EP3338070A1

EP3338070A1 - Temperature measuring device with comparison temperature determination

Info

Publication number: EP3338070A1
Application number: EP16744721.8A
Authority: EP
Inventors: Thomas Härle; Björn WALSER
Original assignee: Endress and Hauser Wetzer GmbH and Co KG
Current assignee: Endress and Hauser Wetzer GmbH and Co KG
Priority date: 2015-08-20
Filing date: 2016-07-26
Publication date: 2018-06-27
Also published as: WO2017029080A1; US20180238744A1; CN108027285A; DE102015113842A1

Abstract

A measuring apparatus for determining a measurement temperature at a measurement point using a first thermal element is described. The measuring apparatus comprises a first connection terminal and a second connection terminal for connecting the first thermal element, a first connecting line and a second connecting line that connect the first and second connection terminals to respective evaluation electronics, a reference temperature sensor that is arranged outside the connection terminals and is designed to sense a reference temperature at the location of the reference temperature sensor, and the evaluation electronics, which are designed to determine the measurement temperature at the measurement point. In the second connection terminal, the second connecting line has an additional connecting line connected to it that consists of a different material from the second connecting line. The additional connecting line forms, together with the second connecting line, a second thermal element that is likewise connected to the evaluation electronics. The second thermal element delivers a second measurement voltage that is dependent on the second temperature difference between the temperature of the second connection terminal and the reference temperature at the location of the reference temperature sensor. The evaluation electronics are designed to take a first measurement voltage delivered by the first thermal element, the second measurement voltage delivered by the second thermal element and the reference temperature sensed by the reference temperature sensor as a basis for determining the measurement temperature at the measurement point.

Description

Temperature measuring device with reference temperature determination

The invention relates to a measuring device for determining a measuring temperature at a measuring point by means of a thermocouple and a temperature measuring device for determining a measuring temperature at a measuring point. Furthermore, the invention relates to a method for determining a measuring temperature at a measuring point.

In automation technology, field devices are often used which serve to detect and / or influence process variables. Examples of such field devices are level gauges, mass flowmeters, pressure and temperature measuring devices, etc., which detect the corresponding process variables level, flow, pressure or temperature as sensors.

In industrial temperature measurement come in many applications

Thermocouples used. A thermocouple provides a measurement voltage that can be used to determine the temperature difference that occurs along the thermocouple. In order to be able to determine the measuring temperature at the tip of the thermocouple with the aid of this temperature difference, the reference temperature at the terminals of the thermocouple must be determined

Thermocouple be known as accurately as possible. The more accurately the reference temperature is known, the more accurate the measurement temperature can be determined.

It is therefore an object of the invention to provide a temperature measuring device and a method for measuring the temperature available, which allows a more precise determination of the comparison temperature for a thermocouple with little additional effort.

This object is achieved by the features specified in claims 1, 7 and 18

Characteristics.

Advantageous developments of the invention are specified in the subclaims.

A measuring device for determining a measuring temperature at a measuring point by means of a first thermocouple according to embodiments of the invention comprises a first terminal and a second terminal for connecting the first thermocouple, a first connecting line and a second connecting line, the first and the second terminal each with a Connect the transmitter, a reference temperature sensor, which is located outside the terminals and to do so is designed to detect a reference temperature at the location of the reference temperature sensor, and the evaluation, which is designed to determine the measurement temperature at the measuring point. In the second terminal, an additional connecting line is connected to the second connecting line, which consists of a different material than the second connecting line. The additional connecting line forms, together with the second connecting line, a second thermocouple, which is also connected to the

Evaluation is connected. The second thermocouple provides a second measurement voltage that depends on the second temperature difference between the temperature of the second terminal and the reference temperature at the location of the reference temperature sensor. The evaluation electronics are designed to determine the measurement temperature at the measurement point on the basis of a first measurement voltage supplied by the first thermocouple, the second measurement voltage supplied by the second thermocouple and the reference temperature detected by the reference temperature sensor. For the accuracy of a temperature measurement by means of a thermocouple, it is important to compare the reference temperature at the connection points or terminals of the

To determine thermocouple with the highest possible accuracy. For this purpose, a separate reference temperature sensor is provided in many cases. However, the reference temperature sensor is usually a bit away from the

Terminals arranged so that the reference temperature detected by the reference temperature sensor does not correspond exactly to the temperature of the terminals. So there is a second temperature difference between the reference temperature measured by the reference temperature sensor and the temperature of the terminals. To detect this temperature difference and the prevailing at the terminals

To be able to determine the reference temperature with high accuracy, the second thermocouple is used. This second thermocouple is formed by an additional connection with the second connecting line of the second terminal

Connecting line is made of a different material than the second connecting line. This second thermocouple detects the second temperature difference between the reference temperature detected by the reference temperature sensor and the

