Multi-channel thermoelectric measuring device and method for fault detection

Abstract

Thermoelectric measuring device, containing - a main thermocouple (1) with a thermocouple consisting of thermowires (1.1 to 1.4) and a thermon node (8), - Comparison points (6.1, 6.2, 6.3), - an electrical evaluation unit (5) with at least one functional unit and - a thermometer fitting, characterized in that the electrical evaluation unit (5) contains at least two internal reference junctions (6.1, 6.2) or at least two thermoelectric connections (11) of at least two separate reference junctions (6.1, 6.2), and the reference junctions (6.1, 6.2) have different, but known reference junction temperatures (T V ) exhibiting the reference junctions (6.1, 6.2) being thermoelectrically connected to a common thermo node (8).

Description

[0001] The invention relates to a thermoelectric measuring device comprising a main thermocouple with a thermocouple consisting of thermowires and a thermon node, reference junctions and an electrical evaluation unit with at least one functional unit as well as a thermometer fitting.

[0002] The thermocouple is suitable for measuring medium and high temperatures and features self-diagnostic capabilities for fault detection. A process connection is generally used as the thermometer fitting.

[0003] For electrical temperature sensors, including thermocouples, the detection of short circuits and sensor breaks is a minimum requirement for self-diagnostics during electronic evaluation. In thermoelectric temperature measurements at high temperatures in conjunction with critical atmospheres, thermoelectric drift becomes the most critical fault compared to short circuits and wire breaks.

[0004] For example, when temperature thermocouples are used in furnace processes or similar thermal processes, the materials are exposed to significant temperature loads for extended periods. Commercially available thermocouples based on nickel / chromium, platinum / platinum-rhodium, or boron carbide / graphite, carbide / carbide, or carbide / titanium carbide exhibit a number of technical shortcomings. Long-term applications, in particular, lead to steadily increasing drift values.

[0005] The temperature drift of a thermocouple is a thermoelectric voltage change over a defined period under constant temperature load and constant electrical connection values ​​and environmental conditions.

[0006] The drift depends on both the material properties and the operating conditions at the measurement location. There are essentially three influencing factors: 1. The drift depends on the magnitude of the temperature load as well as on temporal and spatial temperature changes. Below room temperature, no significant drift effects are observed, primarily because the activation energy required for thermal diffusion effects would be too high. 2. The drift also depends on the thermocouple wire diameter. This is especially true if the thermocouple wires are incompletely protected, i.e., used without protective tubes. A relative drift value can be derived from the ratio of the possible diffusion length of the diffusion partners to the wire diameter. The larger the diameter, the lower the drift. 3. Drift can be increased or decreased by certain atmospheres. For comparative data on drift effects, precise atmospheric information is required, or the drift investigations must be carried out in a neutral atmosphere. Hydrogen atmospheres (for noble elements) and sulfur gases (for nickel-based thermocouples) can lead to considerable drift values.

[0007] One technologically induced cause of drift lies in the change (interdiffusion) of the material composition at or near the thermocouple, which originates in suboptimal welding conditions during thermocouple wire welding. Given the complexity of the drift mechanisms and the numerous influencing factors, reliable drift predictions cannot be made, especially at higher temperatures, and mathematical correction models can only be developed for specific applications.

[0008] Existing drift values ​​can be determined using calibration or comparison procedures. Classical testing and calibration methods are known for this purpose, in which the temperature sensor is removed and measured separately in a test furnace, calibration bath, or similar device. In addition to these separate testing and calibration methods, process-testable methods are used, in which testing is performed without removing the sensor. The following methods are known:

[0009] Fixed-point calibration method: In this method, a mini fixed-point cell is integrated into the thermocouple fitting. Additional electronics compare the fixed point with the measured value at separate measurement intervals.

[0010] Furthermore, a Curie point self-calibration method described in WO 2018 / 127357 A1 is known. In this method, a magnetic component is integrated into the thermocouple fitting. Additional electronics compare the Curie point of the magnetic element with the measured value at short intervals.

[0011] Furthermore, arrangements are known from DE 10 2006 040 135 B4 and DE 20 2009 012 292 U1 in which a separate test channel is integrated into the thermocouple. A comparative measurement can be carried out using a pluggable control sensor without disturbing the measurement process.

