RU2035705C1 - Temperature measurement technique - Google Patents

Abstract

FIELD: temperature measurement in moving and static media. SUBSTANCE: two identical resistance thermometers are installed in medium under measurement and electric power supplied to them for controlling their temperature is measured. Temperature of each thermometer is regulated by supplied electric power at different levels, electric power supplied to each resistance thermometer is measured, respectively, and temperature of medium under measurement is calculated from equation given in description of invention. EFFECT: enlarged functional capabilities. 1 dwg

Description

 The invention relates to techniques for measuring the temperatures of mobile and stationary media with resistance thermal converters, including semiconductor ones.

A known method of compensating for the temperature error of resistance thermometers, which consists in limiting the magnitude of the measuring current. The reduction in temperature error in the known method is achieved due to the fact that the electrical power supplied to the resistance thermometer is kept constant regardless of the change in its resistance in the range of measured temperatures [1]
The disadvantages of the method of measuring temperature should include the formation of measurement errors caused by changes in the heat transfer conditions between the resistance thermometer and the measured medium when the flow rate changes, since the heat transfer coefficient substantially depends on the flow rate of the measured medium.

The closest in technical essence is a method of measuring temperature with a resistance thermometer, which consists in regulating (controlling) the power supplied to it. The use of a thermometer in a wide temperature range while ensuring a given accuracy is achieved by maintaining a constant temperature difference between the resistance thermometer and the environment [2]
The disadvantages of the method include the influence of the properties of the thermometric substance (the spread of static characteristics, deviation from linearity, instability, etc.) on the temperature measurement. The source of a significant error in the temperature measurement is also the outflow of heat through the housing of the resistance thermometer caused by the temperature difference between the temperature of the resistance thermometer R _θ directly and the temperature of the medium being measured θ _cf. Significant disadvantages of the method should also include a component of the error in temperature measurement caused by a change in the flow rate G of the measured medium, which violates the steady state heat transfer.

Indeed, according to the known method for measuring the temperature θ _cf of the medium being measured, it is determined by the power P _θ supplied to the resistance thermometer R _θ , so that the difference θ-θ _cp over a wide range of temperatures θ _cp and the flow rate G of the measured medium remains constant. However, the expression P _θ = K _t xF = (θ-θ _cf ) realized in this case is valid if the surface area of the resistance thermometer Φ is constant and K _{t is} the convective heat transfer coefficient (heat transfer). As it is well known [3] the convective heat transfer coefficient of water (K _{r) tm} ^at 20 ° C in a pipe 300 mm in diameter varies from 370 to 7500 W / (m ² K) for changing a water flow rate of 0.1 to 4 m / with.

Similarly, the heat transfer coefficient of air (K _t) varies _taken ^at 20 ° C in a pipe 400 mm in diameter from 4.3 to 93 W / (m ² K) for changing the air flow rate of 1 to 50 m / s. Therefore, the scope of the known method of measuring temperature with resistance thermometers is limited by the ability to measure the temperature of stationary media.

 The purpose of the invention is to increase the accuracy of measurement and to provide the ability to measure the temperature of a moving flow of the measured medium at variable mass flow rates.

(1)
Исследование известных в науке и технике решений показало, что известен метод двух теплоприемников (термометров сопротивления) [4]
Однако, в отличие от известных методов двух теплоприемников, согласно которым измеряют температуру теплоприемника по зависимости его сопротивления от температуры, в предложенном техническом решении исключается влияние метрологических свойств термометрического вещества, а температура среды определяется при фиксированных температурах термометров сопротивления по отношению подводимых электрических мощностей. Т.е. температуру не измеряют, а стабилизируют и измеряют мощность, подводимую к термометрам сопротивления для их самонагрева до заданного значения температуры.To achieve the goal in the method of measuring temperature, in which a resistance thermometer is installed in the medium to be measured and the electric power supplied to it is changed to control the temperature of the resistance thermometer, in the immediate vicinity of the resistance thermometer, an additional identical resistance thermometer is installed in the measured medium and the supplied electric power is changed to control its temperature, supplied by electric power stabilize the temperature of each thermometer with resistance at different levels θ _{1 and} θ _2, respectively, measure the electrical power supplied to each resistance thermometer, P ₁ and P ₂ , respectively, and the temperature θ _cf of the medium being measured is calculated from the relation:
θ _avg =