Temperature at the terminals. In this way, the to the

Terminals prevailing comparison temperature can be determined with high accuracy. Based on the more accurate than previously known reference temperature can then be determined with high accuracy with the help of the first thermocouple actually interesting measurement temperature. The second thermocouple allows a determination of the temperature within the terminals with very little additional construction costs. It is only necessary to attach an additional connecting line to the second connecting line. The invention is based on the drawing shown in the drawing

Embodiments explained in more detail. Show it:

Fig. 1 shows the principle of a temperature measurement by means of a thermocouple; Fig. 2 shows a transfer characteristic for a thermocouple of type K, the am

Thermocouple applied measuring voltage as a function of the temperature difference between the measuring point and reference junction shows;

3 shows a temperature measuring device according to the prior art;

4 shows a temperature measuring device with a second thermocouple, which allows an accurate determination of the comparison temperature for the first thermocouple. and

5 shows an overview of the different temperatures in the temperature measuring device shown in FIG. 4.

In the field of industrial metrology, temperature measurement is common

Thermocouples used. In Fig. 1, the measuring principle of a temperature measurement by means of a thermocouple is shown schematically. A thermocouple 100 consists of a first conductor 101, which consists of a first metal or a first alloy. This first

Conductor 101 extends from a reference junction 102 to a measuring point 103 whose temperature T _{mess is to be} determined. Parallel to the first conductor 101, a second conductor 104 extends from the comparison measuring point 102 to the measuring point 103. The second conductor 104 consists of a second metal or a second alloy, which differs from the first metal or the first alloy. For the function of the thermocouple 100, it is essential that the two conductors 101, 104 are made of different materials. The two conductors 101, 104 are electrically connected to one another at a contact point 105. This can be done, for example, by soldering or welding the ends of the two conductors 101, 104. The contact point 105 serves as a measuring probe and assumes the temperature T _{mess of} the measuring point 103. This temperature T _mess should by means of

Thermocouple 100 can be determined. The opposite to the contact point 105 ends 106, 107 of the two conductors 101, 104 have the temperature T _ref , which at the

Cold junction 102 prevails. In this respect, the ends 106, 107 are at the

Reference junction 102 at the temperature T _ref , whereas the contact point 105 is at the temperature T _mess . The temperature difference _d i _ff i = (T _me S s _-T _ref ) between the reference _junction 102 and the measuring point 103 is shown in Fig. 1 with.

The function of the thermocouple 100 is based on the Seebeck effect. As a Seebeck effect, the occurrence of a thermal voltage between two points becomes more different

Temperature of a conductor called. As a result of the Seebeck effect, a Seebeck voltage UA forms along the first conductor 101, which consists of the first metal or the first alloy A, which can be expressed in a linear approximation as follows:

UA ⁼ SA '(Tmeas T ^~ _re f) wherein SA represents the Seebeck coefficient, which is usually expressed in microvolts per Kelvin. By the Seebeck coefficient SA, the temperature difference across a conductor is converted into the Seebeck voltage U _A appearing at the ends of the conductor. For the second conductor 104, which consists of the second metal or the second alloy B, one obtains in a linear approximation a Seebeck voltage U _B :

VSS = SSS · (Tmeas ^~ T _ref) wherein S _B denotes the Seebeck coefficient of the second metal or of the second alloy B. The Seebeck voltage U _B occurring at the second conductor 104 is

oppositely poled to the Seebeck voltage UA occurring at the first conductor 101. Between the ends 106, 107 of the two conductors 101, 104 there is therefore a voltage U _TC ("tc" stands for "thermocouple"). This voltage U _TC corresponds to the difference of

Thermoelectric voltages U _A and U _B of the metals or alloys involved: UTC = U _B - U _A = (S _B - S _A) - (T _mess - T _ref)

The voltage U _TC which can be tapped off at the reference junction 102 at the ends 106, 107 thus depends on the one hand on the difference of the Seebeck coefficients SA and S _B and on the second on the temperature difference (T _{mess -T} _ref ). However, the Seebeck coefficients also show SA and S _{B has} a certain temperature dependence. To take into account the

Temperature dependence of the Seebeck coefficients S _A (T) and S _B (T), the voltage Ute applied to the conductors 101, 104 can be determined by means of the following integral: U _tc = j _Ti (S _B (T) -S _A (T )) dT

The relationship between temperature difference and thermoelectric voltage is therefore not completely linear. Depending on the material pairing, more or less pronounced nonlinearities are present.

For evaluation, the thermal voltage U _tc between the ends 106, 107 of the two conductors 101 and 104 is measured. For this purpose, in FIG. 1, a voltage measuring device 108 is connected to the two ends 106 and 107 of the thermocouple 100. These

Thermoelectric voltage U _tc allows a conclusion on the temperature difference T _d i _ff i = (T _{mess -T} _ref ) between the measuring point 103 and the reference junction 102. In order to determine from this temperature difference the actually interesting temperature T _{mess of} the measuring point 103 must in addition, the comparison _temperature T _{ref of} the reference junction 102 may also be known. This comparison temperature T _{ref of} the reference junction 102 is often determined by means of a further temperature sensor, for example by means of a resistance temperature sensor or RTD (Resistance Temperature Device).

