RU1789914C - Method of graduation of heat conduction meter - Google Patents

Abstract

The invention relates to thermal methods for investigating a substance. In order to increase the accuracy of thermal conductivity measurements, the meters are graduated using two standards of different thermal conductivity, on which several values of the heat flux are successively created, and the steady-state values of thermo-EMF converters for flow and temperature difference are measured. The transfer function of each converter is converted to a linearly proportional form and a calibration factor is calculated. 4 tab.

Description

The invention relates to thermal methods for studying a substance, including determining the thermal conductivity by the method of stationary heat flux.

A known method is according to which the calibration of the meter is carried out according to one standard. Calibration is reduced to finding a calibration coefficient and eliminates instrument errors in the vicinity of the thermal conductivity of the standard. The disadvantage of this method is that graduation is valid in a narrow range of thermal conductivity.

A method is known in which the calibration of the meter is carried out according to several standards of different thermal conductivity. At the same time, a calibration factor is found for each reference. For intermediate thermal conductivities, a calibration factor is found by interpolation. This technique allows us to compensate for the measurement error in a wider (compared

with the previous method) thermal conductivity range. The disadvantage of this method is that such compensation cannot be complete. This is because in order to determine a calibration coefficient dependent on thermal conductivity, it is necessary to know the thermal conductivity of the test material in advance. But it is unknown; its definition is the purpose of testing. Thus, the calibration coefficient is determined by the unknown thermal conductivity of the test material. Therefore, the calibration factor cannot be determined exactly.

A common disadvantage of the known methods is that the calibration coefficient depends on the thermal conductivity. The reason for this is as follows. The principle of operation of heat conductivity meters by the stationary heat flux method is based on the Fourier law, i.e., on a linearly proportional relationship

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heat flux from a temperature gradient with constant thermal conductivity. During testing, the desired heat flux and temperature difference (temperature gradient) are determined by the magnitude of thermoEMF of converters (thermocouples, heat meters) of these values. This means that the Fourier law is not violated, that the fraction φ $ {; cb. $ P8HЈt is proportionally linear., The dependence between The corresponding values of the thermoEMF of the converters are the tempera ture and the flux flux. However, in reality this does not happen for a number of reasons, for example, due to an instrument error in measuring thermopower, imperfect boundary conditions on the surface of a sample, etc.

The aim of the invention is to improve the accuracy of measurements of thermal conductivity by achieving a linearly proportional dependence of the inter-thermoEMF of the temperature and heat flux transducers and simplifying the measurements.

This goal is achieved in that in a method of calibrating heat conductivity meters, including creating a constant heat flow through reference samples (standards) of known geometry and heat conductivity, measuring steady-state values of thermoelectric power of heat flux converters and temperature difference, calculating calibration coefficients according to the invention, , sequentially set several values of the heat flux through standards of known geometry and thermal conductivity, measure the established thermopower transducers, adjust the transfer function of each transducer to bring it linearly proportional, and calculate the calibration coefficient of the meter.

Example. Two standards of thermal conductivity AI 0.1 W / (m K) and Aa 0.2 W / (m K) and a thickness of di are used to calibrate a stationary thermal conductivity meter of flat samples. d2 0.05 m. -.

In a material with heat conductivity AI, three heat fluxes of different magnitude are created sequentially, as a result of which three pairs of steady-state values of thermoelectric power of the temperature differential transducers Uti and heat flux Uqi are determined, which are recorded in Table. 1.

A standard with thermal conductivity Ar is tested in the same way, as a result of which three pairs of thermoEMF of heat flux Uq2 and temperature difference Ut2 are obtained, which are recorded in table. 2.

From the table. Figure 1 shows that the thermoelectric power of Uqi and Uti vary linearly proportionally. When testing a standard with AZ thermal conductivity, the linear proportionality condition between Uq2 and Ut2 is not satisfied. Therefore, it is necessary to correct the transfer functions of the converters. For this purpose, a Cartesian coordinate system is constructed, on the ordinate axis of which the scale of thermoEMF of the heat flux transducer Uq is plotted, and on the abscissa axis - the temperature difference Ut. In the constructed coordinate system, points corresponding to those summarized in table. 1 three pa5 frames of thermoEMF (Uqi, Uti), namely (0.55; 0.55), (1.05; 1.05), (1.55; 1.55). A line is drawn through the points, which in this case is a straight line. A similar construction is done at three points (Uq2, Ut2), namely

0 (0.45; 0.25), (0.85; 0.45), (1.25; 0.65). It can be seen that in this case, the points fell on a straight line.

The constructed straight lines intersect at the point (0.05; 0.05). According to Fourier law, this point must correspond to zero

5

5

0

5

heat flux and temperature gradient in the samples. Denote the values corresponding to this point. thermoEMF through Uqo and Uto. Then the correction of the transfer functions of the sensors is reduced to

0 to the fact that instead of Uq, in subsequent measurements it will be necessary to use the value Uq.- Uqo, and instead of Ut, the value Ut - Uto.

Convert the given data to the table. 1, 2 and summarize them, respectively, in table. 3, A.

According to the above, the formula for calculating thermal conductivity will have the form

and where d is the thickness of the sample, m;

K is a calibration coefficient determined from the formula

-Uto)

k ..vvt w (21 d (Uq-Uq0) ... W Substitute the thermal conductivity AI, the thickness di, and the pair of numbers of the first column of Table 3 in formula (2).

k 0,050,5

We are convinced that the value of the calibration coefficient obtained by formula (2) does not depend on the heat flux through the sample, for which the remaining two pairs of numbers of the table should be used. 3, neither from the thermal conductivity of the standards, for which it is necessary to substitute A2 and d2 instead of AI and di, and also substitute Uq2 - Uqo and Ut2 - Uto from Table 2 instead of Uqi - Uqo and Uti-Uto. 4.

Claims (1)

SUMMARY OF THE INVENTION A method for calibrating heat conductivity meters, including creating a constant heat flux through reference samples of known geometry and heat conductivity, measuring steady-state thermoEMFs of heat flux converters and temperature difference, calculating calibration coefficients, characterized in that, in order to improve accuracy and simplify measurements , sequentially create several values of the heat flux through two reference samples and measure the steady-state values of thermopower converters and differential heat flow temperature for each value of the heat flux, after which the calibration coefficient is calculated by the formula D (Uti - Uto) di (Uqi - Uqo) where i 1,2 Ai is the thermal conductivity of the 1st standard; di is the thickness of the iLro standard; Uti, Uqi - steady-state value of thermoEMF of transducers of temperature difference and heat flow, respectively, when testing the i-ro standard UtO, UqO AND Uq2 (Ut2) - POINT COORDINATES intersection of dependencies Uqi (Uti). Table 1 twenty table 2 Table 3 Table 4