RU2269102C1 - Mode of determination of temperature with a semi-conducting thermistor - Google Patents

Abstract

FIELD: the invention refers to measuring technique particular to measuring the temperature of various objects and mediums.

SUBSTANCE: a current is passed through the semi-conducting thermistor located in the controlled medium and its resistance is measured. The current is augmented that provokes additional heating of the thermistor relatively to controlled medium and the second resistance of the thermistor is measured. In accord with the two resistances measured at minimum current for two temperatures set by the frontiers of the diapason the parameters of the temperature characteristic of the thermistor are determined( maximum resistance and constant temperature) for the minimum current and in accord with it for the measured resistance of the thermistor the temperature of the controlled medium is determined. The determined temperature is compared with the second resistance of the thermistor, beforehand the third resistance which is considered to be a norm is measured at the second current and the maximum temperature. In accord with the normalized and the second resistances of the thermistor and corresponding to them temperature parameters of the working temperature characteristic of the thermistor is determined. In accordance with this the temperature of the controlled medium is determined at changing the resistance of the thermistor on augmented current is determined.

EFFECT: increases accuracy of determination of the temperature due to elimination of error of self-heating of the thermistor and increases efficiency due to reducing the number of measuring operations.

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Description

The invention relates to measuring technique, in particular to measuring temperature.

A known method of measuring temperature with a resistance thermometer [a.s. No. 1332158], which is brought into contact with the object of control. The power P _{1 is} supplied to the resistance thermometer and, at time t _1, the first temperature value θ _{1 is} measured and the power is increased to a value of P ₂ . At time t ₂ and t ₃ spend the second and third temperature measurement θ ₂ and θ ₃ . The temperature measurement is organized so that t ₂ -t ₁ = t ₃ -t ₂ . The value of the measured temperature is calculated by the formula

The disadvantages of this method are the uncertainty of the conditions and parameters under which the resistance thermometer is calibrated. When calibrating a resistance thermometer for various values of power dissipation, the calibration characteristics are different. The heat exchange parameters with the medium in which the resistance thermometer is calibrated also have a great influence. The neglect of these factors in the process of measuring temperature leads to an error.

The prototype adopted a method of measuring temperature by a semiconductor thermistor [A.S. No. 1364911] when measuring its resistance. To do this, an electric current is passed through the thermistor, which produces its heating. The temperature of the additional heating is controlled by comparing the obtained resistance with the initial resistance. The second resistance value is measured. Measure the heating current corresponding to the second resistance value of the thermistor, and determine the electric power dissipated on it. The heating temperature of the thermistor is determined from the heat balance equation. Gradually increase the heating temperature by stepwise changes in the heating current with registration of changes in the resistance of the thermistor. The process of increasing the heating current is stopped if the change in the resistance of the thermistor becomes insignificant compared to the previous state. Measure the third resistance value of the thermistor. According to the results of measuring three values of the resistance of the thermistor and the heating current, find the temperature T of the controlled medium according to the formula

where R ₁ is the resistance of the thermistor at the temperature of the controlled medium, R ₂ is the resistance of the thermistor additionally heated by the measuring current, I ₁ is the current of additional heating of the thermistor, C is the heat dissipation coefficient by the heated thermistor, S is the cooling surface of the thermistor.

The disadvantages of the method are the measurement error due to the presence in the calculated expression of parameters C and S, which cannot be determined with high accuracy. Parameter C depends not only on the properties of the medium being measured, but also on contact thermal resistances (for solid materials), the flow regime (for liquid and gaseous media), in a wide temperature range its value cannot be considered constant. The disadvantages also include the long duration of the temperature measurement process, during which the temperature of the controlled medium can change due to external conditions or due to the strong heating of the thermistor by transmitted current.

The technical objective of the method is to increase the accuracy of determining the temperature by eliminating the error of self-heating of the thermistor and increasing efficiency by reducing the number of measurement operations.

