SU1688134A1 - Method of graduation of thermal elements - Google Patents

Abstract

The invention relates to thermometry and can be used to calibrate thermocouples in the operating temperature range. In order to improve the calibration accuracy of the thermocouple in dynamics, shorten the calibration time, the calibration medium and the recorded thermocouple are heated to a temperature corresponding to the maximum temperature that the calibrated thermocouple is measured, keep it constant, and the calibrated thermocouple is cooled to a temperature corresponding to the minimum temperature it measures, puts it in the calibration environment, records the change in the output signal over time and determines make its dynamic response. 4 il. (Ls

Description

The invention relates to thermometry and can be used to calibrate thermocouples in the operating temperature range.

The goal is to improve the accuracy of graduation while reducing time

FIG. 1 shows a device that implements the method of dynamic calibration of thermal converters; Fig, 2 shows the dependence of the EMF on time; in fig. 3 shows the dependence of the scale factor on time; in FIG. 4 - graduation curve.

The device contains; calibration medium 1, thermocouple 2 calibration medium, graduated thermocouple 3, cooler 4, cooler thermocouple 5.

Calibration medium 1 is a heated gas or liquid-metallic medium into which thermoconverter 2 is introduced into the calibration medium. The temperature of the calibration medium 1 is set equal to the maximum temperature that the graduated thermocouple 3 measures. Refrigerator 4 is designed to cool the calibrated thermocouple 3 to a temperature that corresponds to the minimum temperature of the calibrated thermocouple 3. The thermocouple 5 of the refrigerator is introduced into it.

The graduation device works as follows.

A graduated thermocouple 3 is introduced into the refrigerator 4, warming up to the temperature corresponding to the minimum 00 00

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temperature, which is measured by the calibrated thermocouple 3. After that, the calibrated thermocouple 3 is removed from the cooler 4 and introduced into the charging medium 1 and warmed to the temperature of this medium,

From the equation of the dynamics of heating of any thermal converter (TP) for any point in time

TiU (ti) + U (ti) Ki0§

where Ti is the inertia parameter;

KI - scale factor.

The equation holds in any time interval.

At (ti); (tin)

where I 1,2, ..., p.

The outcome of the mathematical approximation of any functions in a small time interval, the parameters P and KI are constant, i.e. TI const; KI const, therefore, the system

TiU (ti) + U (ti) - KI 08

TiU (ti + i) + U (ti + i) KI 0 ° 0 (1)

valid in a small time interval, At ,.

where is 1.2 p.

From system (1), determining Ati is not difficult;

TiU (ti) -K (0g-Ufo): Ti J (tH-i) -Ki 0§ - u (ti + i);

U (ti) Ki0o-U (ti) U (ti + i Kl U (ti + 1)

from where

U (ti) K | 00-U (i) U (ti + i) - U (ti + i) Ki 00-U (ti-H) U (ti);

e 8 KI and (ti) - and (ti, o - and (ti) and (U (ti + i) U {ti);

and (h) and (h +1) - and (ti +1) u (ti)

,,

K, (n) -u (ti + i)

15

from intervals

Ati f (ti; ti-n) 5 or temperature measurement interval

AbR (; 6 + i),

10 where 6P is the temperature of heating of the TP at the moment

where is 1.2 p.

Consequently, there is a dependence of KI in different intervals D0P i. Co - f (03) in the interval of heating of the calibrated TP from its minimum 6ft to the maximum heated temperature of 6 ° corresponding to the temperature of the calibration medium,

With a deliberate increase in the values of U (ti), the ero derivative may decrease. In particular, for the processes described by equation (1), for which the change in U (t)

25 asymptotically grows, and its derivative decreases, tending to zero, on the whole the sum of the increasing and decreasing functions remains constant, i.e. T U (t) HJ (t) Ki (6}

As for the moments of time ti from the time interval

At 4T-5T,

where T is the inertia parameter, the time constant of the thermocouple, then taking into account one of equations (1), at TI T const, Ki K const, the solution gives

thirty

40

U (t) Ki 0g (1-rt / T).

At the end of the transition process

U (t) - KI in ° when G1 / t O, and this occurs when t T-BT

t "4T; r4t / T 0.00012-0.

