Apparatus for carrying out a process of calorimetry according to Claim 1, comprising temperature probes one provided upstream and another downstream of a heat removing device in inlet and outlet conduits for a fluid heat medium, each probe comprising a temperature-sensitive precision electrical resistance measuring device, reference electrical resistance presenting means is provided in an analog part of a measurement circuit, and means for connecting the electrical resistances of the probes alternately in series with the same reference resistance presenting means so as to produce required voltage drop during measurement.

3. Apparatus according to Claim 2, wherein, for deriving voltage proportional to temperature difference, capacitors are associated with the measurement resistances of the probes, which, in a first phase, are connected in parallel to these measurement resistances and, subsequently, are isolated from the measurement resistances and sequentially connected relative to reference potential.

4. Apparatus according to Claim 3, wherein a capacitor is also associated with the reference resistance.

5. Apparatus according to Claim 2, 3 or 4, wherein parallel resistance means is selectively associated with an integration resistance of an analog-digital converter.

6. Apparatus according to any one of Claims 2 to 5, wherein connection means are provided for the production of like reference potentials between the measurement resistance and reference resistance.

7. Apparatus according to any one of Claims'2 to 6, wherein means provided for correction of measurement results depending on the density and enthalpy of the heat medium as well as actual characteristics of the measurement resistance or resistances, the correction means serving to form a current proportional to the temperature or temperature difference and add same to the reference current during the reference integration.

8. Apparatus according to any one of Claims 2 to 7, wherein a threshold voltage switch serves for avoidance of tendency to oscillation of amplifier components.

9. Apparatus according to any one of Claims 2 to 8, wherein a difference amplifier serves for reversal of integration, the difference amplifier having controlled resistances at the measurement input.

10. Apparatus according to any one of Claims 3, wherein for a dual-slope circuit serves as analog-digital converter, an offset branch type has switch means for offset balance current, another switch means which serves to null relative to the balance branch during the measurement step, and further switch means to interrupt connection to offset storage capacitor means.

11. A method or process for electrical calorimetry substantially as herein described with reference to the accompanying drawings or any of them.

12. Apparatus for electrical calorimetry arranged and adapted to operate substantially as herein described with reference to and as shown in Figures 1 and 2, or Figures 3 and 4, or Figures 5 and 6, or Figure 7, or Figures 8, 9a and 9b, or Figure 10 of the accompanying drawings.