(1)
A study of the solutions known in science and technology showed that the method of two heat receivers (resistance thermometers) is known [4]
However, in contrast to the well-known methods of two heat sinks, according to which the temperature of the heat sink is measured by the dependence of its resistance on temperature, the proposed technical solution excludes the influence of the metrological properties of the thermometric substance, and the temperature of the medium is determined at fixed temperatures of the resistance thermometers in relation to the supplied electrical powers. Those. they do not measure the temperature, but stabilize and measure the power supplied to the resistance thermometers for their self-heating to a predetermined temperature value.

 The positive effect is associated with an increase in the measurement accuracy by reducing the temperature difference between the measured medium and the resistance thermometer, by reducing the heat outflow along the heat receiver.

(1+K_c(θ-θ_o), (2) где R

сопротивление термометра сопротивления при температуре θ_o;
К_с температурный коэффициент сопротивления.Another important property associated with the achieved positive effect is the independence of the measurement results from the properties of the metrological characteristics of the thermometric substance. Traditionally, the temperature θ _cf is determined by the change in resistance R _{θ of the} resistance thermometer
R _θ = R

(1 + K _c (θ-θ _o ), (2) where R

resistance of a resistance thermometer at a temperature θ _o ;
K _with temperature coefficient of resistance.

 In this case, the nonlinearity, instability and scatter of the nominal metrological characteristics of the resistance thermometer are converted directly to the measurement error.

The proposed technical solution measures the electrical power P ₁ and P ₂ supplied to resistance thermometers. They are associated with the temperature of the medium θ _cf and the intrinsic temperatures of the resistance thermometers with the following relationships:
P ₁ = K _t xFx (θ ₁ -θ _sr ); (3)
P ₂ = K _t xFx (θ ₂ -θ _sr ) (4)
Dependencies (3) and (4) used in this technical solution are based on a functional relationship between the amount of heat generated by the measuring current (self-heating current) when applying electric power to resistance thermometers and their temperatures, θ ₁ and θ _2, respectively.

 The most important positive effect associated with the implementation of this technical solution is that the temperature measurement method implements an absolute temperature thermodynamic scale (ATTS) in degrees Kelvin.

(5) является уравнением АТТШ, которое не зависит от свойств термометрического вещества, а также не зависит от свойств измеряемой среды (сокращается К_т) и от конструктивных параметров термометра сопротивления (сокращается Ф). Поскольку параметр К_т функционально связан со скоростью потока, то его сокращение исключает влияние изменения расхода измеряемой среды на показания прибора (результат измерения температуры). Поскольку оба термометра сопротивления идентичные и расположены в непосредственной близости, в одинаковых условиях, можно ожидать одинаковую адгезию, что при сокращении Ф исключает влияние налипания частиц измеряемого вещества на показания прибора.Indeed, the basic equation (1), implemented in the method, is obtained by dividing the expression (3) by the expression (4). As a result of this division, K _t (convective heat transfer coefficient) and Ф (surface area of the resistance thermometer) are reduced. The original equation obtained in this way

(5) is the ATTS equation, which does not depend on the properties of the thermometric substance, and also does not depend on the properties of the medium being measured (shortened K _t ) and on the design parameters of the resistance thermometer (shortened Ф). Since the parameter K _{t is} functionally related to the flow rate, its reduction eliminates the influence of a change in the flow rate of the measured medium on the instrument readings (temperature measurement result). Since both resistance thermometers are identical and are located in close proximity, under the same conditions, the same adhesion can be expected, which, when Φ is reduced, excludes the influence of sticking of particles of the measured substance on the readings of the device.

