SU1364911A1 - Method of determining temperature - Google Patents

Abstract

Invention m. used to improve the accuracy and performance of semiconductor resistance thermometers. The purpose of the invention is to improve the accuracy of determining the temperature under long-term operation conditions. Through a semiconductor thermistor (MFR) current is passed, which produces its heating. The temperature of the additional heating is controlled by comparing the obtained resistance with the initial resistance. The heating current corresponding to the resistance to the MTR resistance is measured, and the electric power dissipated on it is determined. The heating temperature is determined from the heat balance equation of the heated MFR. The heating temperature is gradually increased by stepwise changes in the heating current. The process of increasing the heating current is stopped if the decrease in the resistance of the MFR becomes smaller (O, compared to the previous state. According to the measurement results of three values of the resistance to MFR and to the heating current, the temperature of the controlled medium is calculated using a computer. 1 np.

Description

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The invention relates to a technique for measuring the temperature of various media using semiconductor thermal resistors and can be used to improve the accuracy and performance of semiconductor resistive thermometers.

The purpose of the invention is to increase the accuracy of temperature determination by semiconductor thermistors during long-term operation without additional intermediate calibrations.

The drawing shows a structural diagram of the device that implements the proposed method.

The device contains a controlled current source 1, to which are connected in series a semiconductor thermistor (MFR) 2 and a low-resistance calibrated resistor 3 with resistance R, resistance-to-voltage converter 4 and current-to-voltage converter 5, multiplexer 6, analog - a digital converter (ACP) 7, a digital-analog converter (D / A converter 8, microcomputer 9 with digital display 10.

The method is carried out as follows.

The MFR 2 is placed in a controlled medium with a temperature of 0 and its resistance is measured.

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where A is the coefficient having the dimension of resistance; B is a coefficient; an agent that has a dimension of tag-terature and which determines the sensitivity of the MFI. Resistance R, MFR 2 is converted by converter 4 into a voltage that multiplexer 6 is fed to the input of ADC 7. A digital code corresponding to the resistance of thermistor R, at a controlled temperature of 6, enters the micro-computer 9, where it is stored. A current is passed through the NTR, which heats the working medium MFR. As a result of the additional heating of the MFR, its resistance decreases in accordance with its temperature response. The temperature of the additional heating is increased by increasing the current to a value at which the initial resistance to the MFR R decreases by

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5-10%. Resistance Optional Hai peToro PTR

R.

(2)

where db is the temperature of additional heating.

The temperature of additional heating -18 is controlled by comparing the obtained resistance B with the initial resistance R, from the condition:

0.05 R, R, -R, j.O, l R.. (3)

The heating current I ,, is measured, corresponding to the resistivity of the MFR R, and the power dissipated on it is measured.

P, 1,. (4)

The heating current I, flowing through the low-impedance calibrated resistor 3 is converted by converter 5 into voltage, which through multiplexer 6 enters ADC 7, where it is encoded and then stored in the microcomputer 9. The temperature of the additional heating is determined from the heated heat balance equation PTR

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where S is the cooling surface of the MFR; C is the scattering coefficient, which takes into account all types of heat propagation from the working body, LF (convection, thermal conductivity and thermal radiation).

Heating of the thermistor is performed by commands from the microcomputer 9. At the output of the DAC 8, a stepped control voltage is produced, which changes the heating current of the thermistor by the source 1,

For electrical isolation, the measured current of the transducer 4 flowing through the MFR 2 with the heating current from source 1 is made variable with a sinusoidal or rectangular waveform. The microcomputer program provides for the comparison of each new value of the resistance to MFI with the previous one and the determination of the relative increment for each heating stage. Quantity - 1

in steps in the heating current is determined by the achievement of condition (3).

Gradually increase the temperature of the MFR heating by stepwise changes in the heating current 11, fixing at each current increase the decrease in the MFR resistance. At the same time, the current increment jl is equal to the initial value of the heating current. As a result of the heating of the MFR, its resistance decreases, and with each increment of the current 1, the increase in the resistance of LK decreases due to the falling nature of the temperature dependence of the MFR resistance.

