The invention relates to a measuring apparatus for determining a measurement temperature at a measuring point by means of a thermocouple as well as to a temperature measuring device for determining a measurement temperature at a measuring point. Furthermore, the invention relates to a method for determining a measurement temperature at a measuring point.
In automation technology, field devices are often applied, which serve for registering and/or influencing process variables. Examples of such field devices are fill level measuring devices, mass flow measuring devices, pressure- and temperature measuring devices, etc., which as sensors register the corresponding process variables, fill level, flow, pressure, and temperature.
In the case of industrial temperature measurement, thermocouples are applied in many fields of use. A thermocouple delivers a measurement voltage, based on which the temperature difference occurring along the thermocouple can be determined. In order with the assistance of this temperature difference to be able to determine the measurement temperature at the tip of the thermocouple, the reference temperature at the connections of the thermocouple must be known as exactly as possible. The more exactly the reference temperature is known, the more exactly the measurement temperature can be determined.
It is, consequently, an object of the invention to provide a temperature measuring device as well as a method for temperature measurement, which enables with little extra effort a more exact determining of the reference temperature for a thermocouple.
This object is achieved by the features set forth in claims 1, 7 and 18.
Advantageous further developments of the invention are set forth in the dependent claims.
A measuring apparatus for determining a measurement temperature at a measuring point by means of a first thermocouple corresponding to the forms of embodiment of the invention includes a first connection terminal and a second connection terminal for connecting the first thermocouple, a first connecting line and a second connecting line, which connect the first and second connection terminals with an evaluating electronics, an auxiliary temperature sensor, which is arranged removed from the connection terminals and is designed to register an auxiliary temperature at the site of the auxiliary temperature sensor, and an evaluating electronics, which is designed to determine the measurement temperature at the measuring point. Connected at the second connection terminal with the second connecting line is an additional connecting line, which is composed of a material other than that of the second connecting line. The additional connecting line forms together with the second connecting line a second thermocouple, which is likewise connected to the evaluating electronics. The second thermocouple delivers a second measurement voltage, which depends on the second temperature difference between the temperature of the second connection terminal and the auxiliary temperature at the site of the auxiliary temperature sensor. The evaluating electronics is designed, based on a first measurement voltage delivered by the first thermocouple, the second measurement voltage delivered by the second thermocouple and the auxiliary temperature registered by the auxiliary temperature sensor, to determine the measurement temperature at the measuring point.
For the accuracy of a temperature measurement by means of a thermocouple, it is important to determine the reference temperature at the connection locations, or connection terminals, of the thermocouple with best possible accuracy. For this, in many cases, a separate auxiliary temperature sensor is provided.
However, the auxiliary temperature sensor is, as a rule, arranged somewhat removed from the connection terminals, so that the auxiliary temperature ascertained by the auxiliary temperature sensor does not exactly correspond to the temperature of the connection terminals. There is thus a second temperature difference between the auxiliary and temperature measured by the auxiliary temperature sensor and the temperature of the connection terminals. In order to register this temperature difference and to be able to determine the reference temperature reigning at the connection terminals with high accuracy, the second thermocouple is applied. This second thermocouple is formed by connecting to the second connecting line of the second connection terminal an additional connecting line, which is composed of a material other than that of the second connecting line. This second thermocouple registers the second temperature difference between the auxiliary temperature registered by the auxiliary temperature sensor and the temperature at the connection terminals. In this way, the reference temperature reigning at the connection terminals can be determined with high accuracy. Based on the more exactly than previously known reference temperature, then the measurement temperature actually of interest can be determined with high accuracy with the assistance of the first thermocouple. The second thermocouple permits determining the temperature at the connection terminals with very little extra constructional effort. It is only required that an additional connecting line be connected with the second connecting line.
In the following, the invention will now be explained in greater detail based on examples of embodiments illustrated in the drawing. The figures of the drawing show as follows:
In the field of industrial measurements technology, thermocouples are frequently applied for temperature measurement.