[0012] Furthermore, US Patent 2011 / 0 299 567 A1 describes a temperature sensor for measuring temperature in a technical process, comprising a first electrical connection to connect a first wire of a thermocouple to a first and a second electrode, and a second electrical connection to couple a third and fourth electrode to a second wire of a thermocouple, wherein the first and second wires are made of different materials. The temperature sensor further includes a measuring circuit coupled to the first and second connections, which detects the temperature of the thermocouple and evaluates the voltages between the first, second, third, and fourth electrodes.

[0013] Furthermore, US 2012 / 0 197 586 A1 describes an I / O module with multidimensional cold junction compensation. An I / O circuit for temperature measurement within an industrial control system uses a stored empirical model together with input data from at least one reference temperature sensor to determine temperature differences between the terminals and thus enable more accurate cold junction compensation.

[0014] Furthermore, US 2015 / 0 276 498 A1 describes a thermocouple module with wire resistance compensation. An interface circuit is provided for an I / O module in an industrial control system to compensate for the voltage generated by a bias current in a thermocouple. During a calibration process, the interface circuit supplies two known bias currents to a thermocouple and measures the voltage generated by each bias current. The resistance of the thermocouple leads is determined from the measured voltages and the known current values. By using two known bias currents, an accurate measurement of the thermocouple lead resistance is possible when the thermocouple generates a voltage corresponding to the measured temperature.Either the I / O module or the industrial control system can determine a voltage resulting from the bias current applied to the thermocouple during operation as a function of the measured resistance, and compensate for the voltage measured at the thermocouple terminals to accurately determine the measured temperature.

[0015] All of the aforementioned methods have in common that the thermocouple sensor requires the installation of mechanical auxiliary and additional elements, and in some cases, extensive additional electronics. The integration of these mechanical auxiliary and additional elements into the thermoelectric sensor generally leads to an increase in the sensor tube diameter or prevents a reduction in the sensor diameter that would otherwise be possible for improved dynamics. Furthermore, the materials used for the fixed points and the magnetic materials limit the maximum operating temperature of the temperature sensors.

[0016] Regarding functional safety, wire breaks in the leads to the thermocouple are irrelevant. They are easily detected by the electronics. Short circuits in the area of ​​the sensor tip or in the front part of the protective tube, however, are problematic. Depending on the welding technology, they can be particularly prone to failure, and cannot be detected during operation with only one measuring channel. In the event of a short circuit in both thermocouple wires, the electrical signal changes, but it does not necessarily become zero or nearly zero. The short circuit effectively switches off the front thermocouple, and the short circuit point takes over its function. Since the short circuit point is generally located away from the actual measuring point, the new thermoelectric signal does not correspond to the correct measurement signal. It is therefore faulty. Two measuring channels are generally used to detect a short circuit.An electronic circuit compares the signals of both channels and thus detects if there is a short circuit in one channel. However, if there is an external influence that affects both channels or causes a short circuit in both channels, then this fault will not be detected.

[0017] The invention is based on the objective of creating a thermocouple and a method that allows for the electrical evaluation of various possible faults and can therefore be used safely in hazardous areas.

[0018] The problem is solved according to the invention with a thermoelectric measuring device which has the features specified in claim 1 and with a method which has the features specified in claim 7.

[0019] Advantageous embodiments of the invention are the subject of the dependent claims.

[0020] The measuring device according to the invention has an evaluation unit which contains at least one internal reference junction or at least two thermoelectric connections and to which thermoelectric signals from the thermo node are supplied via the reference junction with different reference temperatures.

[0021] The thermoelectric temperature measuring device according to the invention is simple and inexpensive to manufacture. It is characterized by its reliable and accurate measurement across a wide temperature range and, in terms of functional safety, exhibits only "detectable errors".

[0022] A key element of the temperature measuring device consists of a multi-channel evaluation of the thermoelectric voltage signals, whereby the evaluation electronics receive thermoelectric temperature measurement signals with reference to various reference temperatures T. v be supplied.

[0023] The CJC method is generally used. In the CJC method, temperature determination is not based on an ice temperature reference point. Instead, the reference point has an arbitrary temperature, which is measured via an additional temperature channel. This non-zero reference temperature is then combined with the applied thermoelectric measurement signal in the evaluation electronics using a corrective mechanism. This internal electronic correction process ensures that the correct measurement temperature is output despite the non-zero reference temperature.

