JPH0220058B2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to the thermal conductivity, thermal diffusivity, specific heat,
In measurement methods that measure thermophysical property values such as heat capacity in an unsteady state, and then digitally process, calculate, and display, even relatively low-resolution A/D converters can be used with high resolution, making highly accurate measurements possible. This paper relates to a method for measuring thermophysical property values.

Conventionally, methods of measuring thermophysical property values in an unsteady state are known, and one method is to use a linear heating source and apply constant power to the linear heating source from a certain point,
To explain the method of determining the thermal conductivity, which is a thermophysical property value of a sample, from the temperature change at one point within the sample, Figure 1 shows a method in which a thin heating wire b is connected in a straight line to the center of an infinite cylindrical sample a. FIG. 3 is an explanatory diagram of the principle of thermal conductivity measurement by the unsteady hot wire method assuming a state in which the device is placed.

When constant power is continued to be supplied to the heating wire b from time t=0 (t ₀ ), the temperature T at a distance r from the heating wire b increases exponentially with the passage of time. This situation is shown in FIG. When this is analyzed mathematically, it has been found that the thermal conductivity k of the sample is given by the following equation as a good approximation.

k=qln(t ₂ /t ₁ )/4π(T ₂ −T ₁ ) (1) Figure 3 is a diagrammatic representation of the contents of equation (1), and according to this, measurement starts from the start of heating. After the time required for , the temperature rise and ln (time) are obtained as a linear relationship, and the reciprocal of the slope is proportional to the thermal conductivity k of the sample.

Note that after a certain period of time, in an actual sample, it is no longer possible to secure an infinite size as explained in the principle. Therefore, the heat penetrates the outer surface of the sample and absorbs the material around the sample (for example, the desk or the surrounding air).
Heat begins to be supplied to the area, and an area where measurement cannot be made occurs as shown in FIG.

The unsteady hot wire method described above was a method in which a thin heating wire was completely surrounded by the sample for measurement, but this was improved and sample a was placed on one side (half) of heating wire b.
Explaining the method of measuring by placing sample c, which has good insulation properties, elasticity, and known thermal conductivity, on the other side, this is a method of continuously applying constant power to heating wire b, and temperature rise and The relationship with elapsed time, etc. is exactly the same as in the case of thermal conductivity measurement described above, but
The difference is that the following equation provides a good approximation for the thermal conductivity k of the sample.

k=Aqln(t ₂ /t ₁ )/(T ₂ −T ₁ )−B (2) Here, A and B are constants, and are calculated using two or more standard samples with known thermal conductivities. is determined as the test constant.

As another example of measuring thermophysical property values, a method of unsteadily measuring thermal diffusivity, thermal conductivity, heat capacity, specific heat, etc. using a planar heating source will be described with reference to FIG.

This is done by placing flat standard plates c and c with known thermal conductivity on both sides of a flat plate to be measured (sample), respectively, and inputting heat from the direction of the arrow on the other hand. Measure the temperature response at a total of 4 points, one point each on the contact surface of c and c, and one point on the inside or surface of each standard plate c and c.Furthermore, calculate the Laplace integral value of these, and calculate the Laplace integral of the basic equation of heat conduction. Using conversion, it is possible to simultaneously measure the thermophysical properties of a sample.

In this measurement method, the heat input method for heating is to use a planar heat source d (both sides may be heated, but Fig. 5 shows a heat source using one side heating) from a standard plate. c, c and plate to be measured a
In this case, the planar heat source can also be irradiated with a lamp, and the heat input method for heating is different from the above-mentioned unsteady hot wire method and its improvement method. Although it does not have to be constant, if heat is continued to be applied from the heat source for a suitable time and then turned off, the temperatures at the four temperature measurement points in FIG. 5 will change as shown in FIG. 6, for example.

Therefore, based on this temperature change, the Laplace integral value is determined for each, and the thermophysical property value is calculated from the calculation formula.