The problem is achieved in that a minimum current is passed through a semiconductor thermistor placed in a controlled environment and its resistance is measured, the current is increased, which causes additional heating of the thermistor relative to the controlled medium, and the second resistance of the thermistor is measured, characterized in that two resistance measured at the minimum current for two temperature ranges specified by the boundaries, determine the parameters of the temperature characteristic of the thermistor (maximum resistance phenomenon and constant temperature) for the minimum current, according to it, for the measured resistance of the thermistor, determine the temperature of the controlled medium, which is compared to the second resistance of the thermistor, in advance, at the second current and maximum temperature, measure the third resistance, which is taken as the norm, according to the normalized and second resistance of the thermistor and the corresponding they determine the temperature parameters of the operating temperature characteristics of the thermistor, which determine the temperature of the controller medium when measuring the resistance of a thermistor at an increased current.

The thermistor is a parametric sensor - with a change in temperature, it changes its resistance. To measure the resistance of the thermistor, it is included in the circuit for generating an electrical signal — a controlled source of stabilized current (Fig. 1). When a current is passed through a thermistor, it self-heats up - an increase in its temperature relative to the temperature of the medium being measured. The value of self-heating depends on the power dissipated on the thermistor and the parameters of heat exchange with the environment, which are determined by the properties of the medium itself and its state (fixed, mobile - in the case of liquid and gaseous media; smooth, rough - for solid materials). Self-heating is the cause of the methodological error. Reducing its value can be achieved by reducing the power dissipated on the thermistor. At the minimum current, the difference between the self-heating value of the thermistor in various media is insignificant, however, the sensitivity of converting resistance to voltage will be insufficient for reliable recording of small temperature changes. To increase the sensitivity of converting resistance to voltage, the current value must be increased. The resistance of semiconductor thermistors strongly depends on the magnitude of the flowing current, which does not allow the temperature characteristic of the thermistor to be used (the dependence of its resistance on temperature or the dependence of temperature on resistance) obtained at a current of one magnitude for currents of another magnitude. In addition, the temperature characteristics obtained at the same currents, but on materials with different properties, also differ (figure 2). The temperature characteristic of semiconductor thermistors has the form

or

where R is the resistance of the thermistor [Ohm], T is the absolute temperature [K], R ₀ is the ultimate resistance corresponding to the resistance of a semiconductor thermistor as T → ∞:

T ₀ - temperature constant, numerically equal to the temperature of the thermistor, at which its resistance takes the value eR ₀ , where e is the base of the natural logarithm.

The essence of the proposed method is illustrated in figures 1-4.

при минимальном токе I₁. Для этого на токе I₁ при двух известных температурах T₀₁ и Т₀₂ измеряют соответствующие им сопротивления R₀₁ и R₀₂. Температуры T₀₁ и Т₀₂ соответствуют нижней и верхней границам диапазона рабочих температур, которые будут измеряться терморезистором. Параметры температурной характеристики

и

находятся решением системы уравнений:The method is organized as follows. The semiconductor thermistor is placed in a controlled environment, the minimum current I _{1 is} passed through it and the first resistance of the thermistor R _{1 is} measured. Increase the current to a value of I ₂ and measure the second resistance of the thermistor R ₂ . The resistance value R ₁ practically does not depend on the properties of the medium being measured due to the small current I ₁ , and the resistance value R ₂ will be different for each controlled medium. To account for the self-heating of the thermistor at the preliminary stage, it is calibrated - obtaining its temperature characteristic

at minimum current I ₁ . To do this, the current I ₁ at two known temperatures T ₀₁ and T ₀₂ measure the corresponding resistance R ₀₁ and R ₀₂ . Temperatures T ₀₁ and T ₀₂ correspond to the lower and upper limits of the operating temperature range that will be measured by the thermistor. Temperature Characteristics

and

are a solution to the system of equations:

и постоянной температуры

имеют вид:Design dependencies for ultimate resistance

and constant temperature

have the form:

Thus obtained temperature characteristic T ⁰ (R) (figure 2, curve 1) is suitable for any materials and media, however, the sensitivity of the conversion of resistance to voltage is insufficient to measure small changes in temperature.

The resistance R ₁ and the temperature characteristic T ⁰ (R) determine the temperature of the medium T ₁ , which is compared with the resistance R ₂ .

The difference in temperature characteristics obtained with an increased current in various media decreases with increasing temperature (Fig. 2, curves 2). This allows you to choose a certain temperature T *, at which the discrepancy between the temperature characteristics of various media is much smaller than in the range of operating temperatures (figure 2). Using this observation, at a current of I ₂ measure the third resistance of the thermistor R * corresponding to the temperature T *. The obtained values of R * and T * are taken as the norm, that is, they say that the resistance R * corresponds to the temperature T * for any controlled environment.