[0024] The solution according to the invention utilizes this CJC method in two ways, by working with two different reference temperatures. The measurement signal coming from only one thermonode is branched in parallel in the thermometer connection head or in the electrical supply line.

[0025] Let the two parallel branches be labeled A and B, for example. In branch A, the reference point A with temperature T is located. VA and correspondingly in the other branch B, the reference point B with temperature T VB The two comparison temperatures can be preset, e.g., T VA = 0°C and T VB = 500°C. The temperature of 500°C is regulated, for example, by a micro heater.

[0026] If these reference temperatures cannot be precisely defined, their actual values ​​can be recorded by two additional temperature sensors. These two reference temperatures, either existing or determined separately, T VA and T VB According to the normative voltage-temperature characteristic curve of the thermocouple, there are two normative thermoelectric voltage values ​​U. A and U B assigned. In the drift detection method according to the invention used here, the voltage difference U is assigned.A - U B ΔUv is used as a drift indicator. The two assignable normative stresses to the reference temperatures T VA and T VB They are free from drift influences because the two sensors in the parallel measuring branches A and B are located in the connection head or in electronics. They are thus outside the high-temperature environment and outside harmful atmospheric influences. The temperature difference is therefore error-free, and the thermoelectric voltage difference Uv corresponds to the normative setpoint and is thus also error-free.

[0027] The thermoelectric voltages initially present in the two measuring branches without a CJC procedure were compared with U MA and U MB They are named. They are generated thermoelectrically in the front part of the thermocouple, which extends into the measuring medium. Their voltage values ​​correspond to the following temperature differences: U MAFunctionally corresponds to the temperature difference Tm - T VA and U MB according to the temperature difference T M - T VB , if T M The measurement temperature is...

[0028] The voltages U MA and U MB Thermoelectrically generated at the measuring tip of the thermocouple. If the thermocouple is subjected to prolonged thermal stress, material or structural changes can occur in the leading portions of the thermocouple wires. This results in corresponding changes to the characteristic curve. MA and U MB They then no longer correspond to the thermoelectric standard values. The electronics control and maintain the voltage difference between the two parallel signals U. MA -U MB = ΔU M . Only if the thermoelectric properties of the thermocouple used correspond to the normative target characteristic curve, is ΔU M= ΔUv. An inequality between the two values ​​signals the occurrence of a drift.

[0029] For example, the main thermocouple is formed by two thermocouple wires with thermomaterials x and y connected by a thermocouple node. The thermocouple wires are led to two connection points in the terminal head, from which compensating leads extend from the terminal head to a reference junction. These compensating leads form a first measuring channel A. Furthermore, thermocouple leads extend from the connection points to another reference junction located in an electronic component. These leads form a second measuring channel B. In addition to the reference junction, the electronic component also contains a heater and a temperature sensor.

[0030] For example, the main thermocouple is formed by a first thermocouple wire made of thermomaterial x and a second thermocouple wire made of thermomaterial y, as well as the thermocouple node, wherein the thermocouple wires are led to two connection points in the connection head, from which two compensating leads lead out of the connection head to the measuring electronics or to two reference junctions with different reference temperatures, wherein the two compensating leads form a measuring channel A or B.

[0031] In an advantageous embodiment, the main thermocouple is equipped with a ceramic tube and a connection head.

[0032] It is also possible for the evaluation unit to contain two separate components. The first component contains an external reference junction with connection points where the thermocouple wires are connected to external leads. A temperature sensor is also located within this component. The second component contains an internal reference junction whose temperature can be varied relative to that of the external reference junction by means of a heater and measured by a second temperature sensor. This component also contains two connection nodes to which leads are attached. These leads are internally connected to a functional unit for converting the temperature signals into electrical signals.

[0033] In a further advantageous embodiment, the thermocouple has two parallel thermocouple channels made of different materials. One thermocouple channel contains an external reference junction, a first thermocouple node, and thermocouple wires made of two different thermocouple wire materials (x, y), and is connected to a parallel branch which has an internal reference junction. The reference junctions are connected to an evaluation unit. A second thermocouple channel has another reference junction and is connected to another evaluation unit. This second thermocouple channel contains thermocouple wires made of thermocouple wire materials (n, m) that differ from those of thermocouple wire materials (x, y) and form a second thermocouple node.