As mentioned above, in the method of determining the thermal conductivity by the unsteady method using a linear heating source, the thermal conductivity of the sample is determined from transient temperature changes as shown in equations (1) and (2). In both cases, if the temperature difference ΔT at two different times is measured with high accuracy, the thermal conductivity k of the sample can be accurately determined.

Furthermore, in the method of calculating thermophysical property values using the unsteady method using the above-mentioned planar heating source, the calculation formula is (1)
Equation (2) is different from Equation (2), but for example, the temperature change at the four temperature measurement points in Figure 5 is determined with the heating start point as 0, and the point at which the thermophysical property value of the plate to be measured is determined is the reference point (the hot wire method is
The same is true in that the temperature change measurement from t ₁ (t=0 in this method) is an important point.

However, it is extremely difficult to accurately measure the temperature difference between the two different times and the temperature change from the start of heating using conventionally used digital measurement circuits with relatively low resolution.

In other words, _{the total temperature (T 0 +}
In order to measure ΔT), a digital measurement circuit with a wide dynamic range and high resolution is required.

To explain this with respect to measurement using the unsteady hot wire method mentioned above, the degree of temperature rise from the start of heating to time _t2 is from several degrees Celsius to several tens of degrees Celsius at most, and the temperature difference ΔT required for normal measurement is from 0.3 to The temperature is about 2℃.
On the other hand, the ambient temperature T ₀ is measured by heating the sample to the desired measurement temperature and maintaining it at temperature T ₀ , so it may be 800℃, 1000℃, or even
Temperatures of 1500℃ often occur. In other words, the required temperature change ΔT is approximately 1/10 of the temperature change from the start of heating, and approximately 1/1 of the temperature of the atmosphere.
The quantity is 100.

Using conventionally used digital measuring instruments with relatively low resolution, these atmospheric temperatures T ₀
Obtaining thermophysical property values with sufficient accuracy from measured values including
This is extremely difficult.

In view of the above circumstances, this invention has been made with the purpose of developing a measurement method that can obtain highly accurate thermophysical property values even when using a relatively low resolution, inexpensive and practical A/D converter. Researched.

As a result, as mentioned above, when a heat source is applied to the measured substance, the temperature change at that time is detected by the temperature sensor, and the thermophysical property value is calculated using a calculator, the ambient temperature T ₀ of the measured substance is calculated. If you use a circuit that can pre-cut the ambient temperature of the substance to be measured, focusing on the fact that it is not included in the temperature change ΔT, it is possible to create an A/
It has been discovered that measurements can be made with a D converter, and that highly accurate measurements can be made by using a circuit that can correct the zero-level fluctuations inherent in electrical measurement circuits, that is, offset drift, while measuring ΔT. It is.

To explain this about the thermal conductivity measurement method using the unsteady hot wire method of equations (1) and (2), in a circuit consisting of an analog measurement circuit and a digital measurement circuit,
What is necessary is to set the temperature input to zero, perform pre-cut processing in the analog measurement circuit, and send this signal amount to the subsequent digital measurement circuit. Further, as for offset drift correction, it is sufficient to measure the amount of offset drift while measuring ΔT and correct the variation due to this from the measured value of ΔT.

Furthermore, in the method of measuring thermophysical property values by the unsteady method using the above-mentioned planar heating source, the fact that the ambient temperature of the measured substance is not required during calculation is the same as above.
Therefore, as mentioned above, by pre-cutting the ambient temperature of the substance to be measured and setting the temperature at the start of heating to zero, even if an A/D converter with relatively low resolution is used, it can be made to have high resolution and high precision. It becomes possible to measure

In addition, in the method of measuring thermophysical property values applying the Laplace integral value, it is not necessary to start heating at the point when the temperatures at four temperature measurement points show the same temperature. What is necessary is to measure the amount of change with respect to the temperature (initial temperature) at the start of heating at each hot point as zero.

