SU1747945A1 - Method of determining temperature - Google Patents

Abstract

The invention relates to temperature measurement methods and can be used to improve the accuracy of thermoelectric converters under long-term conditions of operation under the influence of factors destabilizing their sensitivity. The essence of the invention is that three thermopower values corresponding to the initial state of the thermocouple are measured. , by heating it with a constant current and cooling it by changing the direction of this current, after which the desired temperature is determined according to the proposed oh Formula determination result does not depend on the temperature coefficient of the thermo-sensitive yl pairs 1

Description

The invention relates to the field of temperature measurement using electrical methods and can be used to improve the accuracy of thermoelectric converters that are in continuous operation at a thermal object.

A known method for measuring the temperature by a thermoelectric thermometer, comprising inserting the thermocouple spa into the medium whose temperature is measured, and recording the thermoelectromotive force (thermopower) arising at the free ends of the thermocouple

The disadvantage of this method is low measurement accuracy due to the instability of the parameters of the thermocouple and the mode of its operation. Thus, as a result of thermoelectrode oxidation, metal evaporation from electrodes, diffusion of alloy components through junctions, thermoelectric deformations, etc., the slope of the calibration characteristic changes and, accordingly, the sensitivity error of the thermometer changes. The temperature of the free ends of the thermocouple causes a parallel shift of the calibration characteristic, which causes a thermometer zero error. These error components are random and difficult to compensate for or take into account. during operation at the facility Significant error is introduced by instability of the parameters of the communication line and the secondary device.

The known method of measuring the temperature by a thermoelectric thermometer, which consists in inserting the junction of the thermocouple into the medium, the temperature of which is measured, and compensating for the thermoelectromotive force arising at the free ends of the thermocouple

v | YU

Ј

ate

the pair, with the help of an auxiliary current source, creating a voltage drop across the compensating resistor, equal to the measured thermoelectric power.

With the compensation method of measurement, the error of the change in the resistance of the thermocouple circuit and the inconsistency of sensitivity of the secondary device are eliminated. However, the error from the instability of the thermocouple itself and the inconsistency of the temperature of its free ends is not excluded, which does not allow for a high accuracy of temperature measurement,

The closest to the invention according to the technical essence is the method of measuring the temperature, which consists in introducing the thermocouple spa into the medium whose temperature is measured, recording the measured thermoelectromotive force, heating the thermocouple spa by passing a current through the thermocouple, and determining the ratio of the registered values thermoelectromotive forces, the temperature in which is determined by the formula

Ratura method of the prototype, for example, in the range from 150-400 ° C there is a significant methodological error (up to 2-5 ° C)

The aim of the invention is to improve the accuracy of determining the temperature by the calculation formula by eliminating the influence of Joule heat entering the junction of the thermocouple due to the thermal conductivity of the thermoelectrodes.

The goal is achieved by the fact that in a method involving heating a thermocouple spa by passing a current through it, measuring the values of thermoelectromotive

force and the calculation of the ambient temperature, additional operations are introduced:

changing the direction of the current flowing through the thermocouple junction;

the thermocouple junction is cooled to a temperature at which the corresponding decrease in the thermoelectromotive force exceeds the rms value of its random deviations by 10–20 times;

the temperature is calculated using a mathematical expression

VP -J

FflT-1)

where P is the Peltier coefficient;

I is the current passed through the junction of the thermocouple,

E

K - is the ratio of the values of the registered thermoelectromotive forces EI and Ea before and after heating;

A- thermal conductivity spa.

As follows from the above formula, the accuracy of temperature measurement in this way is not affected by the sensitivity of the thermocouple, and hence its instability. However, studies of the method have shown that the linear relationship between the temperature measured by the current through junction I is broken. This is due to the fact that in addition to the heat released at the junction due to the Peltier effect, when the current flows through the thermoelectrodes, the heat of Joule, part of which goes to the working junction of the thermocouple, is released. And while the Peltier effect for a thermocouple spa is decisive, the resulting overheating of the spa also depends on the heat of Joule, which is proportional to the square of the current flowing. As a result, the real dependence of the measured temperature $ on the current I passed through the junction of the thermocouple is nonlinear. Therefore, when determining the pace

be2-ez

2P

+ v0.

five

0

five

0

five

where in - temperature of the free ends of the thermocouple;

EI, Еа, Ез - values of the thermoelectromotive force, respectively, before heating, after heating and after cooling the thermocouple spa;

P - Peltier coefficient,

I - current through thermocouple junction;

I - is equivalent to the thermal conductivity of the thermocouple spa, taking into account heat exchange with the environment.

The drawing shows a variant of the thermoelectric device circuit for implementing the proposed method.

