MEASURING ARRANGEMENT COMPRISING FIRST AND SECOND PAIRS OF THERMOWIRES

Abstract

A measuring arrangement comprising a first pair of thermowires wherein the two thermowires of the first pair are insulated completely from one another by an insulation, such as, for example, a material, which serves for electrical insulation between the two thermowires of the first pair.

Claims

1-26. (canceled)

27. A measuring arrangement, comprising: a first pair of thermowires, wherein: the two thermowires of said first pair are insulated completely from one another by an insulation, such as, for example, a material, which serves for electrical insulation between said two thermowires of said first pair.

28. The arrangement as claimed in claim 27, wherein: said two thermowires of said first pair have no junction, which serves for determining temperature, and where they are electrically connected with one another, but, instead, preferably have free ends.

29. The arrangement as claimed in claim 27, wherein: said two thermowires of said first pair serve to register a first thermovoltage, especially a parasitic, first thermovoltage, for example, as a reference voltage for a second thermovoltage.

30. The arrangement as claimed in claim 27, further comprising: a second pair of thermowires, wherein: said two thermowires of said second pair are electrically connected with one another at a junction and serve to provide a second thermovoltage.

31. The arrangement as claimed in claim 30, wherein: said first pair of thermowires is electrically insulated from said second pair of thermowires.

32. The arrangement as claimed in claim 27, further comprising: an evaluating circuit, which serves to register the first thermovoltage and the second thermovoltage.

33. The arrangement as claimed in claim 30, wherein: said second pair of thermowires extends along a path; and said first pair of thermowires extends along at least one part of such path, preferably to the junction of said second pair of thermowires.

34. The arrangement as claimed in claim 27, wherein: said first and/or said second pair of thermowires are embedded at least partially in an electrically insulating material.

35. The arrangement as claimed in claim 34, wherein: said insulating material is surrounded by a shell, preferably a metal shell.

36. The arrangement as claimed in claim 35, wherein: said shell is arranged in a protective tube.

37. The arrangement as claimed in claim 27, wherein: at least three thermowires are provided; and a first and a second of these three thermowires form the first pair; and said first and a third of these three thermowires form the second pair.

38. The arrangement as claimed in claim 27, wherein: four thermowires are provided, and a first and a second of these four thermowires form the first pair; and a third and a fourth of these four wires form the second pair.

39. The arrangement as claimed in claim 27, wherein: said first thermovoltage serves to produce a diagnostic report regarding the state of said second pair of thermowires or the state of a measurement signal.

40. The arrangement as claimed in claim 27, wherein: said first thermovoltage serves for correction of an error contained in said second thermovoltage, which error results especially from the presence of a virtual reference junction between said second pair of thermowires.

41. The arrangement as claimed in claim 27, wherein: said first pair of thermowires forms a first measurement channel; and said second pair of thermowires forms a second measurement channel; and said evaluating circuit is connectable with the first and/or second measurement channel.

42. The arrangement as claimed in claim 32, wherein: said evaluating circuit serves to register as measurement signal a voltage fraction of said second thermovoltage corresponding to a temperature.

43. The arrangement as claimed in claim 32, wherein: said evaluating circuit serves to ascertain an electrical resistance of said second pair of thermowires.

44. The arrangement as claimed in claim 32, wherein: said evaluating circuit serves to register an electrical resistance of said first pair of thermowires.

45. The arrangement as claimed in claim 32, wherein: said evaluating circuit serves, furthermore, to register, event referenced or corresponding to a time specification, for example, recurringly, said second thermovoltage and the electrical resistance of said second pair and the electrical resistance of said first pair of thermowires.

46. The arrangement as claimed in claim 32, wherein: said evaluating circuit serves, furthermore, preferably during operation of the measuring arrangement, to compare the ascertained values of said second thermovoltage, the electrical resistance of said second pair and the electrical resistance of said first pair of thermowires, with at least one furnished value.