The relationship between the occurring along the thermocouple 100

Temperature difference T _diff1 = (T _mess - T _ref ) and the associated thermal voltage U _tc at the two ends 106, 107 is determined by means of a so-called transfer characteristic

described. In Fig. 2 is an example of such a transmission line 200 for a

Type K thermocouple shown. For a type K thermocouple, the

Material pairing nickel-chromium / nickel (NiCr-Ni) used. The first conductor 101 of the thermocouple 100 thus consists of nickel-chromium, while the second conductor 104 consists of nickel. Such thermocouples type K can be used in a temperature range of about 0 ° C to about 1 100 ° C. In the diagram shown in Fig. 2 is along the

_Right axis applied to the thermocouple temperature difference T _diff1 , and along the vertical axis of the thermocouple tapped thermoelectric voltage U _{tc is} applied. It can be seen from FIG. 2 that there is essentially a linear relationship between the temperature difference T _d i _ff i and the associated thermal voltage U _tc , where, for example, deviations from the linear one occur at the point 201

Context are to be recognized. To within a measuring device at a Thermocouple applied thermoelectric voltage U _tc to be able to evaluate as accurately as possible, the transfer characteristic 200 by means of a polynomial 6th, 7th or even higher degree as closely as possible imitated in order in this way, starting from the

Thermoelectric voltage U _{tc to} allow the most accurate determination of the temperature difference T _d i _ff i between the measuring temperature T _mess at the measuring point 103 and the reference temperature T _re at the reference junction 102.

In order to calculate the temperature T _{mess of} the measuring point, the

Temperature T _re the reference point be known. This comparison temperature T _re can, for example, by means of an additional attached to the reference junction 102

Temperature sensor can be measured, for example by means of a

Resistance temperature sensor (RTD, Resistance Temperature Device). The thus determined comparison temperature T _re can then be converted into a corresponding reference voltage U _re f with the aid of the transfer characteristic curve 200 shown in FIG. This is illustrated in FIG. 2 by the arrows 202. The reference voltage U _re f thus obtained corresponds to the comparison temperature T _re f which prevails at the reference junction 102. The reference voltage U _re f is shown in Fig. 2 by the double arrow 203.

Next, the thermal voltage U _tc which can be tapped off at the thermocouple is added to this reference voltage U _re f, which is represented by the double arrow 204. As a result one obtains the summed voltage (U _re f + U _tc ), which is next _translated back into an associated temperature with the aid of the transfer characteristic 200. This is illustrated in Fig. 2 by the arrows 205. Since the reference _voltage U _tc of

Comparison temperature T _re f and the thermal voltage U _{tc of} the differential temperature T _di ffi = (T _mess - Tref), the summed voltage (U _re f + U _tc ) corresponds to the measurement temperature T _mess at the measuring point 103. As a result of the arrows 205 shown implementation therefore obtains the desired measurement temperature T _mess - For those shown in Fig. 2

The procedure is typically that the reference temperature T _re f is first converted into a corresponding reference voltage U _re f and that the temperature evaluation is attributed to a voltage addition.

In addition to the type K thermocouple already described, there are a large number of other standardized thermocouples, which are usually referred to as capital letters. From these types of thermocouples, the user can select the most suitable thermocouple for the particular application. The following are for the most common types of thermocouples listed the respective material pairings and the associated temperature ranges:

• Type K, nickel-chromium / nickel (NiCr-Ni), temperature range 0-1 100 ° C;

· Type J, iron / copper-nickel (Fe-CuNi), temperature range 0-750 ° C;

• Type N, nickel-chromium-silicon / nickel-silicon (NiCrSi-NiSi), temperature range 0-100 ° C;

• Type R, platinum 13 rhenium / platinum (Pt13Rh-Pt), temperature range 0-1600 ° C;

• Type S, platinum 10 rhenium / platinum (Pt1 ORh-Pt), temperature range 0-1600 ° C;

· Type B, platinum 30 rhenium / platinum 6 rhenium (Pt30Rh-Pt6Rh), temperature range

+ 200 ° C to + 1700 ° C;

• Type T, copper / copper-nickel (Cu-CuNi), temperature range -185 to + 300 ° C;

• Type E, nickel-chromium / copper-nickel (NiCr-CuNi), temperature range 0-800 ° C;

• Etc.

This list is not exhaustive, there are a variety of other thermocouples.

In temperature measuring devices, the thermocouple is often connected via a separable connection, for example by means of terminals, with the transmitter. As a result, different thermocouples can be connected to an evaluation depending on the purpose.

