SU1718078A1 - Method and device for complex determination of thermophysical characteristics - Google Patents

Abstract

The invention relates to non-destructive testing and can be used to measure thermal conductivity, thermal diffusivity and thermal activity of substances and materials over a wide range of temperatures. The purpose of the invention is to improve the accuracy and reduce the measurement time. Changing the time constant T0 of the differentiator-inverter, which is included in the measuring circuit of the thermal feedback, excites auto-oscillations in the system. The thermal activity of the sample is determined from the value of T0 at which self-oscillations occur. Measured average values of the power released on the surface of the sample and temperature are the thermal conductivities. The thermal diffusivity of the sample is calculated from the thermal activity and thermal conductivity. This improves accuracy and reduces measurement time. 2 sec. f-ly, 1 ill. Yo

Description

The invention relates to non-destructive testing and can be used to measure thermal conductivity, thermal diffusivity and thermal activity of substances and materials over a wide range of temperatures.

The known method of complex determination of thermophysical characteristics is that the sample under study is included in the automatic control system as a feedback element, and the feedback loop is closed with heat flow through the sample. By changing the gain of the regulator, auto-oscillations are excited in the automatic control system, the frequency of which determines the thermal diffusivity, and the critical value of the gain is the thermal conductivity of the sample.

The disadvantage of this method is the low measurement accuracy.

A device for the complex determination of the thermophysical nature of a stick containing a differential thermocouple, a source of reference voltage, a regulator and a heater, closed into an automatic control system through the sample under study is known.

The disadvantage of this device is low measurement accuracy.

The closest to the proposed technical essence is the method of complex determination of thermal characteristics.

about vj

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the characteristics and the device that implements it. which excites auto-oscillations in an automatic control system with thermal feedback, the thermal element of which is brought into thermal contact with the sample. The device implementing this method comprises a thermostat, a differential thermocouple, a heater, a comparator unit, a recorder, and a source of reference voltage.

The disadvantages of this method and the device implementing it are the low accuracy of measurements and the large amount of time spent on measurements.

The aim of the invention is to improve the accuracy and reduce the measurement time.

The goal is achieved by self-oscillations that excite the system time constant, determine the thermal activity by the critical value of the constant time, determine the thermal diffusivity of the sample by the mean values of the heater power and sample temperature - the thermal conductivity and thermal activity.

In this method, the thermal activity is determined by the critical value of the time constant of the automatic control system (SAR) with thermal feedback, and the thermal conductivity is determined by the method of a flat source by the specific power supplied to the sample and the temperature gradient, the values of which determine the thermal diffusivity sample.

The study of the equation of this SAR, which has the form of a nonlinear boundary value problem of unsteady heat conduction:

t (x, t) -aT (x, t): T (x, t) | xo 0

p1 (

(d, t) -atUo-aa / exp x

°) - 00

x ((d.t) dr, (1)

-00

where T (x, t) о T (x, t) / dt is the rate of temperature change;

T (xt t) is the temperature difference between the sample and the thermostat;

x - coordinate;

t is time; d sample thickness;

K - regulator gain factor;

a - thermal diffusivity of the sample;

And - thermal conductivity of the sample;

S is the heater area;

R is the resistance of the heater;

0

U0 is the reference voltage;

a - thermoelectric coefficient;

r0 is the time constant of the differentiator;

6- power released in the heater;

t is variable integration; The a-Heaviside function gives the following formula for determining thermal activity;

“UK2

SR

/ 2b.

(2)

five

0

five

0

five

0

five

0

five

BUT

(four)

It follows that the relative sensitivity of the method is

c, - d os / Toe to

(3)

where the state is the critical value of the constant time T0.

Thermal conductivity A is determined with sensitivity and is calculated by the formula

S R T (6)

where is the root mean square value of the output voltage of the reference block;

 T (b) is the temperature of the sample thickness d.