The second conductor 104 is composed of a second metal, e.g. a second alloy, which differs from the first metal or the first alloy. For the thermocouple 100 to function, it is essential that the two conductors 101, 104 be of different materials. The two conductors 101, 104 are electrically connected with one another at a contact location 105. This can occur, for example, by soldering, brazing or welding the ends of the two conductors 101, 104. The contact location 105 serves as measuring probe and assumes the temperature Tmeas of the measuring point 103. This temperature Tmeas is to be determined by means of the thermocouple 100. The ends 106, 107 of the two conductors 101, 104 removed from the contact location 105 have the temperature Tref, which reigns at the reference junction 102. Insofar, the ends 106, 107 at the reference junction 102 have the temperature Tref, while, in contrast, the contact location 105 has the temperature Tmeas. The temperature difference Tdiff1=(Tmeas−Tref) between the reference junction 102 and the measuring point 103 is indicated in
The functioning of thermocouple 100 rests on the Seebeck effect. Referred to as the Seebeck effect is the occurrence of a thermovoltage between two locations of a conductor at different temperatures. As a result of the Seebeck effect, there forms along the first conductor 101, which is composed of the first metal, e.g. the first alloy A, a Seebeck voltage UA, which in linear approximation can be expressed as follows:
U
A
=S
A·(Tmeas−Tref)
wherein SA refers to the Seebeck coefficient, which is usually given in microvolt per Kelvin. The Seebeck coefficient SA converts the temperature difference existing along a conductor into the Seebeck voltage UA occurring across the ends of the conductor.
For the second conductor 104, which is composed of the second metal, e.g. the second alloy B, one obtains in linear approximation a Seebeck voltage UB:
U
B
=S
B·(Tmeas−Tref)
wherein SB refers to the Seebeck coefficient of the second metal, e.g. the second alloy B. The Seebeck voltage UB occurring across the second conductor 104 is oppositely poled to the Seebeck voltage UA arising across the first conductor 101. Between the ends 106, 107 of the two conductors 101, 104 there lies, consequently, a voltage Utc (“tc” stands for “thermocouple”). This voltage Utc corresponds to the difference between the thermovoltages UA and UB of the participating metals, e.g. alloys:
U
tc
=U
B
−U
A=(SB−SA)·(Tmeas−Tref)
The voltage Utc tappable at the reference junction 102 across the ends 106, 107 depends thus, on the one hand, on the difference between the Seebeck coefficients SA and SB and, on the other hand, on the temperature difference (Tmeas−Tref). However, also the Seebeck coefficients SA and SB have a certain temperature dependence. To take the temperature dependence of the Seebeck coefficients SA(T) and SB(T) into consideration, the voltage Utc on the conductors 101, 104 can be determined by means of the following integral:
U
tc=∫T1T2(SB(T)−SA(T))dT
The relationship between temperature difference and thermovoltage is thus not completely linear. More or less strongly marked non-linearities are present as a function of the material pairing.
For evaluation, the thermovoltage Utc between the ends 106, 107 of the two conductors 101 and 104 is measured. Thus, in
The relationship between the temperature difference Tdiff1=(Tmeas−Tref) arising along the thermocouple 100 and the associated thermovoltage Utc across the two ends 106, 107 is described with the assistance of a so-called transfer characteristic line.
Based on
In order to calculate therefrom the temperature Tmeas of the measuring point, supplementally the temperature Tref of the reference junction must be known. This reference temperature Tref can be measured, for example, by means of an additional temperature sensor mounted at the reference junction 102, for example, with the assistance of a resistance temperature sensor (RTD, Resistance Temperature Device). The so ascertained reference temperature Tref can then be converted with the assistance of the transfer characteristic line 200 shown in
Next there is added to this reference voltage Uref the thermovoltage Utc tappable across the ends of the thermocouple. Thermovoltage Utc is indicated by the double arrow 204. As a result, one obtains the summed voltage (Uref+Utc), which is next with the assistance of the transfer characteristic line 200 translated back into an associated temperature. This is illustrated in
Besides the already described thermocouple of type K, there exist a large number of additional, standardized thermocouples, which are usually referred to with capital letters. From these types of thermocouples, the user can select the most suitable thermocouple for the particular application. The most common types of thermocouples, with their material pairings and associated temperature ranges, are as follows:
This listing is not exclusive, there are yet a large number of other thermocouples.
In the case of temperature measuring devices, the thermocouple is often connected with the evaluating electronics via a separable connection, for example, by means of connection terminals. In this way, different thermocouples can be connected to an evaluating electronics as a function of the application.
For determining the measurement temperature Tmeas at the contact location 105, supplementally to the thermovoltage Utc between the connection pins 302, 303, also the reference temperature Tref at the reference junction is required. The reference junction here is the interior of the two connection terminals 300, 301, for that is where the transitions between the conductors 101, 104 of the thermocouple 100 and the associated connection pins 302, 303 of the measuring device occur. For correctly determining the measurement temperature Tmeas, the reference temperature Tref would thus be determined within the two connection terminals 300, 301. However, the insertion of an additional temperature sensor within the connection terminals 300, 301 would be complex and, thus, also expensive.