In Fig. 3 is a block diagram of such a temperature measuring device according to the prior art. The temperature measuring device comprises a first connection terminal 300 and a second connection terminal 301, to which a thermocouple 100 can be connected. The first conductor 101 of the thermocouple 100 may be connected to the first

Terminal 300 are clamped, and the second conductor 104 can be clamped to the second terminal 301. The first connection terminal 300 is connected via a first connection pin 302 and the second connection terminal 301 is connected to the evaluation electronics 304 via a second connection pin 303. The evaluation electronics 304 comprises a first measuring unit 305, which is designed to evaluate the tapped at the two connection pins 302, 303 thermoelectric voltage U _tc . The determined thermal voltage U _tc is then supplied to the processing unit 306. For determining the measuring temperature T _mess at the contact point 105, in addition to the voltage applied to the connection pins 302, 303 thermoelectric voltage U _tc also the Comparison _temperature T _ref required at the reference junction. As a reference point, the interior of the two terminals 300, 301 can be seen here, because there takes place in each case the transition between the respective conductor 101, 104 of the thermocouple 100 and the associated connection pin 302, 303 of the measuring device. For the correct determination of the measurement temperature T _mess, the comparison temperature T _{ref would} therefore have to be determined within the two connection terminals 300, 301. However, the attachment of an additional temperature sensor within the terminals 300, 301 would be costly and thus expensive. In the temperature measuring apparatus shown in FIG. 3, outside of the terminals 300, 301, a reference temperature sensor 307 is mounted, preferably at a position close to the terminals 300, 301. For example, the reference temperature sensor 307 may be mounted within the housing of the transmitter 304 proximate the terminals 300, 301. For example, the reference temperature sensor 307 may be mounted on a board of the transmitter near the terminals 300, 301. The reference temperature sensor 307 is assigned a second measuring unit 308, which determines the temperature-dependent variable provided by the reference temperature sensor 307

(For example, the resistance) in an associated reference temperature T _ezug converts. The second measuring unit 308 provides this reference temperature T _beZ ug the processing unit 306.

However, this approach has disadvantages, because the reference temperature _sensor 307 determines a reference temperature T _beZ ug, which differs more or less strongly from the actual reference temperature T _ref within the terminals 300, 301. This temperature difference between the measured reference temperature T _reference and the actual reference temperature T _ref at the location of the terminals 300, 301 enters as an error in the measurement of the measurement temperature T _mess .

This temperature difference between the reference temperature T _ezug at the place of

Reference temperature sensor 307 and the actual reference temperature T _ref within the

Terminals 300, 301 are influenced by various factors. A role, for example, plays the self-heating of the evaluation 304 during operation. In addition, that measured by the reference temperature sensor 307 depends

Temperature also from the heat input by neighboring devices. As a result of

Heat emission from neighboring devices can lead to an additional warming of the

Reference temperature sensor 307 come. Other influential factors to consider are Overall, the temperature difference T _d iff2 between the measured by the reference temperature _sensor 307 reference temperature T _beZ ug and the prevailing at the location of the terminals 300, 301 comparison temperature can be the thermal coupling between the terminals 300, 301 and the reference temperature sensor 307 T _ref up to a few degrees Celsius and thus generate a significant measurement error. As a result of the inaccurate determination of the comparison temperature T _ref , the measurement temperature T _mess is then also subject to a measurement inaccuracy of a few degrees Celsius, which is unacceptable for many measurements. If one does not want to accept this measurement error caused by the inaccurate determination of the reference _temperature T _ref , one would have to use the

Install reference temperature sensor 307 in terminals 300, 301. However, this would mean that you could no longer use standard terminals, but would have to use expensive custom-made. Another approach to reducing the measurement error caused by the reference temperature sensor 307 would be to computationally determine the magnitude of this measurement error from the operating parameters and then computationally compensate. However, it should be noted here that the evaluation electronics 304 shown in FIG. 3 is used in a multiplicity of very different application scenarios.

Temperature gauges are available in a variety of different housing designs, such as head transmitters, DIN rail housings or round metal enclosures, also referred to as "field enclosures"

Design variant and housing type differs, for example, the thermal coupling to neighboring devices. A computational compensation of the temperature difference between T _be ug and T _re f is therefore rather unsuccessful.