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

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сопротивления. В плечи 5 и 6 сравнения резисторных мостов 1 и 2 включены термонезависимые резисторы R₁ и R₂, величины которых найдены в соответствии с номинальной статической характеристикой (НСХ) и выбраны равными соответственно сопротивлениям R

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термометров сопротивления в установившемся режиме при заданных температурах стабилизации θ₁иθ₂. Резисторные мосты 1 и 2 содержат цепи 7 обратной связи, имеющие последовательно включенные усилитель 8 разбаланса резисторных мостов, генератор 9 управляемой частоты, формирователь 10 биполярных импульсов тока и аттенюатор 11 импульсного сигнала. Таким образом, резисторные мосты 1 и 2 охвачены контуром обратной связи, содержащим замкнутую цепь 7 обратной связи, образованную подключением входов усилителей 8 к измерительным диагоналям а-b и выходов аттенюаторов 11 к диагоналям питания резисторных мостов 1 и 2 соответственно. Резисторные мосты 1 и 2 совместно с цепью 7 обратной связи образуют системы 12 стабилизации температур θ₁иθ₂ термометров сопротивления R

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, каждый из которых одновременно является и нагревателем и датчиком температуры. Для гальванической развязки устройство оснащено диодами 13. Питание резисторных мостов может быть как автономным, только по контуру обратной связи, так и совмещенным, при котором к диагоналям питания c-d присоединены параллельно цепь 7 обратной связи и общий (на оба резисторных моста) источник 14 питания установочного напряжения, подключенный посредством переменного резистора 15 и ограничительного резистора 16. Под установочным напряжением в схеме понимается напряжение, необходимое для приведения резисторных мостов в равновесие при нулевых начальных условиях, при нулевом расходе и максимальной температуре измеряемой среды. Генератор 9 импульсов выдает сигналы, пропорциональные величине электрической мощности, подводимой к термометрам R

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соответственно. Генератор 9 имеет две пары параллельно соединенных выходов, к одной из которых подключен формирователь 10 биполярных импульсов, а к другой функциональный блок 17 отношения. Выход функционального блока 17 отношений пропорционален отношению Р₁/Р₂ электрических мощностей, подведенных к термометрам R

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соответственно, и подключен к вычислительному блоку 18, в котором реализован алгоритм вычисления по формуле (1). Выход вычислительного блока 18 соединен с показывающим прибором 19.The device contains two resistor bridges 1 and 2, in the working arms 3 and 4 of which thermometers R are included

and R

resistance. The shoulders 5 and 6 are comparison resistor bridges 1 and 2 are included termonezavisimye resistors R ₁ and R _2, the values of which are found in accordance with the nominal static characteristic (NSH) and respectively chosen equal resistances R

and R

resistance thermometers in steady state at given stabilization temperatures θ _{1 and} θ ₂ . Resistor bridges 1 and 2 contain feedback circuits 7 having a series-connected amplifier 8 of imbalance of resistor bridges, a controlled frequency generator 9, a bipolar current pulse shaper 10, and a pulse signal attenuator 11. Thus, the resistor bridges 1 and 2 are covered by a feedback loop containing a closed feedback circuit 7 formed by connecting the inputs of amplifiers 8 to the measuring diagonals a-b and the outputs of the attenuators 11 to the power diagonals of resistor bridges 1 and 2, respectively. Resistor bridges 1 and 2 together with the feedback circuit 7 form a system of 12 temperature stabilization θ _{1 and} θ ₂ resistance thermometers R

and R

, each of which is simultaneously a heater and a temperature sensor. For galvanic isolation, the device is equipped with diodes 13. The power supply of the resistor bridges can be either autonomous, only along the feedback loop, or combined, in which the feedback circuit 7 and a common (on both resistor bridges) installation power supply 14 are connected to the cd diagonals voltage connected via an alternating resistor 15 and a limiting resistor 16. Under the installation voltage in the circuit refers to the voltage required to bring the resistor bridges into equilibrium at zero initial conditions, at zero flow rate and maximum temperature of the measured medium. The pulse generator 9 generates signals proportional to the amount of electric power supplied to the thermometers R