In the case of the temperature measuring device shown in
This procedure has, however, disadvantages, because the auxiliary temperature sensor 307 ascertains an auxiliary temperature Taux, which differs more or less strongly from the actual reference temperature Tref within the connection terminals 300, 301. This temperature difference between the measured auxiliary temperature Taux and the actual reference temperature Tref at the location of the connection terminals 300, 301 enters as an error into the measuring of the measurement temperature Tmeas.
The temperature difference between the auxiliary temperature Taux at the site of the auxiliary temperature sensor 307 and the actual reference temperature Tref within the connection terminals 300, 301 is influenced by different factors. A role is played, for example, by the self-warming of the evaluating electronics 304 during the ongoing operation. Moreover, the temperature measured by the auxiliary temperature sensor 307 depends also on the heat input by adjoining devices. As a result of heat emission from neighboring devices, an additional warming of the auxiliary temperature sensor 307 can occur. Other influencing factors to be taken into consideration are the position of the connection terminals 300, 301 as well as the thermal coupling between the connection terminals 300, 301 and the auxiliary temperature sensor 307. On the whole, the temperature difference Tdiff2 between the auxiliary temperature Taux measured by the auxiliary temperature sensor 307 and the reference temperature Tref reigning at the site of the connection terminals 300, 301 can amount to several degrees Celsius and, thus, can produce a considerable measurement error. As a result of the inexact determining of the reference temperature Tref, then also the measurement temperature Tmeas is burdened with a measurement inaccuracy of several degrees Celsius. This is unacceptable for many applications.
When one would rather not tolerate the measurement error caused by the inexact determining of the reference temperature Tref, the auxiliary temperature sensor 307 could be incorporated into the connection terminals 300, 301. This would mean, however, that standard connection terminals could no longer be used, but, instead, expensive, custom-made products would be required. A further approach for lessening the measurement error caused by the auxiliary temperature sensor 307 would be to determine the size of this measurement error by calculation based on the operating parameters and then to compensate such during the calculations. Here to be noted, however, is that the evaluating electronics 304 shown in
For ascertaining the temperature difference between the auxiliary temperature Taux registered by the auxiliary temperature sensor 307 and the actual reference temperature Tref, it is proposed according to the invention to register this second temperature difference Tdiff2=(Tref−Taux) with the assistance of an additional, second thermocouple, which can be integrated into one of the connection terminals 300, 301 without much extra effort. A corresponding circuit is shown in
The temperature measuring device shown in
Moreover, the evaluating electronics includes the auxiliary temperature sensor 307, which is designed to determine an auxiliary temperature Taux. The auxiliary temperature sensor 307 is arranged removed from the two connection terminals 300, 400, preferably at a position near to the connection terminals 300, 400. The auxiliary temperature sensor 307 can be placed, for example, within the housing of the evaluating electronics 401 in the vicinity of the connection terminals 300, 400. For example, the auxiliary temperature sensor 307 can be placed on a circuit board of the evaluating electronics in the vicinity of the connection terminals 300, 400. The auxiliary temperature sensor 307 can be, for example, a resistance temperature sensor (RTD, Resistance Temperature Device). Associated with the auxiliary temperature sensor 307 is the second measuring unit 308, which converts the temperature dependent variable (for example, the resistance) provided by the auxiliary temperature sensor 307 into the associated auxiliary temperature Taux and provides this auxiliary temperature to the processing unit 402.
Depending on position of the auxiliary temperature sensor 307, the measured auxiliary temperature Taux differs more or less strongly from the reference temperature Tref reigning within the connection terminals 300, 400, which reference temperature Tref one would actually use for determining the measurement temperature Tmeas, which, however, is metrologically difficulty accessible, because one would have to the register temperature in the interior of the connection terminals 300, 400.
For determining the temperature difference Tdiff2=(Tref−Taux) between the temperature at the site of the auxiliary temperature sensor 307 and the temperature in the interior of the connection terminal in the case of the solution shown in
The additional connecting line 403 is preferably led out from the second connection terminal 400 in the form of a third connection pin 406. In such case, the additional connecting line 403 and the third connection pin 406 are of the same material. Since the material of the second connection pin 303 differs from the material of the additional connecting line 403 and the third connection pin 406, the second connection pin 303 forms together with the additional connecting line 403 and, in given cases, the third connection pin 406 the second thermocouple 405.
