JPS6334970B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a temperature measurement system that measures temperature using a thermocouple.

As is well known, when the temperature of a reference junction (cold junction) of a thermocouple changes, errors occur in the measurement results. Therefore, the following measures have been conventionally taken to compensate for this measurement error.

(1) A method of keeping the reference junction at a constant temperature. FIG. 1 shows a schematic configuration of a temperature measurement system using this method. In this figure, 1 and 2 are thermocouple wires, and A is their contact point, that is, a temperature measuring contact point. A contact point B between the thermocouple wire 1 and the copper conducting wire 4a and a contact point C between the thermocouple wire 2 and the copper conducting wire 4b are both reference junctions, and are inserted into the freezing point bath 3 and maintained at 0°C. According to such a configuration, a thermoelectromotive force determined only by the temperature of the temperature measuring junction A can be obtained at the open end of the copper conducting wire. This thermoelectromotive force is measured by the receiving instrument 5 to measure the temperature.

(2) A method in which temperature compensation of the reference junction is performed using the input circuit of the receiving instrument. As shown in Figure 2, thermocouple wires 1 and 2
is directly input to the receiving instrument 5, and in this case the temperature of the reference junctions B and C cannot be kept constant. To compensate for this, for example, the input circuit is a bridge circuit, and a reference junction temperature compensation resistor Rd having an appropriate temperature coefficient is provided near the reference junctions B and C as part of the bridge resistance.

Such a configuration compensates for measurement errors.
In the case of remote temperature measurement, this method is often used by connecting compensating lead wires 7 and 8 in series with thermocouple wires 1 and 2, respectively, as shown in FIG.

By the way, the conventional temperature measurement system described above has the following drawbacks. In the method (1), the freezing point tank 3 for keeping the temperature of the reference junctions B and C at the freezing point is expensive and troublesome to maintain.Especially in the case of temperature measurement at multiple points, the freezing point tank 3 for keeping the temperature of the reference junctions B and C at the freezing point is required. 3 is often required, which has the disadvantage of increasing costs and maintenance work. The method (2) has the disadvantage that the compensation error caused by the compensation conductors 7 and 8 and the error of the reference junction compensation circuit in the input circuit section of the receiving instrument 5 are added to the inherent error of the thermocouple, reducing measurement accuracy. Furthermore, when the distance between the measuring point and the receiving instrument 5 is long, a large number of expensive compensating conductors are required, resulting in increased costs.

In view of the above-mentioned circumstances, the present invention provides a temperature measurement system that can accurately measure the temperature of a point to be measured without using a freezing point tank, compensation conductor, compensation circuit, etc. a second thermocouple whose temperature measuring junction and reference junction are respectively arranged at the same location as the temperature measuring junction and the reference junction of the first thermocouple, and whose constituent material is different from that of the first thermocouple;
and a first thermocouple corresponding to a temperature difference between the temperature measuring junction and the reference junction obtained from the first thermocouple.
and a second thermoelectromotive force according to the temperature difference between the temperature measurement junction and the reference junction obtained from the second thermocouple, calculate the temperature of the temperature measurement junction. The present invention is characterized in that it is equipped with a calculation means that performs the following steps.

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 4 is a schematic configuration diagram showing the configuration of an embodiment of the present invention. In this figure, α is a thermocouple that has thermocouple wires 1 and 2 and a temperature measuring junction A that is their connection point, and reference junctions B and C of thermocouple wires 1 and 2
are each connected to an input terminal of the receiving instrument 5 via a copper conducting wire 10a. Also, β is thermocouple wire 1, 2
Thermocouple wires 11, 12 made of a material different from
and a thermocouple having a temperature measuring junction D which is a connection point between them, and reference junctions E and F of each of the thermocouple wires 11 and 12 are connected to the input terminal of the receiving instrument 5 via a copper conductor wire 10b, respectively. ing. The receiving instrument 5 is configured using, for example, a microprocessor, and has a thermocouple α.
The thermoelectromotive force E〓 obtained from the thermocouple β and the thermoelectromotive force E〓 obtained from the thermocouple β are measured, and calculations are performed based on the measurement results to obtain the measured temperature t _n .

Next, the calculation operation in the receiving instrument 5 will be explained.
Generally, the thermoelectromotive force E _n V of a thermocouple is a function of the temperature measuring junction temperature t° C. when the reference junction temperature is 0° C., and is expressed by the following equation.

E=f(t) ......(1) The form of the function f(t) is "JIS C1602 thermocouple"
are shown in the form of numerical tables and formulas for each type of thermocouple.