In order to determine the temperature difference between the reference temperature T _beg detected by the reference temperature sensor 307 and the actual reference temperature T _re f, it is proposed to detect this second temperature difference Τ _ά ^ ₂ = (T _re f _-T T ug) by means of an additional second thermocouple. that without great structural

Extra expense in one of the terminals 300, 301 can be integrated. A corresponding circuitry solution is shown in Fig. 4. In Fig. 4 are

Features that are the same or similar to the features already shown in Fig. 3, with the same reference numerals as in Fig. 3, so that will be discussed primarily in the first place on the differences and otherwise referred to the description of FIG. 3 reference becomes. The temperature measuring device shown in Fig. 4 has two terminals 300, 400, to which a thermocouple 100 can be connected. For this purpose, the first conductor 101 of the thermocouple 100 is clamped in the first terminal 300, and the second conductor 104 is clamped in the second terminal 400. About the first

Terminal pin 302 and the second terminal pin 303, the two terminals 300, 400 are electrically connected to a transmitter 401. Between the two connection pins 302 and 303 is supplied by the thermocouple 100 thermoelectric voltage Ute, on the temperature difference between the measuring temperature T _mess on the

Contact point 105 and the reference _temperature T _ref within the terminals 300, 400 is determined. This thermal voltage U _tc applied to the connection pins 302, 303 is evaluated by the first measuring unit 305, and the result becomes

Processing unit 402 supplied.

In addition, the _transmitter includes the reference temperature sensor 307, which is _configured to determine a reference temperature T _beZ ug. The reference temperature sensor 307 is disposed outside the two terminals 300, 400, preferably at a position close to the terminals 300, 400. The reference temperature sensor 307 may be mounted within the housing of the transmitter 401 near the terminals 300, 400, for example. For example, the

Reference temperature sensor 307 on an evaluation board near the

Terminals 300, 400 attached. The reference temperature sensor 307 may, for example, be a resistance temperature sensor (RTD). The reference temperature sensor 307 is associated with the second measuring unit 308, which provides the information provided by the reference temperature sensor 307

Temperature-dependent variable (for example, the resistance) in the associated

_Reference temperature T _ezug converts and this reference temperature of the processing unit 402 provides.

Depending on the position of the reference temperature _sensor 307, the measured reference temperature T _Zug more or less differs from that within the terminals

300, 400 prevailing reference temperature T _re f, which would actually have to use to determine the measurement temperature T _mess , but the metrology is difficult to access, because you would need to detect the temperature inside the terminals 300, 400. For determining the temperature difference T _d iff2 = (T _re f - T _beZ ug) between the temperature at the location of the reference temperature sensor 307 and the temperature inside the Terminal is in the solution shown in Fig. 4 in the interior of the second

Terminal 400 at the second terminal pin 303 an additional connecting line

403 attached. The additional connection line 403 is electrically connected at a connection point 404 to the second connection pin 303. The material of this additional connection line 403 differs from the material of the second connection pin 303, so that the second connection pin 303 together with the additional connection line 403 acts as a second thermocouple 405. The two materials are at the junction

404 electrically connected. For example, the additional

Connecting lead 403 may be soldered or welded to the second terminal pin 303.

The additional connection line 403 is preferably in the form of a third

Connection pins 406 led out of the second terminal 400. In this case, the additional connection line 403 and the third connection pin 406 are made of the same material. Since the material of the second connection pin 303 differs from the material of the additional connection line 403 and the third connection pin 406, the second connection pin 303 forms together with the additional connection line 403 and

optionally, the third terminal pin 406, the second thermocouple 405th

This second thermocouple 405 is configured to measure a temperature difference T _d iff2 between the temperature at the location of the reference temperature sensor 307 and the temperature inside the second terminal 400. When the temperature T _ezug at the location of the reference temperature sensor 307 of the reference temperature T _ref inside the second connection terminal 400 is different, then is formed between the connection pins 303 and 406, a thermal voltage U _tC 2, = of this second temperature differential T _d IFF2 (T _re - T _deduction ) depends. With the help of this second thermocouple 405 so can the second temperature difference T _diff2 between the temperature in the interior of the second

Terminal 400 and the temperature at the location of the reference temperature sensor 307 are measured accurately. For this purpose, the two connection pins 303, 406 with a third

Measuring unit 407 is connected, which is designed to evaluate the tapped between the two connection pins 303, 406 thermo voltage U _tC 2 and the processing unit 402 supply. It is important to ensure that the ends of the two connection pins 303 are to be arranged in the immediate vicinity of the reference temperature sensor 307 406 so that the temperature measured by the reference temperature sensor 307 reference temperature T _respect exactly the

Temperature at the ends of the two connection pins 303, 406 corresponds and the

Reference temperature sensor 307 thus the comparison temperature for the second thermocouple

405 recorded. Based on the reference temperature T detected by the reference temperature _{sensor 307} and the thermal voltage U _tC 2 applied to the second thermocouple 405, the temperature inside the second terminal 400 can be determined by means of the transfer characteristic of the second thermocouple 405. In this way, one obtains the actual reference temperature T _re inside the second terminal 400.

In a second step, the temperature T _mess at the measuring point is determined on the basis of the thus determined comparison temperature T _re f and the voltage applied to the first thermocouple 100 thermoelectric voltage U _tc i by means of the transfer characteristic of the first thermocouple 100.