and R

respectively. The generator 9 has two pairs of parallel connected outputs, one of which is connected to the shaper 10 of the bipolar pulses, and to the other functional unit 17 of the relationship. The output of the relationship function block 17 is proportional to the ratio P ₁ / P _{2 of the} electrical capacities supplied to the thermometers R

and R

respectively, and connected to the computing unit 18, which implements the calculation algorithm according to the formula (1). The output of the computing unit 18 is connected to the indicating device 19.

 A device for measuring temperature is as follows.

, включенного в рабочее плечо 3 резисторного моста 1. Резисторный мост 2 также со своей системой 12 стабилизации поддерживает постоянство температуры θ₂ термометрa R

, включенного в рабочее плечо 4. Стабилизация обеспечивается в широком диапазоне внешних возмущающих факторов, в том числе при изменении расхода G и температуры θ_ср измеряемой среды.The temperature stabilization system 12 maintains a constant temperature θ _{1 of the} thermometer R

included in the working arm 3 of the resistor bridge 1. The resistor bridge 2 also with its stabilization system 12 maintains a constant temperature θ _{2 of the} thermometer R

included in the working arm 4. Stabilization is ensured in a wide range of external disturbing factors, including when the flow rate G and temperature θ _{cf of the} measured medium are changed.

 The device can work with a source 14 of the installation voltage, or without it, in stand-alone power mode. The combined power mode has strength advantages, since in this mode the stabilization system 12 operates in a mode of compensating for small deviations of the relative predetermined value.

 In the drawing, a combined power mode is considered, in which a feedback circuit 7 and a common installation voltage source 14 are connected in parallel in the diagonal of the cd power supply through the regulation resistors 15 and the limiting resistors 16. Before starting operation, a preliminary setting is provided, during which the resistor bridges 1 and 2 are brought into equilibrium with the help of resistors 15 due to the power supply 14 with the feedback circuit 7 disconnected, at zero flow rate of the medium being measured. When steady state is reached, the voltage at the output of amplifier 8 in the feedback circuit 7 is set to zero, after which the feedback circuit 7 can be connected to the cd diagonal for normal operation of the resistor bridges 1 and 2, respectively. Further balancing of the resistor bridges 1 and 2 is carried out automatically by electric signals in the feedback circuit 7.

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выбраны одинаковыми и установлены на одном участке трубопровода с одинаковыми температурными и гидродинамическими характеристиками измеряемой среды.Thermometers R

and R

selected identical and installed on the same section of the pipeline with the same temperature and hydrodynamic characteristics of the medium being measured.

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в рабочих плечах 3 и 4 резисторных мостов 1 и 2. Изменение сопротивлений рабочих плеч 3 и 4 приводит к нарушению равновесия резисторных мостов 1 и 2, которое компенсируется приращениями подводимых электрических мощностей Р₁ и Р₂ за счет изменения сигналов управления в цепи обратной связи 7 систем 12 стабилизации температур θ₁иθ₂термометров сопротивлений R

,R

.When the temperature θ _{cf of the} measured medium changes, the heat transfer conditions between the measured medium and resistance thermometers R

and R

in the working arms 3 and 4 of the resistor bridges 1 and 2. A change in the resistances of the working arms 3 and 4 leads to an imbalance in the resistor bridges 1 and 2, which is compensated by increments of the supplied electric powers P ₁ and P ₂ due to changes in the control signals in the feedback circuit 7 systems 12 stabilization temperature θ _{1 and} θ ₂ resistance thermometers R

, R

.

