SU1661635A1 - Device for measuring thermal diffusivity - Google Patents

Abstract

The invention relates to thermophysical measurements and can be used in the study and production of new materials, as well as in thermal non-destructive testing. The purpose of the invention is to improve the accuracy of determining the thermal diffusivity by taking into account the finite duration of the heating pulse and improving the measurement performance. The device contains a pulse heating source, a temperature sensor, four differentiators, two zero-organs, an RS flip-flop, a pulse generator, a coincidence circuit, a pulse counter, a digital indicator, a heating pulse recorder, two frequency divider counters, a register, a digital comparator. The device makes it possible to measure the thermal diffusivity of the sample under investigation in accordance with the expression A = 0.09 L ² / (T _MP - MΤ _P ), where L is the thickness of the flat test sample

T _MP is the time to reach the first derivative of the chronological thermogram of the maximum value.

Τ _P is the time of maximum density of the flow of energy of the heating pulse

M - coefficient taking into account the final duration of the heating pulse. 2 Il.

Description

The invention relates to thermophysical measurements and can be used in the research and production of new materials, as well as in thermal non-destructive testing.

The purpose of the invention is to improve the accuracy of determining the coefficient of thermal diffusivity by taking into account the finite duration of the heating pulse and improving the measurement performance.

Figure 1 shows the functional diagram of the device; 2 shows time diagrams for measuring the temperature and density of the energy flow of a heating pulse.

The device contains a pulsed heating source 1, sample 2, temperature sensor 3, first 4 and second 5 differentiators 5, first zero-body 6, trigger 7, pulse generator 8, coincidence circuit 9, pulse counter 10, digital indicator 11. recorder 12 heating pulses, the third differentiator 13, the fourth differentiator 14, the second zero-body 15, the first 16 and the second 17 Counters frequency dividers, register 18, digitON

J

CS

ABOUT

SP

a level comparator 19 and an input trigger terminal 20,

The pulsed method for determining the thermal diffusivity is a pulsed heating of the front surface of a flat test specimen. In this case, the curve of the increase in temperature T (t) (T is the temperature, r is the time) of the back surface of the test sample is recorded (Fig. 2). If the heating pulse is instantaneous (theoretical pulse), then measure the time to reach a level with a chronological thermogram T (t), for example, 0.5 Tmax, determine the thermal diffusivity of the sample using the formula

, 139

T 1/2

where L is the thickness of the flat sample under study;

P / 2 - time to reach the curve T (t)

LEVEL 0.5 Tmax.v

Under actual conditions, heating pulses have a finite duration and the neglect of this fact leads to the appearance of methodological errors, especially large for thin samples of high-temperature materials. For example, for heating pulses of a xenon flash lamp, a calculation formula was obtained that takes into account the final duration and shape of the heating pulse:

a 0.139

(g 1/2 -1,28tr)

where Tr is the time of the maximum energy flux density of the heating pulse F (g) (figure 2).

The shape of the heating pulse affects the type of correction introduced. Instead of the characteristic time p / 2, the time Hmp can be used to reach the first derivative of the chronological thermogram T (t} maximum value.

For an instantaneous heating pulse, the expression for the thermal diffusivity in this case is

and 0.09 12 / tmp.

The calculated expression for thermal diffusivity is obtained, taking into account the final duration of the heating pulse of a xenon flash lamp:

ten

| 5

20

25

It is with the help of this mathematical expression that the coefficient of thermal diffusivity is determined for the sample under study in a device that measures time.

(gmp - 1.7 g). Accounting for the finite duration of the heating pulse allows one to eliminate the methodological error and thereby increase the measurement accuracy. Definition for one measurement of the complex

(Tmp - 1.7 g) allows for improved test performance.

The device works as follows (figure 1).

After pressing the Start button, the luminous flux of the pulsed heating source 1 produces heating of the flat test sample 2. A portion of the luminous flux illuminates the recorder 12 of heating pulses, and a generator of 8 pulses is started with a frequency f. The pulses from the generator output 8 pulses simultaneously arrive at the inputs of the first and second counter-dividers, the frequency division factors of which are M and N, respectively, are in ratios

 ,

thirty

where m is the coefficient at gr (expressions 2m3.

At the time Tp of the maximum energy flux density, which reaches the heating pulse recorder 12, the output 35 of the second zero-body 15 appears a pulse that arrives at the input of the register record 18, the digital input code of the output of the first divider 16 frequencies equal to the value of Tf / (M / f), and stored in register 18. Since the coefficient, the counters of the second counter-frequency divider 17 continue counting pulses until the moment of tcr equality of the digital codes of both counters-frequency dividers, and so like digital code tue one of the counter-frequency divider, at this moment is determined by the expression

40

45

50tcp / (N / f), then

(four)

at the moment tcr, the output of the pulse counting output is generated at the output of the comparator 19, which is fed to the S input of the RS flip-flop 7. Thus, starting from the moment of time

a 0.09

(Tmp - 1J Tr)

(3)

 N tsr - -jyj- Tp - m Gy

(five)

the counting pulses begin to flow from the pulse generator 8 through the coincidence circuit 9 to the input of the pulse counter 10.

The heat flux from the rear surface of the flat test sample 2 is supplied to the temperature sensor 3, the output of which is proportional to the temperature of the rear surface of the flat test sample 2 through the first 4 and second 5 differentiators and the null organ 6 to the R input of the RS flip-flop 7 and sets it at the moment bp to the state of prohibiting the counting of pulses by the counter of 10 pulses. At this point in time, the digital display shows the value of the time interval ("l., N - m rp}. The output potential of the trigger 7 allows or prohibits the passage of counting pulses from the generator output 8 pulses through the coincidence circuit 9 to the counter 10 input. - The counting of the counter 10 at the time (Hmp - tpr) is recorded and displayed by the digital indicator 11.

Thus, the device makes it possible to determine the corrected heat transfer process time (Hmp - m g), the value of which is necessary to determine a more accurate value of the coefficient of thermal diffusivity. The coefficient m determines the value of the division factors M and N of the frequency divider counters. The device is implemented in the form of a set of equipment, including a flash lamp as a source of pulse heating, an infrared pyrometer — a temperature sensor and a pulse digital dummy block.

Coefficient, 7 according to the expression (3). In this case, the division factors of the frequency divider counters at a frequency of 8 MHz generators are equal to 1000 and 1700, respectively.

0

five

0

0

five

five

0

Claims (1)

Invention Formula A device for determining the thermal diffusivity, containing a pulsed heating source optically coupled to a heating pulse recorder and through a test sample with a temperature sensor, which is connected via serially connected first and second differentiators and the first null organ to the R input of the RS flip-flop, the output of which connected to the first input of the coincidence circuit, the second input of which is connected to the output of the pulse generator; the output of the coincidence circuit is connected to the input of the pulse counter, the output of which is Dinene with a digital indicator, characterized in that, in order to improve the accuracy of determining the thermal diffusivity by taking into account the finite duration of the heating pulse and increasing the measurement performance, the device additionally introduced the third and fourth differentiators, the second zero-organ, the register, the first and second counters frequency dividers, a digital comparator, the output of the recorder of heating pulses is connected through the third and fourth differentiators connected in series and the second zero-organ to during the register recording, the register data input is connected to the output of the first counter-frequency divider, and the register data output is connected to the first digital comparator input, to the second input of which the second counter-frequency divider counter is connected, and the digital comparator output is connected to the S-input RS-trigger, the output of the pulse generator is connected to the inputs of the first and second counters-frequency dividers, the input of the pulse generator is connected to the input of a pulse heating source and the input start terminal. G / w fy FIG. 2