As mentioned above, in the method of measuring thermophysical property values using the unsteady method, it is necessary to set the ambient temperature at the start of heating to zero and continuously measure the temperature change. It is troublesome to adjust the temperature component to zero, and in particular, it is difficult to adjust the initial temperature components of four temperature measurement points to zero at the same time, such as when measuring thermophysical property values using the Laplace integral value mentioned above. Almost impossible.

As soon as the temperature conditions for each initial measurement point are satisfied, the heating start OK lamp will automatically light up, and when the measurer presses the start button, the initial temperature at each point will be automatically pre-cut. A circuit that measures only the subsequent temperature change is required. It is also necessary to automatically correct the offset drift of the measurement circuit. This necessity increases as the time for measuring temperature changes increases.

Therefore, this invention uses a circuit that can automatically perform pre-cut processing and offset drift detection and correction processing to send only the required signal amount change to the digital measurement circuit through the analog measurement circuit. This paper proposes a method for measuring physical property values.

Specifically, a heat source such as a planar heat source, a linear heat source, or a point heat source is used to heat the sample for a certain period of time.
Heat conduction that applies heat from t ₀ , detects transient temperature changes at one or more points inside or on the surface of the sample, guides the temperature changes to an analog measurement circuit, and then digitally processes, calculates, and displays them. In the method of measuring thermophysical property values such as thermal diffusivity, thermal diffusivity, specific heat, and heat capacity, an input amount Then, this stored amount is converted from a digital amount to an analog amount and is input to the adder provided in the analog measurement circuit described above, together with the input amount (X + ΔX) corresponding to the temperature (T + ΔT) after the required time. In addition, X is pre-cut here, and only ΔX is automatically guided through the analog measurement circuit to the digital processing section for calculation processing, and in the calculation process through the analog measurement circuit described above, the input of the analog measurement circuit is The offset drift of the analog measurement circuit is read by a digital processing section by intermittently shorting the line, and the drift is subtracted from the measured input change to perform the above-mentioned pre-cut calculation process.

Therefore, according to the present invention, in addition to correcting the drift caused by the analog measuring circuit, the signal amount can be used to significantly improve measurement accuracy.

The present invention will be explained below based on the illustrated embodiments.

FIG. 7 is a circuit diagram used to implement the present invention, and FIG. 8 shows its timing chart.

In the circuit shown in FIG. 7, 1 is an analog measurement circuit, 2 is a digital processing section, and the analog measurement circuit 1 is composed of a first-stage amplifier 3, a second-stage amplifier 4, an adder 5, a pre-cut amplifier 6, and an analog multiplexer 7. be done.

The first-stage amplifier 3, the next-stage amplifier 4, and the pre-cut amplifier 6 have gains n ₁ , n ₂ , and n _3, respectively, and are amplifiers capable of obtaining outputs in opposite phases.

On the other hand, the digital processing section 2 includes a digital input multiplexer 8, a μ-CPU 9, and a memory device 1.
0, a digital output circuit 11, a display device 12, etc., an A/D converter 13 is provided between the analog multiplexer 7 and the digital input multiplexer 8, and the digital output circuit 11 and an adder. Feedback circuit 14 connecting 5
A D/A converter 15 is provided.

In the analog measuring circuit 1, the next-stage amplifier 4 and the pre-cut amplifier 6 are normalizing amplifiers used to amplify the respective inputs to the amount required by the A/D converter 13, and the analog multiplexer 7 is the first-stage amplifier. 3 input switch SW1, switch SW2 provided between the input side of first stage amplifier 3 and ground, switch SW3 provided between next stage amplifier 4 and A/D converter 13, and pre-cut amplifier 6 and A/D converter 13. A switch is configured to automatically switch the switch SW4 provided between the D converter 13 and the D converter 13 at regular intervals.