F (6) где Е в аналоговом (непрерывном) режиме работы уровень сигнала напряжения в цепи обратной связи, в дискретном (импульсном) режиме работы амплитуда импульса;
Т, Т_и период подачи импульсов, частота подачи импульсов соответственно;
F 1/T частота подачи питающих импульсов.The method of measuring temperature is based on the fact that between the magnitude of the electric power supplied to the resistance thermometer and the control signals in the feedback circuit 7, there is a well-defined relationship, which in both continuous and pulsed systems can be represented by the following expression:
P

F (6) where E in the analog (continuous) mode of operation, the level of the voltage signal in the feedback circuit, in the discrete (pulsed) mode of operation, the amplitude of the pulse;
T, T _{and the} period of the pulse, the frequency of the pulse, respectively;
F 1 / T feed frequency.

 From the above expression (6) it follows that the control in the feedback circuit 7 can be carried out by the following three methods.

 Analog control mode.

If in expression (6) we take T T _and (the duration of the supply pulse and the period of the pulse supply are equal to each other), and by the value of E we mean the level of the voltage signal in the feedback circuit 7, then expression (6) is reduced to:
P = E ² / R _θ (7) i.e. equation (6) is a general form of expression relating the electric power supplied to a resistance thermometer with its resistance R _θ
Frequency control mode.

F, (8) где постоянная величина
K

=E²T/R_θ.(9)
Широтно-импульсный режим управления.In this mode, maintain the constant amplitude E and the duration of the supply pulse T _and . Stabilization _{θ and} R _{θ is} achieved by changing the frequency F of the pulse in the feedback circuit 7. In this case, the expression
P = K

F, (8) where is a constant
K

= E ² T / R _θ . (9)
Pulse width control mode.

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достигается за счет изменения длительности питающего импульса Т_и. В системе 12 стабилизации реализуется выражение
P=K

Т_и,(10) где K

=E²˙F/R_θ(11)
Любой из рассмотренных методов реализации обратной связи может быть использован для реализации устройства по предложенному способу измерения температуры. В приведенном примере технической реализации рассмотрен частотный вариант, согласно которому приращение температуры δθ_сризмеряемой среды вызывает пропорциональное приращение δ_P подводящей электрической мощности и соответствующее приращение частоты в цепи 7 обратной связи.In this mode, a constant signal level E (amplitude of the supply pulse) and signal (frequency of the supply pulse) is maintained. Stabilization of temperatures θ ₁ and θ ₂ and resistances R

and R

achieved by changing the duration of the supply pulse T _and . The stabilization system 12 implements the expression
P = K

T _and , (10) where K

= E ² ˙F / R _θ (11)
Any of the considered methods for implementing feedback can be used to implement the device according to the proposed method for measuring temperature. In the above technical implementation example, a frequency variant is considered, according to which the temperature increment δθ _{sr of the} measured medium causes a proportional increment δ _{P of the} supply electric power and the corresponding frequency increment in the feedback circuit 7.

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)соответственно). Чтобы обеспечить согласование обоих резисторных мостов в рабочем режиме и взаимозаменяемость, необходимую при разбросе номинальных статических характеристик (особенно в случае использования полупроводниковых и тонкопленочных термометров сопротивления), между формирователем 10 биполярных импульсов и диагоналями c-d питания резисторных мостов 1 и 2 устанавливают аттенюаторы 11, которые используют для регулировки амплитуды импульсов питания. Для исключения аддитивной составляющей погрешности в цепях c-d мостов 1 и 2 в цепь 7 обратной связи вводят формирователь 10 биполярных импульсов. Генератор 9 управляемой частоты имеет два параллельно соединенных выхода. Один из них используется в цепи 7 обратной связи и соединен с входом формирователя 10. Кроме того, учитывая выражение (8), согласно которому частота F на выходе генератора 9 пропорциональна электрической мощности Р, подводимой к термометру сопротивления, генератор 9 используется как измерительный преобразователь мощности термометра сопротивления, в связи с чем вторые выходы генераторов 9 подключены к входам функционального блока 17 отношений, выходной сигнал которого пропорционален отношению Р₁/Р₂электрических мощностей, подводимых к термометрам сопротивления R