For two different types of thermocouples α and β, the respective thermoelectromotive forces E〓 and E〓 are E〓=f〓(t) ………(2) E〓=f〓(t) ……… (3) becomes. Now, the temperature measurement junction temperatures of thermocouples α and β are both
Assuming that both t _n ℃ and the reference junction temperature are t _r ℃,
The thermoelectromotive force E〓, E〓 of each thermocouple is E〓=f〓(t _n )−f〓(tr) ………(4) E〓=f〓(t _n )−f〓(tr) … ...(5) It is expressed by the formula. Since thermoelectromotive forces Eα and Eβ can be obtained by measurement, equations (4) and (5) are expressed as unknowns t _n and t _r
It becomes a simultaneous equation with , and there is generally a solution.
Eliminating _tr from equations (4) and (5), f ^-1 〓(f〓(t _n ) −E〓)−f ^-1 〓(f〓(t _n )−E〓)=0... ...(6) Therefore, the measured temperature t _n can be obtained by solving equation (6) by the arithmetic unit consisting of a microprocessor or the like in the receiving instrument 5. In equation (6), the unknown t _r
Since (reference junction temperature) is erased, it can be seen that the calculation result t _n is determined by the thermoelectromotive force E〓, E〓 and is not influenced at all by the reference junction temperature _tr .

Note that if the receiving instrument already has a calculation function, it can be used as is, and there is no need to take the trouble to provide a new receiving instrument for this embodiment.

In addition, in the embodiment described above, thermocouples α and β
If one of the thermocouple wires 2, 11 is made of the same material, the thermocouple wires 2, 11 made of the same material may be used in common, as shown in FIG.
In this three-wire configuration, the commonly used thermocouple wire 2,
The thermoelectromotive force error of both thermocouples α and β based on the uniformity, heterogeneity, stability of thermoelectromotive force characteristics, etc. in 11 affects both thermocouples α and β equally, so the thermoelectromotive force error is the final It has the advantage of having almost no effect on the results. Further, since it has a three-wire configuration, there is an advantage that the number of extension wires (copper conductive wires) and input terminals of the receiving instrument 5 can be reduced. In addition, specific combinations of thermocouples that can be configured like this are:
An example is a combination of E thermocouple and J thermocouple according to JIS standards. In addition, in the case of such a three-wire structure, measure the voltage between two wires that are not common, and from this result,
It is also possible to know the measured temperature t _n .

In addition, in this embodiment, the measured temperature t _n was obtained by performing various calculations based on the thermoelectromotive force E〓, E〓 measured _by the receiving instrument 5. Create a table or formula in advance,
It is also possible to determine the measured temperature t _n from this.
Figure 6 shows an example of such a diagram. This chart uses the E thermocouple and J thermocouple specified in "JIS C 1602 Thermocouple" and calculates the measured temperature t _n from the thermoelectromotive force of both. For example, the thermoelectromotive force of thermocouple E is 32.06mV, and the thermoelectromotive force of thermocouple J is 23.52mV.
mV, the measured temperature is determined from the coordinates of the intersection.
It can be seen that t _n is 475℃. Also, if you enlarge this chart and create it, you can increase the measured temperature t _n by 1 to 2 degrees Celsius.
can be measured with an accuracy of

In addition, in this embodiment, when there are many measurement points, if the output of the thermocouple installed at each measurement point is switched immediately after each thermocouple or near the input terminal of the receiving instrument 5, the receiving instrument 5 can be Since only one stand is required, the cost of the system per measurement point can be further reduced.

By the way, if both f〓 and f〓 are functions very close to straight lines, the values of the measured temperature t _n and the reference junction temperature t _r cannot be uniquely determined from the thermoelectromotive force E〓, E〓. Needless to say. In addition, the above-mentioned temperature measurement system has the disadvantage that the measurement error becomes large when the measured temperature t _n and the reference junction temperature t _r are extremely close (for example, immediately after the temperature of the measurement target starts to rise). There is.

In other words, when t _n ≒ t _r , E〓≒0 and E〓≒0 in equations (4) and (5) above, and while maintaining the condition that t _n and t _r are almost equal, the value itself is indefinite. .
Therefore, mathematically, even if a solution to equations (4) and (5) exists, the obtained value will have a large error. This is also clear from the fact that the smaller the thermoelectromotive force is in FIG. 6, the denser the isothermal lines are.

On the other hand, when t _n ≒ t _r , an accurate value of t _n is usually not required, and it is often sufficient as long as it does not deviate extremely from the true value. Therefore, the following method can be considered as a temperature measurement method when t _n ≒ t _r .

Set the thermoelectromotive force of either thermocouple corresponding to an appropriate t _n and t _r between t _n −t _r ≫ 0 and t _n − t _r = 0,
If this value is Eo, E〓≦Eo (or E〓≦Eo)
The reference junction temperature t _r is set in advance to _a certain constant value t _rp within the range of
Substitute t _r in the formula to calculate the measured temperature t _n . (In addition,
The measured temperature t _n may be determined from both equations (4) and (5), and these values may be averaged. ) That is, when the calculation result t _n is set to t _n 1, equation (7) is obtained.

t _n = f ^-1 〓 (f 〓 (t _rp ) + E〓) ………(7) When E〓>E _p , t _np is obtained from equation (6), and when E〓≦E _p , equation (7) is obtained. Let t _n be t _n 1 obtained from .