47. A method for operating a measuring arrangement, comprising: a first pair of thermowires, wherein: the two thermowires of said first pair are insulated completely from one another by an insulation, such as, for example, a material, which serves for electrical insulation between said two thermowires of said first pair, the method comprising the steps of: registering a first thermovoltage, especially a parasitic, first thermovoltage by means of said first pair of thermowires, for example, as reference voltage for a second thermovoltage.

48. The method as claimed in claim 47, wherein: a second pair of thermowires is provided; and a second thermovoltage is registered by means of the second pair of thermowires, which are electrically connected with one another at a junction.

49. The method as claimed in claim 48, wherein: the first thermovoltage, especially the first parasitic thermovoltage, and/or the second thermovoltage are/is registered by means an evaluating circuit.

50. The method as claimed in claim 48, wherein: a diagnostic report is ascertained as regards the state of the second pair of thermowires based on the first thermovoltage, especially the first parasitic thermovoltage.

51. The method as claimed in claim 48, wherein: an error contained in the second thermovoltage is determined by means of the first thermovoltage, especially the first parasitic thermovoltage.

52. The method as claimed in claim 51, wherein: said error is used for correction of a measurement signal determined based on the second thermovoltage, which error results especially from the presence of a virtual reference junction between the thermowires of the second pair of thermowires.

Description

[0041] The invention will now be explained in greater detail based on the appended drawing, the figures of which show as follows:

[0042] FIG. 1 a schematic representation of a measuring arrangement, such as, for example, installed in a kiln, in the case of which, for example, heating chamber and combustion chamber are isolated from one another,

[0043] FIG. 2a a schematic representation of a thermocouple having a junction, via which the thermocouple forms a closed line,

[0044] FIG. 2b a schematic circuit diagram of the thermocouple of FIG. 2a,

[0045] FIG. 3a a schematic representation of a thermocouple having a junction and a virtual reference junction, by which a parasitic thermovoltage is introduced,

[0046] FIG. 3b an equivalent circuit diagram of the thermocouple of FIG. 3a,

[0047] FIG. 4 a schematic representation of the insulation resistance of a mineral insulated, sheathed cable as a function of temperature,

[0048] FIG. 5 a schematic representation of the measurement error in the thermovoltage as a function of the insulation resistance,

[0049] FIG. 6a a schematic representation of first and second pairs of thermowires in the case of which a parasitic thermovoltage is present due to a virtual reference junction,

[0050] FIG. 6b an equivalent circuit diagram of the second pair of thermowires of FIG. 6a, which have a junction,

[0051] FIG. 6c an equivalent circuit diagram of the first pair of thermowires of FIG. 6a, which has no junction.

[0052] FIG. 1 shows a thermocouple TC having first and second thermowires LT3, LT4. The two wires LT3, LT4 are galvanically connected with one another, for example soldered or brazed, at one of their ends to form the so-called junction (hot junction) HJ. The junction HJ does not, in such case, absolutely have to lie at one of the ends of the thermowires LT3, LT4. Junction HJ forms at the same time the measuring point, where the temperature T1 is to be registered. Additionally, the junction HJ does not have to lie at one end of a shell S serving as measuring insert, in which the thermowires LT3, LT4 are arranged. The junction HJ can be arranged at any location along the length of the shell S.

[0053] At the end of the shell S lying opposite the junction HJ, the thermowires LT3, LT4 are connected, for example, via connection terminals, with an evaluating circuit EC, which at the same time forms the reference junction (cold junction) CJ. If a temperature difference exists between the junction HJ and the reference junction CJ, then there results at the reference junction CJ a thermovoltage, which can be measured by means the evaluating circuit EC. The evaluating circuit EC is preferably a temperature transmitter.

[0054] Present, by way of example, is the surface of a container B, in which a measured material is located. Known, however, are also other applications, such as, for example, a protective tube, which protrudes inwardly into the container B containing the measured material. In such case, the measuring insert is inserted into the protective tube and comes in contact with its floor.

[0055] Thermowires LT3, LT4 are arranged in this case in the shell S, which is filled with an insulation F, such as, for example, a mineral insulation, such as, for example, MgO or Al.sub.2O.sub.3. For example, of concern can be a mineral insulated cable.