по формулам:The second and third resistances of the thermistor R ₂ and R * and the corresponding temperatures T ₁ and T * receive the parameters R ₀ and T ₀ working temperature characteristics

according to the formulas:

The thus obtained operating temperature characteristic T (R) (FIG. 2, curve 3) takes into account the self-heating value of the thermistor of any controlled medium in which it is located. According to the characteristic T (R), the temperature of the controlled medium is determined when measuring the resistance R of the thermistor at current I ₂ (figure 2).

To assess the effectiveness of the proposed method conducted experimental studies. A semiconductor thermistor type CT1-18 with a nominal resistance of 22 kOhm at 150 ° C was used as a temperature sensor. In a high-accuracy heat chamber of the type TBT-1 at temperatures of 5 and 35 ° С on TF-1 glass, polymethylmethacrylate (PMM) and RIPOR, the resistance of the thermistor was measured by measuring the voltage drop on it at stabilized currents I ₁ = 5 and I ₂ = 50 μA . Based on the obtained values of the resistances and the corresponding temperatures according to formulas (4) and (5), the temperature characteristic parameters T ° (R) for current I ₁ and the real temperature characteristics of the thermistor for glass, PMM and RIPOR for current I _{2 were} calculated respectively: T ₀ (R), T ₁ (R), T ₂ (R). As a normalized point, the resistance of the thermistor R * = 45 kOhm obtained at a temperature of T * = 329 K at a current of I _{2 was used} .

Evaluation of the proposed method for accuracy was carried out by the error in determining the absolute temperature values and by the error in determining small temperature changes relative to the initial value. As a comparative method was used to determine the temperature by translating the measured resistance of the thermistor using a single temperature characteristic T ₀ (P) obtained on glass at a current of I ₂ .

The estimation of the error in determining the absolute temperature in the case of using a single temperature characteristic was calculated as the difference between the temperature values obtained from the temperature characteristics for glass and the actual temperature values at the PMM and RIPOR determined by the corresponding calibration characteristics by the formula:

A change in resistance R from 100 to 300 kOhm corresponds to a change in temperature from 35 to 5 ° C.

, ПММ

и РИПОРа

в соответствии с предлагаемым способом от реальных температурных характеристик по формуле:The error in determining the absolute temperature in accordance with the proposed method was determined as the deviation of the operating temperature characteristics obtained for glass

PMM

and RIPOR

in accordance with the proposed method from the actual temperature characteristics by the formula:

Measurements of the resistances R ₁ and R ₂ at currents I ₁ and I ₂ necessary to determine the parameters of the operating temperature characteristics were carried out at temperatures of 10, 20, and 30 ° C. Figure 3 shows the error curves obtained by formulas (8) and (9). The efficiency of the error in the determination of absolute temperatures was determined by the formula:

не превышает 0,06°С. Таким образом, η_T=4...26, то есть предлагаемый способ позволяет определять температуру при увеличенном токе, в среднем, на порядок точнее, по сравнению со способом с единственной градуировочной характеристикой.ΔT _i lies in the range from 0.26 to 1.6 ° C, a

does not exceed 0.06 ° C. Thus, η _T = 4 ... 26, that is, the proposed method allows to determine the temperature at an increased current, on average, an order of magnitude more accurate, compared with the method with a single calibration characteristic.

, соответствующее изменению сопротивления на величину в 20 кОм по формулам:When measuring temperature increments relative to any initial value, neglecting variations in the value of self-heating on various materials also leads to an error due to the non-linearity of the temperature characteristic of the thermistor. The value of this error was estimated as follows. The resistance value was fixed (R _Ф ) and, using real calibration characteristics for PMM and RIPOR, the temperature increment was determined

corresponding to a change in resistance by 20 kOhm according to the formulas:

The error in determining the value of superheat in a method with a single temperature characteristic was determined as the relative deviation of the superheat values obtained at the PMM and RIPOR with respect to the superheat obtained on the glass:

, соответствующий изменению сопротивления на 20 кОм по рабочим градуировочным характеристикам, рассчитывали по формуле (11). Погрешность предлагаемого способа оценивали как относительное отклонение перегрева, полученного по рабочей характеристике материала, по отношению к перегреву, полученному по реальной температурной характеристике того же материала, по формуле:Overheat

corresponding to a change in resistance by 20 kOhm according to the working calibration characteristics was calculated by the formula (11). The error of the proposed method was evaluated as the relative deviation of the superheat obtained by the working characteristic of the material, in relation to the superheat obtained by the real temperature characteristic of the same material, according to the formula:

The results of calculating the errors in determining overheating by formulas (12) and (13) are shown in Fig. 4. A comparative assessment of the proposed method for accuracy in determining the amount of overheating was carried out according to the formula

не превышает 0,3%. Таким образом, η_ΔT=3...13, то есть предлагаемый метод позволяет определять перегрев относительно начальной температуры при увеличенном токе точнее, в среднем, на порядок, по сравнению со способом с единственной градуировочной характеристикой.δT _i lies in the range from 1 to 4%, a

does not exceed 0.3%. Thus, η _ΔT = 3 ... 13, that is, the proposed method allows us to determine the superheat relative to the initial temperature at increased current more accurately, on average, by an order of magnitude, compared with the method with a single calibration characteristic.

Evaluation of the proposed method for performance was carried out according to the formula:

, где n - число необходимых ступенчатых изменений тока. Как показали опыты, число n получается не менее 10 (τ₁≥10τ₀), в то время как для предлагаемого способа достаточно всего одного изменения тока, то есть τ₂=τ₀. Длительность процесса градуировки терморезистора в расчет не включена, так как она выполняется однократно. Таким образом, предлагаемый способ не менее чем в 10 раз превосходит по оперативности способ-прототип.where τ ₁ is the duration of the temperature determination process in accordance with the prototype method, τ ₂ is the duration of the temperature determination in accordance with the proposed method. Durations τ ₁ and τ _{2 are the} sum of the durations of elementary operations of measuring the resistance of a thermistor. The duration of the elementary operation of measuring resistance consists of the duration of the transition process τ ₀ with a step change in current and directly from the duration of the process of measuring resistance. The latter, due to its smallness in comparison with the transition process, can be neglected. Therefore, the duration of the measurement process in accordance with the prototype method can be written as

where n is the number of necessary stepwise current changes. As experiments have shown, the number n is obtained at least 10 (τ ₁ ≥10τ ₀ ), while for the proposed method, only one change in current is sufficient, i.e., τ ₂ = τ ₀ . The duration of the calibration process of the thermistor is not included in the calculation, since it is performed once. Thus, the proposed method is at least 10 times faster than the prototype method.

The proposed method is implemented in a measuring and computing system for determining the thermophysical properties (thermal conductivity and thermal diffusivity) of solid materials “TEMP-075” and allowed to reduce the error in measuring absolute temperatures to 0.05 ° C, and the error in measuring temperature increments relative to the initial to 0.3% , while the use of a single temperature characteristic led to an error of up to 1.6 ° C when determining absolute temperatures and 4% when measuring overheating.

Thus, the proposed method for determining the temperature, including determining the operating temperature characteristics of the thermistor, taking into account the value of self-heating of the thermistor by current, unlike the known solutions, allows one to increase the accuracy and efficiency of determining the temperature by an order of magnitude. The application of the proposed method in devices for determining the thermophysical properties of various materials and media, as well as other devices requiring accurate temperature measurements, can improve their metrological characteristics.

Claims (1)

The method of determining the temperature, which consists in passing a current through a semiconductor thermistor located in a controlled environment and measuring its resistance, additionally heating the thermistor relative to the controlled medium by current and measuring the second resistance of the thermistor, characterized in that it is based on two resistances measured at a minimum current for two specified range boundaries temperature, determine the parameters of the temperature characteristic of the thermistor (ultimate resistance and constant temperature) for the minimum current, the temperature of the controlled medium is determined from it for the measured resistance of the thermistor, which is compared with the second resistance of the thermistor, the third resistance is measured in advance at the second current and maximum temperature, which is taken as the norm according to the normalized resistance and resistance of the additionally heated thermistor and the corresponding the temperatures determine the parameters of the operating temperature characteristics of the thermistor, which determine the temperature controlled environment when measuring the resistance of a thermistor on a second current.

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