Record the data of the temperature change of the calibrated thermoconverter 3 from the time -U (t), as well as the thermoconverter of the calibration medium 2, according to which

the temperature of the calibration is determined; the values of K (m) are determined; based on

the dependences of Ui KI to f in each time

Ati interval,

where i 1,2, ..., 8, determine

No ul

-Rt

whereupon the calibration curve Urf (ft) is made (Fig. 4),

Example. Thermocouple - THA (chromel-alumel) was introduced into the terri

medium whose temperature was 0f 1000 ° C. A change in the emf of the thermocouple over time was recorded using a recording device, the heating of which continued for 120 s. The dependence of the EMF on time - U (t) is represented by the graph in FIG. 2. U (ti) data for different ti moments

at regular intervals At, where 1 1.2

At 10 s, are:

U (ts) 0.34 mU |; U (t6) 0.36MB; and (t7) 0.37 mV; U (ts) 0.375 mV.

Taking into account the values of U (ti),

where i is 1.28, and solving for each I system

of equations

TU (ti) + U (ti) K0g TU (tH-i) + U (tl + i) K 9g

where b ° 1000 ° C, we get

Ki 0.401 µV / s; Ks 0.409 µV / s;

K2 0.406 µV / s; K6 0.404 µV / s;

Cs 0.403 µV / s; K 0.408 µV / s;

“4 0.408 µV / s; KV 0.410 µV / s.

According to the data of Ki, Kakv, we build the dependence K f (t) shown in FIG. 3. The output from the graph in FIG. 2 and data (t)

considering that the values of f

Ki

where i 1.26, we determine the temperature 0Р

corresponding to each Ui value:

U (ti) 0.12 mV corresponds to 0i ° 299.3 °;

U (t2) 0.210 mV corresponds to 02 ° 517.3 °;

U (ts) 0.284 mV corresponds to 0 ° ° 704.7 °;

U (t4) 0.323 mV corresponds to 04 ° 791.7 °;

U (ts) 0.341 mV corresponds to 0s ° 833.7 °;

U (te) 0.360 mV corresponds to & ° 891.0 °;

U (ty) 0.370 mV corresponds to 927.0 °;

U (ts) 0.380 mV corresponds to 0 ° 950.0 °. These data allow the calibration of TXA from 0 ° C to 950 ° C. Graph of EMF THA from

temperature 6P is shown in FIG. 2, and the dependence K f (t) in FIG. 3 in the range from O to 950 ° C. The last circumstance is explained by the fact that in the range of 950-1000 ° C, the determination of the value of K is difficult, since reaching a temperature of 1000 ° C thermocouple

TXA is too slow. For calibration of TXA in the range of 0 to 1000 ° C, the temperature of the calibration medium can be selected on the order of 1100 ° C, and warming up should be completed when reaching 1000 ° C. After analyzing the obtained results, I compared the K data with the average K values equal to 0.4 µV / s, it will be possible to determine the error K equal to AKi 0.001; DK5 0.009;

DK2 0.006; AKe 0.004;

DKZ 0.003; DK7 0.008;

DK4 0.008; DK 0.010;

where the relative error is 0.25-2.50%.

Claims (1)

Invention Formula The method of dynamic calibration of thermocouples, including heating the calibration medium, measuring the temperature of the medium, placing the calibrated thermoconverter into the medium, recording its output signals and determining the true temperature of the calibrated thermocouple, in order to improve the accuracy while reducing the calibration time, heating the calibration the medium is carried out up to the temperature maximum for the calibrated thermocouple and is kept constant, while the gradient is emy thermal converter before being placed in the calibration medium is cooled to the minimum temperature measured by them, recording its output signals exhibit osuschest- at fixed intervals, and the actual temperature of the calibrated Thermocouples (i) is determined by the formula 40 "-Ј where (ti + i) -U (ti) -U (ti) -U (ti + l) (ti) -U (ti + i) where U (i) is the recorded output signal for the 1st time interval; U (ti + i) is the recorded output signal for the (1 + 1) -th time interval; U (ti) U (tn-i) are derivatives of the recorded output signal, respectively, for the 1st and for (i + 1} -ro time intervals; b§ is the temperature of the calibration medium. And 01   MS "18891 K (j (m. A3 111111111IG 23156789 W " futJ