[0056] Especially in the case of heating chambers, it can happen that the thermowires LT3, LT4 in the distance between the evaluating circuit EC, i.e. the reference junction CJ, and the junction HJ, which serves as measuring point, are, in given cases, exposed to higher temperatures T2 than at the measuring point T1. This can be true, for example, in the case of a kiln having a heating chamber separated from a combustion chamber. The temperature T2 increased relative to the measuring point is shown symbolically by the flame pictured in FIG. 1. In such a situation, a higher thermovoltage and, thus, a higher temperature is measured than is actually present at the measuring point. A goal of the present invention is to avoid this error and provide a more exact measuring of the temperature T1 by means of a thermocouple TC. Additionally, the proposed invention can provide a diagnosis of a thermocouple TC for evaluating the reliability of the measuring and the probability of failure of the thermocouple TC, i.e. of the thermowires LT3, LT4 utilized therefor.

[0057] FIG. 2a shows a measuring insert, which is connected with an evaluating circuit EC and includes a thermocouple TC such as described in connection with FIG. 1. The measuring insert includes, in such case, the two thermowires LT3, LT4, a shell S and an insulation F, which, at least partially, fills the shell S. The thermowires LT3, LT4 are embedded in the insulation F. FIG. 2b shows an equivalent circuit diagram of the measuring arrangement of FIG. 2a. Due to the temperature difference between the junction HJ of the thermowires LT3, LT4 and the reference junction CJ, a thermovoltage U1 results. It is, in such case, assumed that the thermovoltage U1 arises due to the temperature difference between the temperature at the junction HJ, which is arranged at the tip of the shell S, and the temperature at the reference junction CJ.

[0058] Furthermore, the equivalent circuit diagram contains the line resistance R.sub.L of the thermowires LT3, LT4 and the internal- or measuring resistance R.sub.M of the evaluating circuit EC. Line resistance R.sub.INS e4and internal resistance R.sub.M are, in such case, arranged in series with the thermovoltage U1 illustrated as voltage source.

[0059] FIG. 3a shows a conducting connection between the thermowires LT3, LT4 brought about as a result of the temperature difference between the junction HJ and the reference junction CJ. This results, such as above explained, from the fact that the thermowires LT3, LT4, in given cases, including shell S, in which they are arranged, are led a certain distance from the reference junction CJ or evaluating circuit EC to the measuring point through an installation, such as, for example, an industrial plant. As a result of an increased temperature T2 present at a location of the installation, the insulation resistance R.sub.INS of the insulation F between the thermowires LT3, LT4 sinks, in given cases, so strongly that an additional conducting connection arises between the thermowires LT3, LT4 at the location of the increased temperature T2. This is also referred to as a virtual junction VJ. In such case, the resistance R.sub.INS of the material used as insulation F sinks, for example, from some giga-ohm by about one power of ten per 100° temperature difference, such as shown in FIG. 4. A corresponding behavior results in the case of other known materials used for the insulation of thermowires LT3, LT4, such as glass or Al.sub.2O.sub.3. The influence of the sinking insulation resistance R.sub.INS and therewith the meaning of a parasitic thermovoltage U2 resulting therefrom on the measurement signal registered by means of the evaluating circuit EC, such as, for example, a transmitter, can, however, be determined by means of the thermowires LT1, LT2.

[0060] The measurement error resulting from the parasitic thermovoltage U2 in % is shown in FIG. 5 as a function of the insulation resistance between the thermowires. In the case of small insulation resistance R.sub.INS of the insulation between the thermowires LT3, LT4, thus, a correspondingly high measurement error results. In the case of an insulation resistance R.sub.INS of 30 ohm, there results, accordingly, a measurement error of 20%. By means of a suitable insulation resistance R.sub.INS, this error can be minimized and when correspondingly higher insulation resistances are achieved by suitable materials. For such purpose, suitable materials, such as, for example, boron nitride or beryllium oxide are, however, either very expensive or toxic.