The device contains a four-electrode thermoelectric converter 1, consisting of two-electrode thermoelectric converters with a common working junction, the first 2 and second 3 pairs of extension thermoelectrodes, the first 4 and second 5 thermostats with the free ends of the first 2 and second 3 pairs of extension thermoelectrodes placed in them , a two-channel switch b, a two-pole switch 7, an adjustable source of a constant voltage 8 and a digital millivoltmeter 9.

The proposed method for determining the temperature is as follows.

Measured temperature and temperature; Spa overheating temperature is determined by

total dissipated electrical power and thermal conductivity spa

The emf arising at the free ends of a thermocouple is related by a non-linear relationship5

Е А (01 - 00) + В (01 - в0} 2 + С (01 - 00) 3, (1)

where A, B and C are the coefficients of the equation depending on the materials of the thermoelectrodes 10 and the technology of making the thermocouple;

в0 is the temperature of the free ends of the thermocouple.

In the vicinity of the working point of the thermocouple, set by the temperature of 15 01, the adjacent section of the calibration characteristic (Л0 «0i) can be replaced with a straight line (chord) passing through the beginnings of the calibration characteristic (zero) and the working point B1 ,. The dependence of the thermopower on the temperature in the vicinity of the operating point can be represented by a linear dependence

Av

K-FH2 + n-l

-g

(7)

where R is the ohmic resistance of the thermal electrodes;

K - coefficient taking into account part of the heat Joule, heating junction;

P is the Peltier coefficient, depending on the spa materials;

I is equivalent to the thermal conductivity of the spa, taking into account the heat exchange with the medium;

I - direct current power

Measured steady-state value of thermopower E2

E2 - S2 & +

K-r-l2-f GN

T

--- - in 1

VOJ,

(eight)

E S (01 - in),

where s

LU

-7c-n is the average sensitivity of the thermocouple, depending on the measured temperature.

The average sensitivity of the thermocouple can be determined from the ratio

S & - B0) - A & - B0) + B & - B0} 2 + + C (01 - B0) 3,

from where

S А + В (0i - в) + С & - в0) 2.

First, the thermopower value is measured, corresponding to the measured temperature 01 and the temperature of the free ends of the thermocouple during

Ei Si (0i-0o),

(five)

where Si is the average sensitivity of the thermocouple at the temperature difference d in 01 - 0. Then, a constant current I from the source of electrical voltage is passed through the junction of the thermocouple. Due to the Peltier effect in the junctions and the release of heat, the Joule in the electrodes of the thermocouple heats the thermocouple spa to temperature.

02 01-D01,

(6)

where A01 is the temperature of the overheating of the spa relative to the controlled environment.

Av

K-FH2 + n-l

-g

(7)

where R is the ohmic resistance of the thermal electrodes;

K - coefficient taking into account part of the heat Joule, heating junction;

P is the Peltier coefficient, depending on the spa materials;

I is equivalent to the thermal conductivity of the spa, taking into account the heat exchange with the medium;

I - direct current power

Measured steady-state value of thermopower E2

K-r-l2-f GN

T

--- - in 1

VOJ,

(eight)

25

thirty

where $ 2 is the average sensitivity of the thermopair at a temperature difference of d at 02 - v0 The strength of current I is chosen such that the increased thermopower value exceeds the previously measured EI value by an amount exceeding the maximum random changes in thermoEMF

E2- Ei a

AE2

(9)

35

40

45

where UDE2 is the standard deviation (C KO) of thermoEMF;

a is a coefficient depending on the confidence probability.

With a normal distribution of random deviations of thermoEMF from the average value with probability, 997 the maximum random deviation cannot exceed the standard deviation more than 3 times. Therefore, the coefficient a is chosen in the range of 10-20. When choosing а and 10-20, thermopower E2 is measured with almost the same systematic error as EI

50

E2 Ei CHY ... 20)

(ten)

Next, the direction of the current flowing through the junction is changed. Since the Peltier effect is recursive, when the direction of the current through the junction is reversed, its cooling occurs, and the thermo-emf decreases to

Ez Зз (+ - 00), d 1)

where Z3 is the average sensitivity of the thermocouple at the temperature difference d & -

take

Measured steady-state value of thermopower Ez.