Altogether one can thus by means of a two-stage evaluation of the reference temperature Tbezug, supplied by the second thermocouple 405 second thermoelectric voltage U _tC 2 and the first thermoelement 100 supplied by the first thermal voltage U _tc i the

Determine the measurement temperature T _mess at the contact point 105.

The advantage of the circuit shown in FIG. 4 is that the second thermocouple 405 can be realized without significant additional construction costs. It is only necessary, at the existing second terminal pin 303 an additional

Connecting line 403 to be made of a different material than the second

Connection pin 303 exists. This additional connection line 403 is then led out of the second connection terminal 400 by means of a third connection pin 406. This third connection pin 406 may be integrally formed with the additional connection line 403, but the third connection pin 406 may also be formed as a separate connection pin, which is electrically connected to the additional connection line 403. The second thermocouple 405 thus formed is suitable for the required

Temperature difference between the reference temperature T _re f and measured outside the second terminal 400 reference temperature T _beZ ug to determine exactly. Because of its simplicity, the second thermocouple 405 shown in Fig. 4 can be easily integrated into different types of terminals. Temperature transmitters are usually available in a variety of different housing variants.

For example, a temperature transmitter can be realized as a so-called head transmitter and installed in a disk-shaped housing of about 4 cm in diameter. With a head transmitter, the terminals are accessible from the outside.

For example, the terminals can be formed integrated into the housing. In addition, temperature transmitters are offered, which are installed in a DIN rail housing, which is designed for mounting on a DIN rail. In such DIN rail housings, the terminals for a thermocouple in the form of a pin tray are formed. Even in such a pin tray can be an additional second

Insert thermocouple 405. Another housing variant for thermotransmitters is a round field housing with a housing diameter of about 10 cm, which is often designed as a two-part explosion-proof housing. In this type of housing, a separate connection space is provided, in which the terminals are arranged. In all the housing variants mentioned, the connection terminals can be modified such that the required second thermocouple 405 is formed in the connection terminal.

The materials of the first connection pin 302 and the second connection pin 303 on the one hand and the additional connection line 403 and the third connection pin 406

on the other hand, must be chosen so that one for measurements at

Operating temperature suitable thermocouple results. In addition, the

Materials should be chosen so that the second terminal 400 can be finished without problems. The connection pins 302, 303 may be made of copper, for example. In this case, a second material is needed, which forms a suitable thermocouple together with copper, for example, here Konstantan in question. For example, it would be possible to produce the first connection pin 302 and the second connection pin 303 from copper and manufacture the additional connection line 403 and the third connection pin 406 from constantan. In this way, a thermocouple of type T (copper / copper-nickel) is formed. This is only meant as an example. As a second thermocouple 405 z. B. also a thermocouple type J (iron / copper-nickel), type K (nickel-chromium / nickel) or type E (nickel-chromium / copper-nickel) can be used. Finally, for the second thermocouple 405 any material pairing can be used, with which temperature differences at the operating temperature can be meaningfully measured.

To fulfill different measuring tasks, different thermocouples 100 with different material pairings can be connected to the connection terminals 300, 400 of the temperature measuring device shown in FIG. 4. In general, therefore, it can be assumed that the material pairing of the first thermocouple 100 does not match the material pairing of the second thermocouple 405. Therefore, the

Translation of the voltage applied to the first thermocouple 100 thermoelectric voltage U _tc in a corresponding first temperature difference T _d i _ff i requires a first transfer characteristic that fits the material pairing of the first thermocouple 100. To implement the am second thermocouple 405 applied thermoelectric voltage U _tC 2, however, a

Transfer characteristic that matches the material pairing of the second thermocouple 405 is needed. For each of the two thermocouples 100, 405, a separate transfer characteristic is therefore generally required in each case. In this case, the same transfer characteristic is always used for the second thermocouple 405.

The various temperatures that occur in the temperature measuring device shown in Fig. 4, are shown in Fig. 5 again in overview. FIG. 5 shows the reference temperature _T.sub.zug , which is at the location of the reference temperature sensor 307 outside of

Terminals 300, 400 is determined by the reference temperature sensor 307. In addition, the comparison temperature T _re f is shown, which prevails within the connection terminals 300, 400 and which serves as a comparison temperature for the first thermocouple 100. In addition, the measurement temperature T _mess at the tip of the first thermocouple 100 is located, which is to be determined by means of the temperature measurement.