The process of increasing the heating current is stopped if the decrease in the MFR resistance becomes less than 0.5-1% compared with the previous state. The steady-state heating current 1, where n is the number of current increments, causes a higher temperature for heating the MFR, corresponding to the area of low sensitivity of the MFR to temperature changes.

Achieving a weak dependence of the MFR resistance on the heating current increment means reaching the gently sloping portion of the MFR temperature characteristic (), where the exponential term of equation (1) has the following {th form:

gB / () l

In this mode, the MFR resistance approaches its minimum value.

, (7)

which is measured with an acceptable error.

In this case, the power dissipated by the MFR, taking into account the maximum measured temperature, should not exceed the maximum permissible value for the type of thermistor used.

According to the results of measuring the three values of the resistance of the thermistor R, R, j, R 5 to the heating current, the temperature of the controlled medium 6 is calculated using a microprocessor of the computer, which is displayed in digital form on the indicator 10;

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The values of S and C are stored in the memory of the microcomputer as constants. At the same time, it is possible to take into account the dependence of the scattering coefficient on the thermal properties of the controlled medium and its temperature by specifying the scattering coefficient of the array of experimental data stored in the permanent storage device. COMPUTER. From (8) it follows that the result of the calculation of the temperature does not depend on the coefficients of thermoresistor A and B, but is determined only by the measured resistances H, R. and E, the heating current, the cooling surface B, and the dissipation factor C PTR.

The choice of the initial heating of the MFR from the condition (G) and the cessation of its further heating, based on the achievement of a monotonous decrease in the MFR resistance in the range of less than 0.5-1%, is made from the following considerations. For low-inertia bead MFR, such as type CT1-18, CT1-19, STZ-25, etc., the temperature coefficient of resistance, which characterizes the sensitivity of the MFR, varies from 3-8% / K to 0.6-0.9% / K at b 500-600 ° С, i.e. decreases by about 10 times.

The initial heating from condition (3) ensures the correctness of the application of formula (5), which is valid for small differences between the heated body and the environment. At the same time, such overheating significantly exceeds the level of thermal fluctuations of the MFR and the controlled medium. Therefore, a tenfold decrease in the sensitivity of the MFR in the process of gradual heating of the MFR indicates that a flat section of its temperature characteristic is reached.

Thus, an increase in the accuracy of temperature determination by a thermistor resistive thermometer is achieved by eliminating the influence of the instability of the coefficients of the thermistors; determining its temperature characteristics and sensitivity, as well as non-constant voltage supply and sensitivity of the resistance transducer of the thermistor to electrical voltage, measured by the output device of the thermistor.

Claims (1)

Formula of the invention cess of increasing the heating current while decreasing the thermal resistance The method of determining the temperature, which consists in passing current through a semiconductor thermistor located in a controlled environment and measuring its resistance, additional heating the thermistor relative to the controlled environment, and measuring the resistance of an additionally heated thermistor, characterized in that , in order to improve accuracy, the temperature of additional heating of the thermo-15 resistor is increased to a value In which the initial resistance of the thermistor decreases by 5-10%, the current generating the indicated heating of the thermistor is measured, gradually the heating temperature of the thermistor is increased by 20 stepwise changes in the heating current, fixing the magnitude of the decrease in resistance of the thermistor with each increase in current, while 25 increments of the current are chosen equal the initial value of the heating current of the thermistor, stop the protector relative to the previous value of less than 0.5-1%, measure the minimum resistance of the heated a resistor, and the temperature Θ of the controlled medium is determined by the formula θ _ / lnR ^ -lnRs \ I? Rz • ^k InRlnR ₂ “S '' where R, is the resistance of the thermistor at the temperature of the controlled medium; R 2 is the resistance of the thermistor, additionally heated by current; R is the minimum resistance of a thermistor heated by current; I _? - current additional heating of the thermistor; C is the coefficient of heat dissipation by a heated thermistor; S is the cooling surface of the thermistor.