When t _n obtained in this way is plotted as a graph with E〓 as an independent variable, it becomes as shown in Fig. 7. In addition,
In this figure, the curve L ₁ represents equation (6), and the curve
L ₂ represents equation (7). Also, in this figure, the value of E〓 in equation (6) is the value when t _r is fixed at a certain constant value, but when t _r changes, the curve L ₁ changes as shown by the broken line in the figure. Change.

As can be seen from the figure, when E = E _p , t _np and t _n1 obtained from equations (6) and (7), respectively, are generally discontinuous, and this discontinuity can be used, for example, to control the temperature of the measurement target. This is inconvenient in some cases. For this reason, a function is set that gives t _n2 that is continuous with t _n0 at E=E _p . For example, the following formula satisfies this.

t _n2 = E〓t _n0 + (E _p −E〓) t _n1 /E _p ………(8) When this equation (8) and the above-mentioned equation (6) are graphed, it is as shown in Figure 8. It is continuous at the point E = E _p , but (8)
As can be seen from the formula, t _n2 is near E = 0,
In this vicinity, the influence of t _n0 , which has a large error, does not become zero at all, which causes an error in the measurement results. Therefore, in order to avoid this, 0<E _D <E ₀
Set E _D that satisfies E D and use equation (7) where there is no influence of t _n0 in the range E〓≦E _D , and use equation (7) where E _D <E〓≦E _p , E〓=E _p and t _n0 Introduce a function t _n3 that is continuous and continuous with t _n1 with E = E _D.

For example, the following formula satisfies this condition.

t _n3 = (E〓−E _D )t _n0 + (E _p −E〓)t _n1 /E _p −E _D ………(
9) In this way, the range is divided into three depending on the value of E〓, and the measured temperature t _n is calculated using a different formula in each range. Expression (6) in the range E〓＞E _p Expression (9) in the range E _D <E〓＜E _p , Equation (9) in the range E〓＜E _D
FIG. 9 shows a graph when the measured temperature t _n is calculated using equation (7). As can be seen from this figure, the measured temperature t _n is continuous over the entire range of the thermoelectromotive force E〓 (or E〓) to be measured. In the above explanation, the measured temperature t _n was treated as a function of the thermoelectromotive force E〓, but of course it may be treated as a function of the thermoelectromotive force E〓.

In this way, the above method uses thermoelectromotive force Eα (or
Since the formula for calculating the measured temperature t _n is changed depending on the value of E〓), the measured temperature t _n can be calculated extremely accurately regardless of the value of the thermoelectromotive force E〓 (or Eβ). Can be done.

As explained above, according to the present invention, the first thermocouple, the temperature measuring junction and the reference junction are arranged at the same location as the temperature measuring junction and the reference junction of the first thermocouple, and a second thermocouple whose constituent material is different from that of the thermocouple; a first thermoelectromotive force according to a temperature difference between the temperature measuring junction and the reference junction obtained from the first thermocouple; calculation means for calculating the temperature of the temperature measurement junction based on a second thermoelectromotive force corresponding to the temperature difference between the temperature measurement junction and the reference junction obtained from the thermocouple; The following effects can be obtained.

A reference junction device such as a freezing point tank is not required,
The temperature measuring system is therefore easy to maintain and its construction is simple. Compensating conductors are not required; instead they can be replaced by ordinary copper conductors, which reduces the cost of constructing the system, especially in the case of telemetry systems. Since there is no need for a compensation conductor and no need for a reference junction compensation circuit in the input circuit of the receiving instrument, there is no compensation error caused by these, and therefore temperature measurement can be performed with high precision.

[Brief explanation of the drawing]

1 to 3 are schematic configuration diagrams showing the configuration of a conventional temperature measurement system, FIG. 4 is a schematic configuration diagram showing the configuration of an embodiment of the present invention, and FIG. 5 is a thermocouple α,
A diagram showing the configuration when β has a three-wire configuration, Figure 6 is a diagram for determining the measured temperature t _n from each thermoelectromotive force of the E thermocouple and J thermocouple, and Figures 7 to 9 are each FIG. 3 is a diagram showing changes in measured temperature t _n with respect to thermoelectromotive force E〓. α... Thermocouple (first thermocouple), β... Thermocouple (second thermocouple), A, D... Temperature measuring junction, B, C,
E, F... Reference junction, 5... Receiving instrument (calculating means).

Claims (1)

[Scope of Claims] 1. A first thermocouple, a temperature measuring junction and a reference junction are arranged at the same locations as the temperature measuring junction and the reference junction of the first thermocouple, respectively, and have a configuration with the first thermocouple. a second thermocouple made of a different material; a first thermoelectromotive force according to the temperature difference between the temperature measuring junction and the reference junction obtained from the first thermocouple; and a first thermoelectromotive force from the second thermocouple. Calculating means for calculating the temperature of the temperature-measuring junction based on the obtained second thermoelectromotive force corresponding to the temperature difference between the temperature-measuring junction and the reference junction; Temperature measurement system.