In FIG. 5, the first temperature difference T _d = T IFFI _measured - T _re f inscribes detected by the first thermocouple element 100. In addition, the second temperature difference T _diff2 = T _re _{-T Zzug is} drawn, which is detected by means of the second thermocouple 405. The determination of the measurement temperature T _mess can be carried out, for example, as follows:

First, the reference temperature T _beZg supplied from the reference temperature _sensor 307 is converted into an associated reference voltage U _be train by means of the transfer characteristic associated with the second thermocouple 405. The measuring voltage U _tC 2 supplied to the second thermocouple 405 is then added to this reference voltage U _beZ . The summed voltage U _beZμg ⁺ U _tC 2 thus obtained is translated into a temperature by means of the transfer characteristic associated with the second thermocouple 405, and as a result, the comparison temperature T _{ref is obtained} . With the aid of the measuring voltage U _tC 2 supplied by the second thermocouple 405, it is thus possible to determine the reference temperature T _re f very accurately from the reference temperature T _reference .

With this reference _temperature T _ref , one now goes into the transfer characteristic to the first thermocouple 100 and translates the comparison _temperature T _ref by means of this characteristic into an associated reference voltage U _re f. The measuring voltage U _tc supplied by the first thermoelement 100 is then added to this comparison voltage U _re f, and as a result If one obtains the summed voltage U _ref + U _tc - This summed voltage U _ref + Um is converted by means of the transfer characteristic into an associated temperature and thus obtains the measurement temperature T _mess - In this way, starting from the high-precision comparison temperature T _ref the measurement temperature T _{mess are} determined.

Claims

claims

A measuring device for determining a measuring temperature at a measuring point (103) by means of a first thermoelement (100), the measuring device comprising:

a first connection terminal (300) and a second connection terminal (400) for connecting the first thermocouple (100),

- A first connecting line (302) and a second connecting line (303), the first (300) and the second terminal (400) each having a

Connect evaluation electronics (401),

a reference temperature sensor (307) disposed outside of the terminals (300, 400) and configured to detect a reference temperature at the location of the reference temperature sensor (307),

- The evaluation (401), which is designed to measure the temperature at the

To determine the measuring point (103)

characterized in that

- In the second terminal (400) to the second connecting line (303) an additional connecting line (403) is connected, which consists of a different material than the second connecting line (303), wherein the additional

Connecting line (403) together with the second connecting line (303) forms a second thermocouple (405), which is also connected to the transmitter (401),

- wherein the second thermocouple (405) provides a second measurement voltage that depends on the second temperature difference between the temperature of the second terminal (400) and the reference temperature at the location of the reference temperature sensor (307), and

- wherein the transmitter (401) is adapted, starting from a first measuring element supplied by the first thermocouple (100), the second measuring voltage supplied by the second thermocouple (405) and the from

Reference temperature sensor (307) detected reference temperature to determine the measuring temperature at the measuring point (103).

Measuring device according to claim 1, characterized in that the second

Thermocouple is provided to detect a second temperature difference between the temperature of the second terminal and the reference temperature at the location of the reference temperature sensor. Measuring device according to claim 1 or claim 2, characterized by at least one of the following:

- The first terminal is connected to the via a first connection pin

Evaluation electronics electrically connected;

- The second terminal is connected to the via a second connection pin

Evaluation electronics electrically connected;

- The additional connection line is led out by means of a third connection pin from the second terminal and is electrically connected via the third terminal pin to the transmitter, wherein the additional connection line and the third terminal pin made of the same material;

- The second terminal is connected via the second terminal pin and the third pin to the transmitter;

- The second terminal is connected via the second terminal pin and the third terminal pin to the evaluation, wherein between the second terminal pin and the third terminal pin supplied by the second thermocouple second measurement voltage is applied.

Measuring device according to one of claims 1 to 3, characterized by one of the following:

- The measuring device is designed as a head transmitter, wherein the terminals are arranged on top of the head transmitter;

- The measuring device is housed in a round field housing, wherein the terminals are arranged in a connection space of the field housing;

- The measuring device is housed in a DIN rail housing, wherein the

Terminals are realized by means of a pin tray.

Measuring device according to one of claims 1 to 4, characterized by at least one of the following:

the reference temperature sensor is mounted inside a housing of the measuring device;

- The reference temperature sensor is designed as part of the evaluation;

- The reference temperature sensor is arranged on a circuit board of the transmitter;

- The reference temperature sensor is arranged in spatial proximity to the terminals; - The transmitter is designed to operate from one of the

Reference temperature sensor supplied to determine a reference temperature at the location of the reference temperature sensor;

- The reference temperature sensor is a resistance sensor.

Measuring device according to one of claims 1 to 5, characterized in that it is a temperature transmitter in the measuring device, wherein optionally different thermocouples can be connected to the terminals depending on the measurement situation.

A temperature measuring device for determining a measuring temperature at a measuring point (103), which has

a measuring device according to one of claims 1 to 6,

- A first thermocouple (100) which is connected to the first terminal (300) and the second terminal (400) of the measuring device.

Temperature measuring device according to claim 7, characterized by at least one of the following:

the first thermocouple is intended to provide a first temperature difference between the temperature at the measuring point and the temperature of the

To detect terminals;

- The first thermocouple provides a first measurement voltage, which depends on a first temperature difference between the temperature at the measuring point and the temperature of the terminals.