Since the two additionally measured thermopower values (Ea and Ez) lie in the vicinity of the working point of the thermocouple given by the initial value of thermoEMF Ei, it is reasonable to assume that S1 $ 2 5z Taking into account the latter, the relative difference in thermoEMF is determined

E2-E3 2GN Ei A (ft-ft,)

(12)

Since the temperature is determined by the formula derived from the expression (12),

a l

f 1 - C / o

(13)

From the expression obtained, it can be seen that the measurement result does not depend on the inadequacy of the sensitivity S of the thermocouple to temperature and its instability during operation. In this case, the dependence of the measured temperature on the current passed through the junction is strictly linear. The measurement result does not depend on the systematic measurement error of thermoEMF either, which can be seen by substituting in (13) instead of Ei, Ј2 and Ez of the values Ui (1+ + X) Ei , U2 (1+ y) Ј2 and Us (1+ Ez, where 1H, U2, 1) s are the measured values of the thermo-EMF, and y is the relative measurement error. The required current strength I entering into the expression (13) can be determine from the difference of thermoEMF E2 and Ез and the recommended ratio (10), which reduces the random component of the measurement error or thermoelectric power to an acceptable value,

Ez-E2

25GI

(ten

(14)

From the relation (14) the minimum current value

m

IN

 2TU5 {10..20) VAf

(15)

where S is the average sensitivity of the thermocouple at the measured temperature ft.

The proposed method for determining the temperature can be implemented with thermocouples of various calibrations which during long-term operation

0

five

0

under the influence of a controlled environment change their sensitivity

The method is implemented by the device shown in the drawing in the following sequence.

The working junction of the thermocoupler 1 is placed in the zone of the measured temperature ft, and two pairs of extension thermoelectrodes 2 and 3 are attached to the free ends of the thermoconverter, respectively, the thermostat 4 maintains a constant temperature 00 and the thermostat 5 has a temperature close to .

First, the key 6 is opened and a millivoltmeter 9 makes a series of measurements of the thermoEMF generated by the temperature difference —sp ft and the free ends of the ends, determine its rms distance.

five

0

five

, as well as the arithmetic mean- Ej / n Then, by the gradation of

The thermocouple characteristic and the arithmetic mean value of its thermopower determine the average sensitivity S of the thermocouple for the approximate value of the detected temperature ft. Then the required value of the current passed through the slurry is used for all thermocouples of this graduation that work under similar conditions The thermopower (5) takes the arithmetic average value of the measurement series taken as the Ei value

Ei

Ei / n

five

five

0

Then the key 6 is closed and by adjusting the voltage of the source 8 through the junction of the thermoconverter 1, a current is passed, the value of which corresponds to the expression (15). The established thermoelectric value corresponds to the measured value E2 (8) and condition (10).

Then switch 7 changes the direction of the current through the junction of the thermocouple 1 and uses the millivoltmeter 9 to measure the steady-state value of thermopower Ez.

Further, using the obtained values of Ei, Ea, 3, the unknown temperature is determined by formula (13).

For example, when measuring the temperature of the working zone of a hydrogen furnace of recrystallization annealing of parts under the influence of hydrogen as a reducing medium, the sensitivity of the thermocouple XA calibration

changes on average by 10-15 per 100 working hours, which leads to a temperature deviation of 300 ° C in the working zone of the hydrogen furnace, controlled by the thermo-emf value, by 30 ... 45 ° C. According to the proposed method, the determination of temperature using such a thermocouple gave the following results.

The temperature of the working zone of the hydrogen furnace was set to 300 ° C. The thermal emf of a sample thermocouple calibration PP-1. The standard deviation of thermocouple thermocouple of the XA calibration at the temperature of the free ends of the thermocouple B0 60 ° C, placed in the thermostat, was 154 times the measurement of 0.004 mV, the average sensitivity determined by the calibration characteristic of the thermocouple in the temperature range 200 .... 400 ° C 0.004 (mV / ° C).

Thus, the difference in the temperature determined by the inventive method using an XA thermocouple calibration, whose sensitivity during operation has changed uncontrollably, as compared with the reference thermocouple PP-1 calibration was only 0.5 ° C,

The economic effect of introducing the proposed method is to improve the quality level of the products, in the manufacturing process of which there are heat treatment operations or specified thermal conditions.

0

Claims (1)

Claim method for determining temperature, including heating a thermocouple thermocouple transducer by passing a current through it, measuring thermoelectromotive force values before and after heating, measuring the temperature of free thermocouple ends and calculating the ambient temperature, characterized in that the junction of the thermocouple is cooled to improve accuracy changing the direction of the current through it and to a temperature at which a change in the measured value of the thermoelectromotive force is 10–20 times its value ednekvadratichnogo deviation, and the ambient temperature is calculated by the mathematical expression 20 0 EI E2 - Ez where in - temperature of the free ends of the thermocouple; EI, Ez, Ez are the values of the thermoelectromotive force, respectively, before heating, after heating, and after cooling the thermocouple spa; P - Peltier coefficient; I - current through thermocouple junction; I - is equivalent to the thermal conductivity of the thermocouple spa, taking into account heat exchange with the environment