[0061] FIG. 3b shows an equivalent circuit diagram of the measuring arrangement of FIG. 3a. The virtual reference junction VJ is composed, in such case, of the insulation resistance R.sub.INS and the parasitic thermovoltage U2. The insulation resistance R.sub.INS is arranged in parallel with the measuring resistance R.sub.M, while the parasitic voltage U2 shown in the equivalent circuit diagram as a voltage source is arranged in parallel with the actual measurement voltage U1, likewise shown as a voltage source.

[0062] FIG. 6a shows an arrangement of thermowires LT1, LT2, LT3, LT4, by which a deterioration of the thermowires LT3, LT4 and/or a sinking insulation resistance R.sub.INS between the thermowires LT3, LT4 can be compensated.

[0063] In such case, a mineral insulated line is used, in which e.g. two type K thermocouples are embedded, wherein, however, a junction HJ is formed only between one pair of thermowires LT3, LT4. An arising measurement error can then be compensated by calculation by means of a temperature transmitter EC, with which the thermowires LT3, LT4 are connected.

[0064] A first pair of thermowires LT1, LT2, which are not connected with one another, forms, in such case, an open line. This first pair LT1, LT2 forms, in given cases, a parasitic thermocouple VJ.

[0065] A second pair of thermowires LT3, LT4, which are connected with one another at a junction HJ, forms, in such case, a closed line. In this closed line, there forms besides the measuring thermocouple, in given cases, also the parasitic thermocouple VJ. Instead of the here shown four thermowires LT1, LT2, LT3, LT4, an arrangement of the invention can also be composed of only a single pair of thermowires LT1, LT2, which are not connected with one another. Likewise, an embodiment with only three thermowires is possible wherein two of these thermowires form the first pair LT1, LT3 and two the thermowires LT3, LT4 of the second pair.

[0066] The thermowires LT1, LT2, which form the first pair, are, in the region in which they are arranged in the shell S, completely insulated from one another by an insulation F. In the illustrated case, the first and second pairs of thermowires LT1, LT2, LT3, LT4 extend along the total length of the shell S spaced from one another in the shell S.

[0067] The temperature transmitter EC can have different inputs K1, K2 and, for measurement signal processing, for example, switch between these inputs, also referred to as channels. In such case, a first input K1 can be connected with the first pair LT1, LT2 and a second input K2 with the second pair of thermowires LT3, LT4. By means of the transmitter EC, a resistance measurement and/or a voltage measurement can occur between the thermowires LT1, LT2, and LT3, LT4, connected to the respective inputs K1, K2. The inputs K1, K2 of the transmitter EC can especially be alternately sampled. This can occur corresponding to a predetermined schedule, which is furnished, for example, in software of the transmitter EC.

[0068] At the second input K2 of the transmitter, which forms a second measurement channel, the total voltage between the second pair of thermowires LT3, LT4 and/or the line resistance R.sub.L of the closed line can be measured. The resistance R.sub.L of the closed line can, in such case, be determined with a first polarity and then with reverse polarity and then averaged.

[0069] At the first input K1 of the transmitter, which forms a first measurement channel, the insulation resistance R.sub.INS between the first pair of thermowires LT1, LT2 can be measured. When this falls below a predetermined threshold value, for example, 1000 ohm, either a corresponding report can be output and/or a compensation of the measurement signal or of the measured value, as determined by means of the second pair of thermocouples LT3, LT4, performed.

[0070] If the predetermined threshold value is achieved or exceeded, the influence of the parasitic thermovoltage U2 can be compensated, for example, based on the measured insulation resistance R.sub.INS. For determining the insulation resistance R.sub.INS, for example, electrical current and voltage across the open line can be cyclically measured.

[0071] Since the input resistance R.sub.M of the temperature transmitter is known, the resistances R.sub.L, R.sub.INS and the voltage U.sub.2 can be determined. From these variables, then the voltage U1 caused by the junction between the second pair of thermowires LT3, LT4 and, thus, the temperature T1 at the measuring point can be determined.