[0034] Another advantageous embodiment involves arranging the second thermocouple channel in an additional protective tube.

[0035] For example, the main thermocouple has two thermocouple wires made of materials (x, y) and a thermocouple node. The thermocouple wires are routed through a process connection to connection points in the terminal head, with the thermocouple wires made of materials (x, y) of the main thermocouple being connected to two material-related thermocouple wires at the same height as the process connection. The first thermocouple wire made of thermocouple material x is connected to the third thermocouple wire made of thermocouple material y, and the second thermocouple wire made of thermocouple material y is connected to the fourth thermocouple wire made of thermocouple material x.

[0036] Exemplary embodiments of the invention are explained in more detail below with reference to drawings.

[0037] It shows: Fig. 1. a cut through the measuring device, Fig. 2 a schematic representation of the electrical operation of an evaluation unit, Fig. 3 a schematic representation of short-circuit and drift detection in a diverse double thermocouple and Fig. 4 a circuit arrangement for a thermoelectric measuring device with drift and installation error detection.

[0038] Corresponding parts are marked with the same reference symbols in all figures.

[0039] In Fig. Figure 1 shows a high-temperature sensor with a terminal head 9 and outgoing connecting leads 11 in a longitudinal section. The main thermocouple 1 is located in the lower part of the sensor and is arranged in a ceramic tube 1.5. It consists of a first thermocouple wire 1.1 made of thermomaterial x and a second thermocouple wire 1.2 made of thermomaterial y, as well as the thermocouple node 8. The thermocouple wires 1.1 and 1.2 are led to two connection points in the terminal head 9. Compensating leads 12 extend from these connection points out of the terminal head 9. These leads, referred to as compensating leads 12 or thermocouple leads, form the measuring channel A and lead to an external reference junction 6.1 (not shown) with a temperature T. VAand further to evaluation unit 5. From the connection nodes, lines also lead to an internal reference point 6.2, which is located in evaluation unit 5. This arrangement forms a measuring channel B. In evaluation unit 5, in addition to the internal reference point 6.2 with temperature T, there are VB Another heater 7, an auxiliary thermon node 8.1 and a temperature sensor S. The signals from channel B are routed to the evaluation unit 5 via a connecting line 11.

[0040] The Fig. Section 2 explains the electrical operation of a multi-channel evaluation unit 5 located in the connection head 9. In the example shown here, the evaluation unit 5 contains two components. The first component forms an external reference junction 6.1 with temperature T VA. This component contains connection points where the thermoelectric wires 1.1 and 1.2 are connected to externally leading connecting lines 11 and form the first measuring channel A, at which the thermoelectric voltage U is measured. A is located in this component. The first temperature sensor S1 is also located in this component.

[0041] The second component forms an internal reference junction 6.2, whose reference temperature T VBThe temperature is increased by means of the heater 7 and is detected by the second temperature sensor S2. Furthermore, this component contains two connection nodes that establish an electrical connection with the thermocouple wires 1.1 and 1.2 of the main thermocouple 1 and with external connecting leads 11. It is necessary to insulate at least one of the electrical connections with the thermocouple wires 1.1 and 1.2, as otherwise it would form a short circuit with the crossing thermocouple wire. The external connecting leads 11 form the second measuring channel B, on which the thermoelectric voltage U is measured. B is present. The signals U A , U B The signals from the temperature sensors S1 and S2 are further processed in a functional unit located internally in the evaluation unit 5.

[0042] This means that the external reference junction 6.1 with temperature T is located in the first measuring channel A. VAand in the second measuring channel B the internal reference junction 6.2 with the temperature T VB The two comparison temperatures can be fixed, e.g., T VA = 0°C and T VB = 500°C. The temperature T VB The temperature of 500°C is regulated, for example, by a micro heater 7.

[0043] If these reference temperatures cannot be predefined, their actual values ​​can be recorded by temperature sensors S1 and S2. These two reference temperatures, either existing or determined separately, are then used to... VA and T VB According to the normative voltage-temperature characteristic curve of the thermocouple 1, there are two thermoelectric voltage values ​​U. A and U B assigned. In the innovative drift detection method used here, the difference U A - U B = ΔU V used as a drift indicator. The two are related to the reference temperatures T. VA and T VBThe associated normative voltages are free from drift influences, since the two sensors are arranged in the parallel outgoing measuring branches A and B in the terminal head 9 or in the electronics. They are thus located outside the high-temperature stress and outside harmful atmospheric influences. The thermoelectric voltage difference ΔU V Therefore, its value corresponds to the normative target value and is thus error-free.