The device for complex determination of the thermophysical characteristics of sample 1, contains a thermostat 2, a differential thermocouple 3, a heater 4, a differentiator-inverter 5, a comparison unit 6, a reference source 7, a recorder 8 and a switch 9.

One of the junctions of the differential thermocouple 3 is fixed on the thermostat 2, and the second on the sample under study 1. The outputs of the differential thermocouple 3 are connected to the corresponding inputs of the differential-inverter 5 connected to the first and second outputs respectively to the first and second inputs of the switch 9. Block 6 Comparison is connected by the first input to the output of the source 7, the second input to the third output of the differentiator-inverter 5 and the output to the thermocouple 3 and to the third input of the switch 9, the output of which is connected to the input of the recorder 8.

A device for complex determination of thermophysical characteristics works as follows.

At small values of the time constant of the comparator unit 6, the constant power is released in the heater 4, and a stationary temperature field is established in sample 1:

T (x)

K2UЈ

At certain values of Uo. K and T0. called critical, auto-oscillations occur in the system. These values are related to thermal activity by the relation (2), which can be used to determine it. The thermal conductivity of the sample can be calculated from the average values of the power released in the heater and the temperature T (5, t) using formula (5).

To measure the thermophysical characteristics, the parameters Do and K should be chosen such that J (d, t) does not exceed a certain limit imposed by the experimental conditions T0 (in this case T0 2 K). At the same time, it may turn out that the range of variation of r0 is not enough to induce self-oscillations. Therefore, before measurements, it is necessary to establish the values of Uo and K, which would satisfy the following conditions:

l K UQ t

DntT |

about

where Gomax - the maximum value of the constant time th

For this, the first switch of the differentiator-inverter 5 is set to the middle position and auto-oscillations are excited by decreasing U0, while simultaneously adjusting K so that T (g, t) remains within T0. The measurements of the thermophysical characteristics were carried out as follows.

In accordance with the requirements formulated above, the values of the parameters U0 and K were chosen. Then the time constant r0 was changed from zero to a certain critical value H0c, at which auto-oscillations occurred in the system. The value of the state is determined by measuring by the recorder the resistance of the resistors of the RC-chain of the differentiator-inverter 5, at which the self-oscillations are excited. Since block 6 and the source 7 of the reference voltage were calibrated in advance, and subsequently U0 and K did not change, thermal activity was measured, only one parameter Go was determined experimentally. To find the thermal conductivity, the effective values of the output voltages of the comparator unit 6 and diff

ten

15

20

thirty

35

40

45

50

55

of the inverter 5. The thermal activity and the thermal conductivity were calculated using formulas (2) and (4).

The use of the invention makes it possible to increase the measurement accuracy by increasing the sensitivity of measuring thermal activity at a critical constant time by about 4 times. The measurement time is shortened, since it is not necessary to establish a certain amplitude of oscillations and to register the necessary number of full cycles of oscillations.

Claims (2)

Claims 1. A method for complex determination of thermophysical characteristics, which consists in exciting self-oscillations in an automatic control system with thermal feedback, the thermal element of which leads to thermal contact with the sample, characterized in that, in order to improve accuracy and reduce measurement time, self-oscillations excite, to compensate for the phase shift of the thermal feedback signal by changing the constant time of the system, while the critical value of the constant time is determined by oic activity by an average value m of the heater power and the sample temperature - thermal conductivity, and thermal conductivity of the thermal activity - thermal diffusivity of the sample. 2. A device for complex determination of thermophysical characteristics, comprising a thermostat, a differential thermocouple, a heater, a comparison unit, a recorder and a voltage source, the output of which is connected to the first input of the comparison unit; one of the junctions of the differential thermocouple is fixed on the thermostat and the other is fixed on the sample , characterized in that it is equipped with a switch and an inverter-differentiator connected by the first and second inputs to the terminals of the differential thermocouple, the first and second outputs are respectively Respectively to the first and second inputs of the switch and the third output to the second input of the comparison unit, the output of which is connected to the heater and to the third input of the switch connected by the output to the input of the recorder.