This second thermocouple 405 is designed to measure a temperature difference Tdiff2 between the temperature at the site of the auxiliary temperature sensor 307 and the temperature in the interior of the second connection terminal 400. When the temperature Taux at the site of the auxiliary temperature sensor 307 differs from the reference temperature Tref in the interior of the second connection terminal 400, then there arises between the connection pins 303 and 406 a thermovoltage Utc2, which depends on this second temperature difference Tdiff2=(Tref−Taux). With the help of this second thermocouple 405, thus the second temperature difference Tdiff2 between the temperature in the interior of the second connection terminal 400 and the temperature at the site of the auxiliary temperature sensor 307 can be measured exactly. In this regard, the two connection pins 303, 406 are connected with a third measuring unit 407, which is designed to evaluate the thermovoltage Utc2 tappable between the two connection pins 303, 406 and to supply the result to the processing unit 402. In such case, it is to be heeded that the ends of the two connection pins 303, 406 are arranged in the immediate vicinity of the auxiliary temperature sensor 307, in order that the auxiliary temperature Taux measured by the auxiliary temperature sensor 307 corresponds exactly to the temperature at the ends of the two connection pins 303, 406 and the auxiliary temperature sensor 307, thus, registers the reference temperature for the second thermocouple 405.
Starting from the auxiliary temperature Taux registered by the auxiliary temperature sensor 307 and the thermovoltage Utc2 across the ends of the second thermocouple 405, the temperature in the interior of the second connection terminal 400 can be determined by means of the transfer characteristic line of the second thermocouple 405. In this way, one obtains the actual reference temperature Tref in the interior of the second connection terminal 400.
In a second step, then, based on the so ascertained reference temperature Tref and the thermovoltage Utc1 across the ends of the first thermocouple 100, the temperature Tmeas at the measuring point is determined by means of the transfer characteristic line of the first thermocouple 100.
In summary, thus by means of a two-stage evaluation, one can determine the measurement temperature Tmeas at the contact location 105 from the auxiliary temperature Taux, the second of thermovoltage Utc2 delivered by the second thermocouple 405 and the first thermovoltage Utc1 delivered by the first thermocouple 100.
The advantage of the circuit shown in
Because of its simplicity, the second thermocouple 405 shown in
The materials of the first connection pin 302 and of the second connection pin 303, on the one hand, and the additional connecting line 403 and the third connection pin 406, on the other hand, must be so selected that a thermocouple suitable for measurements at operating temperature results. Moreover, the materials must be so selected that the second connection terminal 400 can be manufactured without problems. The connection pins 302, 303 can be, for example, of copper. In this case, a second material is required, which forms a suitable thermocouple with copper, wherein here, for example, constantan is a possibility. One could thus, for example, manufacture the first connection pin 302 and the second connection pin 303 of copper and the additional connecting line 403 and the third connection pin 406 of constantan. In this way, a thermocouple of type T (copper/copper-nickel) is formed. This is, however, only one example. The second thermocouple 405 could e.g. also be a thermocouple of type J (iron/copper-nickel), of type K (nickel-chromium/nickel) or of type E (nickel-chromium/copper-nickel). Ultimately, used for the second thermocouple 405 can be any material pairing, with which temperature differences at operating temperature can be sensibly measured.
For fulfillment of the different measuring tasks, different thermocouples 100 with different material pairings can be connected at the connection terminals 300, 400 of the temperature measuring device shown in
The different temperatures, which occur in the case of the temperature measuring device shown in
The determining of the measurement temperature Tmeas can be performed, for example, in the following way:
First, the auxiliary temperature Taux delivered by the auxiliary temperature sensor 307 is converted with the assistance of the transfer characteristic line for the second thermocouple 405 into an associated auxiliary voltage Uaux. To this reference voltage Uaux is then added the measurement voltage Utc2 delivered by the second thermocouple 405. The so obtained summed voltage Uau+Utc2 is translated by means of the transfer characteristic line for the second thermocouple 405 into a temperature, and, as a result, one obtains the reference temperature Tref. With the help of the measurement voltage Utc2 delivered by the second thermocouple 405, one can, thus, starting from the auxiliary temperature Taux, very exactly determine the reference temperature Tref.
With this reference temperature Tref, one then uses the transfer characteristic line for the first thermocouple 100 to translate the reference temperature Tref into an associated reference voltage Uref. To this reference voltage Uref is then added the measurement voltage Utc delivered by the first thermocouple 100, and as result one obtains the summed voltage Uref+Utc. Using the transfer characteristic line, one converts this summed voltage Uref+Utc into an associated temperature and so obtains the measurement temperature Tmeas. In this way, the measurement temperature Tmeas can be determined based on the reference temperature Tref determined with high accuracy.
Number | Date | Country | Kind |
---|---|---|---|
10 2015 113 842.5 | Aug 2015 | DE | national |
Filing Document | Filing Date | Country | Kind |
---|---|---|---|
PCT/EP2016/067741 | 7/26/2016 | WO | 00 |