[0044] Fig. Figure 3 shows an embodiment in which dual drift detection is applied to a double thermocouple, utilizing the diversity principle. Two different thermocouple channels A1 and A2 are used. One thermocouple channel A1 has a parallel branch B with two reference junctions, namely an external reference junction 6.1 and an internal reference junction 6.2, a first thermocouple node 8a, and an electronic evaluation unit 5 adapted to the method. The second thermocouple channel A2 contains another reference junction 6.3 and another electronic evaluation unit 14. It is arranged in a protective tube 13 and has a thermocouple wire 1.3 made of material m and a thermocouple wire 1.4 made of material n, as well as a second thermocouple node 8b. Both channel assemblies are installed in an additional outer protective tube 16.

[0045] For temperature measurements relevant to high safety, the detection of temperature drift plays a crucial role. Therefore, two drift detection methods are employed. Firstly, the diversity principle is applied, with diversity being implemented multiple times in the thermocouple. This means that the thermocouples used must be different; furthermore, if sheathed thermocouples are used, their sheathing materials must differ. Additionally, differently manufactured electronic assemblies are used. The diverse elements will not drift in the same or simultaneous manner, thus ensuring drift detection even under load. Furthermore, the different materials x, y, m, n result in varying susceptibility of the elements to short circuits.By comparing the signals of the two elements and assuming a short circuit does not occur simultaneously (which is highly unlikely due to the additional protective tube), the short-circuit fault can be detected. Drift can also be detected via the two channels A and B using the previously described methods. This dual monitoring ensures a high level of reliability.

[0046] In Fig. Figure 4 shows an embodiment in which the thermocouple wires 1.1, 1.2, made of materials x and y, of the main thermocouple 1 are connected to two material-specific thermocouple wires 1.3, 1.4 at the height of the process connection 10 of the thermocouple. The process connection temperature T prevails in the area of ​​the process connection 10. PThe long thermocouple wire 1.1 of the main thermocouple 1, made of material x, is connected to a short thermocouple wire 1.3 of material y at the process connection level, and vice versa. This creates, in addition to the main thermocouple 1 with the thermonode 8, which measures one of the temperature differences T. M -T V relevant voltage U A for temperature T M outputs at the measuring point, a short intermediate thermocouple 2, which measures one of the temperature differences T P -T V between temperature T P at process port 10 and the reference temperature T V relevant voltage U V outputs, and a secondary thermocouple 3, which measures one of the temperature difference T M -T P relevant voltage U B outputs. Using the three voltage signals U A, U V, U B and an available reference temperature signal to T VBoth drift control and installation error verification can be performed based on VDI / VDE guideline 3511. While the general application of this method involves measuring the temperatures of two reference junctions, requiring one of them to be heated, this version eliminates the need to heat the junction at temperature T. P (also at process port 10). The heat rising from the measuring tip to process port 10 heats the thermonode 8.2 of the intermediate thermocouple 2 or the secondary channel thermocouple 3 with the thermonode 8.3 due to the process.

[0047] Under relatively constant process and environmental conditions and at reference junction 6.2 in connection head 9, the temperature T P at process port 10, the difference T is primarily taken into account. M -T Vand depends on the distance between the associated measuring points or the sensor design. The following results for T: Changes in the medium temperature or the measuring temperature P a plausibility field. If a short circuit occurs, as shown by short circuit point 15 in the example, the distance to the measuring points changes, usually shortens, so the temperature value T P no longer within this plausibility range. This is how a short circuit can be identified. REFERENCE MARK LIST 1 main thermocouple 1.1 First thermocouple wire 1.2 second thermocouple wire 1.3 third thermocouple wire 1.4 fourth thermocouple wire 1.5 ceramic tube 2 Intermediate thermocouple 3 secondary channel thermocouple 5 evaluation unit 6 Comparative point 6.1 External reference point with temperature T VA 6.2 Internal reference junction with temperature TVB 6.3 further comparison point 7 stokers 8 thermo nodes 8.1 Auxiliary thermon nodes 8.2 Thermonode on the first and third thermowire 8.3 Thermonode on the second and fourth thermowire 8a first thermon node 8b second thermonode 9 Connection head 10 Process connection 11 Connection line 12 Standard compensating cables or thermocouple cables 13 Protective tube 14 additional evaluation units 15 Short circuit point 16 outer protective tube x first thermowire material y second thermoelectric wire material m third thermowire material fourth thermoelectric wire material A first measurement channel A1, A2 Thermocouple channels AS1...AS5 junctions B second measuring channel, parallel branch S temperature sensor S1 first temperature sensor at the external reference point 6.1 S2 second temperature sensor at the internal reference point 6.2 T P Temperature at the process connection T M Temperature at the measuring point T V Reference point temperature T VA Temperature of reference point 6.1 T VB Temperature of reference point 6.2 U A thermoelectric measuring voltage of measuring channel A U B thermoelectric measuring voltage from measuring channel B U V thermoelectric voltage associated with the reference temperature