[0072] Thus, in a first operating mode of the temperature transmitter EC, the influence of the parasitic thermovoltage U2 can be compensated.

[0073] Furthermore, a second operating mode of the transmitter EC can be provided, in which the parasitic thermovoltage U2 is monitored, without a compensation of the measuring occurring. In a third operating mode, a measuring of the thermovoltage of the second pair of thermowires LT3, LT4 occurs, without measuring resistance, electrical current and/or voltage on the open line LT1, LT2.

[0074] In the first operating mode, the transmitter EC can determine on the first channel K1 the insulation resistance R.sub.INS and the parasitic voltage U2. In such case, the first channel K1 can be used once as measurement channel for resistance measurement R.sub.INS and once for registering the parasitic thermovoltage U2.

[0075] Via the second channel K2, a measuring of the line resistance R.sub.L and the voltage U.sub.3 falling across the internal resistance R.sub.M of the transmitter EC can be registered. From the measured variables, the voltage U.sub.1 arising from the junction HJ between the second pair of thermowires LT3, LT4 can be calculated as follows:

U.sub.1=U.sub.3(1+R.sub.L/R.sub.M)−R.sub.L(U.sub.2−U.sub.3)/R.sub.INS

[0076] The sinking of the insulation resistance depends, however, not only on the instantaneous value of the temperature but on the length of time, over which a temperature acts on the thermowires LT1, LT2, LT3, LT4, and on the insulation F. The measuring and registering of the insulation resistance R.sub.INS of the line resistance enables, thus, a performing of a diagnosis of the thermowires, for example, of the second pair of thermowires LT3, LT4, and the outputting of a report characterizing the state of the thermowires LT3, LT4.

[0077] Experiments have additionally shown that the parasitic voltage U2 can even increase above the value of the thermovoltage actually expected at this temperature. This is an indication that chemical conversion of the mineral insulated line or generally a deterioration of the thermowires LT1, LT2, LT3, LT4 is happening. Such conversion likewise limits the durability of the used thermowires LT1, LT2, LT3, LT4. Thus, the insulation between the thermowires LT1, LT2, LT3, LT4 becomes increasingly conductive and begins to behave similarly to an electrolyte in a battery. This can then lead to dissolution of the material of the thermowires LT1, LT2, LT3, LT4. The evaluation of the parasitic voltage U2 measured by means of the first pair of thermowires LT1, LT2 can, thus, be utilized, in order to determine the state of the used thermowires, especially their (remaining) life.

[0078] The parasitic thermovoltage U2 is shown in FIG. 3a and FIG. 6a as an additional connection between the thermowires LT1, LT2, LT3, LT4, which connection lies between the junction HJ and the reference junction CJ—this is indicated by an oval, in which a connection between the first pair of thermowires LT1, LT2, respectively the second pair of thermowires LT3, LT4, is located.

[0079] The first and second pairs of thermowires LT1, LT2, LT3, LT4 can be embedded in a material, which serves as insulation F. As a result of this insulation F, both the first pair of thermowires LT1, LT2 as well as also the second pair of thermowires LT3, LT4 are completely insulated from one another (at least in the region, in which they are embedded in the insulation). Insulation F thus not only insulates the individual thermowires of a pair LT1, LT2 but also the pairs from one another. It is, however, also possible for practicing the invention to use only one pair of thermowires LT1, LT2, which are completely insulated from one another.

[0080] The first and second pairs LT1, LT2, LT3, LT4 are, in such case, preferably arranged, such as shown, for example, in FIG. 6a, in one and the same shall S, which is at least partially filled with the insulation F. The first and second pairs LT1, LT2, LT3, LT4 extend, in such case, preferably parallel to one another in the region, in which they are arranged in the shell. The shell can have one end closed, for example, by a floor especially in the form of a cap. Furthermore, the shell S can have an open end, through which the thermowires LT1, LT2, and LT3, LT4, preferably the free ends of the thermowires LT1, LT2, and LT3, LT4 or connection lines (not shown) connected to the thermowires are led.