Temperature measuring device according to claim 7 or claim 8, characterized by at least one of the following:

- The second terminal serves as reference junction for one of the first

Thermocouple detected first temperature difference;

- The temperature of the second terminal serves as a comparison temperature for a detected by the first thermocouple first temperature difference;

- The second thermocouple is intended, starting from the of

Reference temperature sensor detected reference temperature to determine the temperature of the second terminal as the reference temperature for the first thermocouple.

10. Temperature measuring device according to one of claims 7 to 9, characterized in that the evaluation is designed to, starting from the of

Reference temperature sensor detected reference temperature at the location of

Reference temperature sensor and the second measuring voltage supplied by the second thermocouple to determine the temperature of the second terminal.

Temperature measuring device according to claim 10, characterized in that the

Transmitter is designed based on the temperature of the second terminal and the first supplied by the first thermocouple first

Measuring voltage to determine the measuring temperature at the measuring point.

12. Temperature measuring device according to one of claims 7 to 1 1, characterized in that the evaluation is designed to

the reference temperature detected by the reference temperature sensor at the location of the

Reference temperature sensor by means of a belonging to the second thermocouple

To convert the transfer characteristic into a corresponding reference voltage,

to add to the reference voltage thus obtained the second measuring voltage supplied by the second thermocouple, and

the voltage obtained as a result of the addition by means of the second

Thermoelement belonging transfer characteristic to a corresponding temperature and thus to determine the temperature of the second terminal.

13. Temperature measuring device according to claim 12, characterized in that the

Transmitter is designed to

- The temperature of the second terminal by means of one to the first

Thermocouple associated transfer characteristic into a corresponding

To convert the reference voltage,

to add to the comparison voltage thus obtained the first measuring voltage supplied by the first thermocouple and

the voltage obtained as a result of the addition by means of the first

To convert thermocouple transfer characteristic curve in a corresponding temperature and thus to determine the measurement temperature at the measuring point.

14. Temperature measuring device according to one of claims 7 to 13, characterized by at least one of the following: the first thermocouple has a first material pairing, and the second thermocouple has a second material pairing that does not necessarily coincide with the first material pairing;

- The material of the second connection line and the material of the additional connection line are chosen so that the second thermocouple

Can detect temperature differences at operating temperature.

Temperature measuring device according to one of claims 7 to 14, characterized by at least one of the following:

- The temperature measuring device is a field device;

- The temperature measuring device comprises a field bus interface for connection to a fieldbus system;

- The temperature measuring instrument is designed to exchange data with a fieldbus system according to one of the following fieldbus protocols: HART, Profibus, Fieldbus Foundation, an Industrial Ethernet protocol.

Method for determining a measuring temperature at a measuring point (103) by means of a temperature measuring device, wherein the temperature measuring device comprises:

a first connection terminal (300) and a second connection terminal (400) for connecting a first thermoelement (100),

a first thermocouple (100) connected to the first connection terminal (300) and the second connection terminal (400),

Connect evaluation electronics (401),

- The evaluation (401), which is designed to measure the temperature at the

To determine the measuring point (103)

- wherein in the second terminal (400) to the second connecting line (303) an additional connecting line (403) is connected, which consists of a different material than the second connecting line (303), wherein the additional connecting line (403) together with the second Connecting line (303) forms a second thermocouple (405), which is also connected to the transmitter (401), the method being characterized by the following steps:

- evaluating a voltage applied to the first thermocouple (100) first measurement voltage;

- Evaluating a second thermocouple (405) adjacent second

Measuring voltage;

- Determining a reference temperature at the location of the reference temperature sensor (307) by the reference temperature sensor (307);

- Determining the measuring temperature at the measuring point (103) starting from the

Reference temperature at the location of the reference temperature sensor (307), the first measurement voltage supplied by the first thermocouple (100) and the second

Thermocouple (405) supplied second measurement voltage.

Method according to claim 16, characterized by the following steps:

Converting the reference temperature detected by the reference temperature sensor at the location of the reference temperature sensor into a corresponding reference voltage by means of a transfer characteristic associated with the second thermocouple,

Adding the second measuring voltage supplied by the second thermocouple to the thus obtained reference voltage and

Converting the voltage obtained as a result of the addition by means of the transfer characteristic associated with the second thermocouple into a corresponding temperature which corresponds to the temperature of the second connection terminal.

Method according to claim 17, characterized by the following steps:

- Converting the temperature of the second terminal by means of a first thermocouple associated transfer characteristic into a corresponding

Comparison voltage

Adding the first measuring voltage supplied by the first thermocouple to the thus obtained comparison voltage and

Converting the voltage obtained as a result of the addition by means of the transfer characteristic associated with the first thermocouple into a corresponding temperature which corresponds to the measuring temperature at the measuring point.