Claims

[1] Thermoelectric measuring device, comprising - a main thermocouple (1) with a thermocouple consisting of thermowires (1.1 to 1.4) and a thermon node (8), - Comparison points (6.1, 6.2, 6.3), - an electrical evaluation unit (5) with at least one functional unit and - a thermometer fitting, characterized by , that the electrical evaluation unit (5) contains at least two internal reference junctions (6.1, 6.2) or at least two thermoelectric connections (11) of at least two separate reference junctions (6.1, 6.2), and the reference junctions (6.1, 6.2) have different, but known, reference junction temperatures (T V ) exhibiting the reference junctions (6.1, 6.2) being thermoelectrically connected to a common thermo node (8). [2] Measuring device according to claim 1, characterized by , that the measuring device is designed as a built-in thermocouple with a connection head (9). [3] Measuring device according to claim 1 or 2, characterized by , that the main thermocouple (1) is designed as a built-in thermocouple with a ceramic tube (1.5). [4] Measuring device according to one of the preceding claims, characterized by , that the evaluation unit (5) contains two components, wherein a first component contains an external reference junction (6.1) and has connection points in this component where the thermocouple wires (1.1, 1.2) are connected to external connecting leads (11) and a temperature sensor (S1) is also arranged in this component, and wherein a second component contains an internal reference junction (6.2) whose reference temperature (T VB) can be changed by means of a heater (7) and detected with a temperature sensor (S2), and wherein there are two connection nodes in this component which establish an electrical connection with the thermowires (1.1, 1.2) and with external connecting lines (11). [5] Measuring device according to claim 1 or 2, characterized by , that the thermocouple (1) contains two different parallel thermocouple channels (A1, A2) with different thermomaterials (m, n, x, y), - wherein a thermocouple channel (A1) has an external reference junction (6.1), contains a first thermo node (8a) and thermowires (1.1, 1.2) made of two thermowire materials (x, y) and is connected to a parallel branch (B) which has an internal reference junction (6.2) and the reference junctions (6.1, 6.2) are connected to the evaluation unit (5), - and wherein a second thermocouple channel (A2) has a further reference junction (6.3) and is connected to a further evaluation unit (14), wherein the second thermocouple channel (A2) has thermowires (1.3, 1.4) made of the thermowire materials (n, m) which differ from the thermowire materials (x, y) and form a second thermo node (8b). [6] Measuring device according to claim 5, characterized by , that the second thermocouple channel (A2) is arranged in a protective tube (13). [7] Method for fault detection on a measuring device according to one of the preceding claims, characterized by , that at two reference junctions (6.1, 6.2) belonging to a thermonode (8) different reference junction temperatures (T) VA , T VB ) are generated, the associated thermoelectric voltages (U) A , U B ) are measured and their voltage difference ΔU V = U A -U Brecorded and compared to a difference ΔU M uncorrected measured value U MA (T M , T VA ) and U MB (T M , T VB ) with ΔU M = U MA - U MB is compared, with an inequality of ΔU V opposite ΔU M as a thermoelectric drift of the thermoelectric measuring device is detected. [8] Method according to claim 7, characterized by , that the temperature (T VB ) at a reference point (6.2) is set to a predefinable value by means of a heater (7). [9] Method according to claim 7 or 8, characterized by , that the temperatures (T VA , T VB ) at the reference points (6.1, 6.2) are recorded with separate temperature sensors (S1, S2) and compared with temperatures that are determined normatively from a thermoelectric function table.

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