In addition, an input filter 16 is provided before the first stage amplifier 3 to cut out noise components contained in the external input signal, but when the input signal is caused by temperature changes as in the present invention, the frequency characteristics of the signal change considerably. Since the frequency is low, the input filter 16
A low-pass filter, which is often used in ordinary measurement circuits, is used.

On the other hand, a thermometer 17 composed of an ordinary thermocouple, resistance temperature detector, thermistor, etc. is used in the sample, and an input signal of mV order is applied from the thermometer 17 to the first stage amplifier 3 via a filter 16. The input signal is further amplified by the necessary amount by the next-stage amplifier 4 and applied to the A/D converter 13, where the input signal is converted into a digital quantity and applied to the digital input multiplexer 8. That is, if X is the input amount of the signal corresponding to the temperature T (or τ ₀ ) of the sample at the measurement reference time (time t ₁ in the unsteady hot wire method, heating start time t = 0 in the Laplace integral method), _The input amount X that is amplified and output from the first stage amplifier 3 has an opposite phase, so it becomes -n ₁
is in the same phase as the input to the analog measurement circuit 1, so it becomes n ₁ n ₂
It is input to the D converter 13. Constant values necessary for measuring thermophysical properties (for example, constant values corresponding to q, A, and B in equations (1) and (2) above) are inserted into the digital input multiplexer 8, and the start button is pressed. The input signal and constant value digitized by 18 are sent to μ-CPU 9, where thermophysical property values are calculated. Note that the digital input multiplexer 8 also provides instructions to the sample heating circuit. The memory device 10 stores the amount of temperature change that is input from time to time.

In addition, the input amount n ₁ n ₂ X is returned to n ₁
Output to. Note that the input quantity n ₁ and added to adder 5.

The above is the operation when the analog multiplexer 7 closes the switches SW ₁ and SW ₃ and opens the switches SW ₂ and SW ₄ , but the analog multiplexer 7 closes the switches SW ₁ and SW ₄ . The operation when the switches SW ₂ and SW ₃ are opened will be described in detail below.

Switch by analog multiplexer 7
By closing SW ₁ and SW ₄ , the input amount (X + ΔX) corresponding to the temperature of the sample (T + ΔT) at (t ₁ + Δt ₁ ), which is Δt ₁ after the measurement reference time t ₁ , passes through the first stage amplifier 3 and becomes - amplified by n ₁ (X+ΔX),
It is input to adder 5.

Therefore, since the input amount _{n 1} When added, −n ₁ ΔX remains,
Only the amount of change −n ₁ ΔX after Δt seconds has passed is obtained.

That is, it is possible to perform a pre-cut to obtain only ΔX by subtracting the input amount at the measurement reference time t from the input amount (X+ΔX) after Δt seconds have elapsed from the measurement reference time.

When the amount of change -n ₁ ΔX obtained by the pre-cut is amplified by the pre-cut amplifier 6, the phase is inverted, and becomes n ₁ n ₃ ΔX, which is sent to the digital processing section 2.
It is output to the A/D converter 13 of
The CPU 9 calculates the thermophysical property value using the amount of change ΔX in Δt seconds from the measurement reference time, and displays this value digitally on the display device 12.

In addition, in the arithmetic processing described above, the input signals X and (X+ΔX) pass through amplifiers 3, 4, and 6, so these input amounts are affected by the offset drift of each amplifier, but in order to correct this, SW1
is closed and the external signal input amounts X and (X+ΔX) are passed through the first stage amplifier 3 to perform arithmetic processing as described above, then open SW1 and close SW2 to read the offset drift of the circuit system at this time. By taking this difference, arithmetic processing can be performed using a signal amount that is not affected by offset drift.