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. Выходы блока 17 отношений подключены к входам вычислительного блока 18, в котором согласно выражению (1) рассчитывается измеряемая температура θ_ср среды, которая отображается показывающим прибором 19.The device in this case works as follows. A change in temperature θ _cp leads to an imbalance of the resistor bridges 1 and 2. In both stabilization systems 12, an amplified unbalance signal is fed through an amplifier 8 to the input of the controlled frequency generator 9. The unbalance voltage of the resistor bridge (respectively 1 and 2) increases the frequency F of the generator 9 by δF to a value at which, under the influence of rectangular pulses entering the resistor bridge from the shaper 10 of the bipolar pulses through the attenuator 11, the equilibrium state of the resistor bridge occurs. Thus, the imbalance signal of the resistor bridge (1 and 2, respectively) leads to a change in δF of the frequency of the generator 9, and, consequently, to a change in δ _{P of the} power supplied to the resistance thermometer R _θ , which causes it to warm up to a given temperature θ _{1 and} θ _2, respectively . Thus, the resistor bridges 1 and 2 with a change in the measured temperature θ _sr come to a new balanced state with new values of the supply frequency and with new values of the electric power P _θ supplied to the resistance thermometers R _o (R

and R

)respectively). In order to ensure matching of both resistor bridges in the operating mode and the interchangeability necessary when scattering the nominal static characteristics (especially in the case of using semiconductor and thin-film resistance thermometers), attenuators 11 are installed between the bipolar pulse generator 10 and the cd diagonals of the power supply of resistor bridges 1 and 2, which use to adjust the amplitude of the power pulses. To exclude the additive component of the error in the cd circuits of the bridges 1 and 2, a bipolar pulse shaper 10 is introduced into the feedback circuit 7. The controlled frequency generator 9 has two outputs connected in parallel. One of them is used in the feedback circuit 7 and is connected to the input of the driver 10. In addition, taking into account expression (8), according to which the frequency F at the output of the generator 9 is proportional to the electric power P supplied to the resistance thermometer, the generator 9 is used as a measuring power converter resistance thermometer, in connection with which the second outputs of the generators 9 are connected to the inputs of the functional block 17 of the relations, the output signal of which is proportional to the ratio P ₁ / P _{2 of the} electric power supplied to the therm resistance meters R

and R

. The outputs of the relationship unit 17 are connected to the inputs of the computing unit 18, in which, according to expression (1), the measured temperature θ _{cf of the} medium is calculated, which is displayed by the indicating device 19.

The present method eliminates the influence of the coefficient K _{t of} convective heat transfer, Ф of the effective surface area of the resistance thermometer, thereby eliminating the effect of changes in flow rate and adhesion of the heat receiver on the readings of the device.

 When implementing the method, an absolute temperature thermodynamic scale (ATTS) is implemented, which is not associated with the metrological properties of a thermometric substance. Thus, the method allows temperature measurement using highly sensitive semiconductor and thin film sensors, which are characterized by low metrological characteristics.

 The advantage of the method is its high speed due to the coverage of the resistance thermometer with deep negative feedback.

 A device for implementing the method can be made entirely of typical blocks of complexes "CASCADE" or AKZSR.

 When implementing the device and method based on the Cascade complex, a regulating device of the P25 type can be used as a stabilization unit, and computational operations can be implemented using function blocks A04, A31, A33. All of these blocks have a normalized output signal that can be used for remote input into a computer.

Claims (1)

A method of measuring temperature, in which a resistance thermometer is installed in the medium to be measured and the electric power supplied to it is changed to control the temperature of the resistance thermometer, characterized in that, in the immediate vicinity of the resistance thermometer, an additional identical resistance thermometer is installed in the medium to be measured and the supplied electrical power is changed for control its temperature, supplied electric power stabilize the temperature of each thermometer rotations at different levels θ ₁ and θ _2, respectively, measure the electric powers supplied to each resistance thermometer, respectively, P ₁ and P ₂ , and the temperature θ _{cp of the} measured medium is calculated from the relation

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