To explain this based on a timing chart for smoothly performing the above calculation processing, first
By closing SW1 and SW3, the external input signal is read by the A/D converter 13, and by opening SW1 and closing SW2 and SW3 in the B interval, the A/D converter 13 reads the external input signal from the amplifiers 3 and 4. Read offset drift. In section C, SW2 is closed, SW3 is opened, and SW4 is closed.
is closed and the D/A converter 15 is set to zero voltage (point a), and the A/D converter 13
Read more. In section D, open SW2 and vice versa.
Close SW1 and SW4, set the value read in section A to the D/A converter 15 (point b),
Read by A/D converter 13.

After performing the above operations, the data read in section B is subtracted from the data read in section A, and the data is converted into input and used as data for displaying the measured temperature.

Next, the data read in section C is subtracted from the data read in section D, and the data is inputted to obtain the true value used in thermophysical property value calculation.

In the present embodiment, the phase of the input quantity is suitably inverted and amplified, and the pre-cut is performed by adding two quantities having mutually opposite phases using an adder, but the present invention is not limited to this. Instead, it is also possible to perform pre-cutting using, for example, a subtracter.

As explained above, according to the present invention, the input amount at the measurement reference time t is subtracted from the input amount (X + ΔX) after Δt seconds have elapsed from the measurement reference time, and ΔX
The amount of change ΔX used for the thermophysical property value can be obtained within the analog measurement circuit by pre-cutting to obtain only the value of the thermophysical property. Therefore, by directly converting the amount of change ΔX from analog to digital in the digital processing section, even with an A/D converter with relatively low resolution, it is possible to obtain accurate thermophysical property values with sufficient accuracy. This allows the minute amount of change ΔX to be converted into a digital amount.

In addition, the offset drift of the analog measurement circuit when the input amount By correcting the values of the input amount X and the amount of change ΔX, and obtaining the thermophysical property value using the corrected amount of change ΔX, it is possible to further improve the measurement accuracy.

Furthermore, calculation processing such as correction of the input amount Thermophysical property values can be measured using a simple circuit.

[Brief explanation of drawings]

Figure 1 is a diagram to explain the principle of the unsteady hot wire method, Figure 2 is a relationship curve between temperature and time in the same method, Figure 3 is a relationship curve between ln (time) and temperature rise, and Figure 4 is a diagram to explain the principle of the unsteady hot wire method. is a diagram for explaining the principle of the improved unsteady hot wire method, Figure 5 is a diagram for explaining the principle of the unsteady measurement method using the Laplace transform method, and Figure 6 is a diagram for explaining the principle of the unsteady measurement method using the Laplace transform method.
The figure shows the temperature-time relationship curve in the same method as above.
FIG. 7 is a diagram showing an example of a circuit used in the measurement of the present invention, and FIG. 8 is a diagram showing an example of a timing chart in the measurement of the present invention.

Claims (1)

[Claims] 1 Heat is applied to the sample from a heat source, a transient temperature change that occurs in the sample due to the heat is detected, the temperature change is guided to an analog measurement circuit, and further digitally processed and calculated. In the method of measuring thermophysical property values to be displayed, an input quantity X corresponding to the temperature T of the sample at the time of measurement reference is led to an analog measuring circuit, amplified, converted to a digital quantity by a digital processing part, and then sent to a digital processing part. The digitized input quantity X is stored in a storage device, reconverted from digital quantity to analog quantity, and fed back to the analog measuring circuit described above, and the temperature of the sample (T + ΔT) after Δt seconds has elapsed from the measurement reference time. A pre-cut is performed to subtract the input amount X from the input amount (X+ΔX) corresponding to , so that only the change amount ΔX is guided to the digital processing section, and the input amount X or the change amount ΔX is sent to the digital processing section through the analog measuring circuit. When deriving , the input line of the analog measurement circuit is intermittently short-circuited, and the offset drift of the analog measurement circuit when the input amount X and the variation ΔX are led to the digital measurement circuit is read by the digital processing section. Subtracting the offset drift from the measured input amount X and change amount ΔX, correcting the input amount A method for measuring thermophysical property values, characterized in that the thermophysical property values of a sample are obtained.