RU2198384C2 - Method of temperature measurement with use of temperature-sensitive resistor - Google Patents

Abstract

FIELD: electric temperature measurement. SUBSTANCE: there is proposed method of temperature measurement of object by change of specific resistance of material of object without resistance coupling or thermal contact of sensor with measurement object. EFFECT: increased accuracy and productivity of measurement of temperature of object. 4 dwg

Description

 The invention relates to methods for electrical temperature measurements and can be used as a means of non-destructive temperature control of the metal walls of vessels, or metal products, both stationary and in motion. In particular, the invention can be used to control the processes of oil and gas transport of gases or liquids in closed vessels by measuring temperature on characteristic sections of the walls of these vessels.

 Known methods of non-destructive temperature measurement, which include pyrometric measurement methods [L.1, p. 273-280], based on the registration of electromagnetic radiation excited by the thermal motion of atoms and molecules that make up the objects of measurement. But the length of the electromagnetic wave of thermal radiation is about 1.0 ~ 5.0 μm, therefore, the main disadvantage of the analogue is the dependence of the readings on the actual state of the surface of the measurement object. It is enough to have a small screen in the form of a protective film, moisture or pollution, as this method is practically unsuitable for measurement.

As a prototype, a thermoresistive method was chosen [L.1, p. 260-265], based on the dependence of the specific resistance of the material on temperature. For metals, this dependence is expressed by the formula:
ρ _t = ρ ₀ (1 + α • Δt ^° ), (1)
where ρ _t is the resistivity of the metal at the desired temperature;
ρ ₀ is the specific resistance of the same metal at a known temperature t ₀ ⁰ (reference point);
α is the temperature coefficient of resistance of the material (TCS);
Δt ^o = T = T ₀ = t ^o -t ₀ ^o
Moreover, for most metals, TCS is approximately the same and amounts to α ≅ 4 • 10 ^-3 1 / T (the region from the end of the superconducting state to the melting temperature).

при этой температуре, которую и принимают за искомую. Температуру вычисляют по соотношению:

которое следует из (1), если заменить удельное сопротивление на интегральное.In the prototype, a coil of copper conductor was used as a temperature sensor. In this case, measure the resistance of this coil (R _{cat · 0} ) at a known temperature (t ₀ ^o ), then bring the coil into contact with the measurement object. Further, they withstand some time sufficient to ensure that the coil temperature is almost equal to the temperature of the object, and measure the resistance of the coil

at this temperature, which is taken as the desired one. The temperature is calculated by the ratio:

which follows from (1), if we replace the resistivity with the integral.

 The disadvantage of the prototype is the lack of measurement accuracy and its inertia due to the fact that the temperature of the sensor is measured, and not the object itself. Due to the fact that the sensor material (in this example, a copper wire) must have electrical insulation, thermal resistance inevitably arises between the sensor and the object, which depends on the size and quality of the surfaces of their contact and on the thermal diffusivity of the electrical insulation. It is possible to use this method of measuring temperature if the thermal resistance is constant, i.e. the sensor with the object should be a single design and, in addition, the temperature of the object should not change during the measurement.

где α - температурный коэффициент сопротивления материала стенки объекта измерения;
t₀ ^o - известная температура объекта;
R₀ - активное сопротивление катушки при известной температуре;

- активное сопротивление катушки при искомой температуре;
R₁ - сопротивление катушки до помещения ее рядом с объектом измерения.The technical result that can be obtained by implementing the proposed method is to increase the accuracy and productivity of measuring the temperature of the object. The specified technical result is achieved due to the fact that in the method of thermoresistive temperature measurement, the active resistance of a multi-turn coil is pre-measured, which is then placed next to the object, an alternating current is supplied to the coil with a frequency that penetrates the electromagnetic wave into the metal wall of the object to a depth equal to or less wall thickness of the object, measure the active resistance of the coil at a known temperature of the object, and then the same resistance at the desired temperature and you calculate the desired temperature by the formula:

where α is the temperature coefficient of resistance of the material of the wall of the measurement object;
t ₀ ^o is the known temperature of the object;
R ₀ is the active resistance of the coil at a known temperature;

- active resistance of the coil at the desired temperature;
R ₁ - the resistance of the coil before placing it next to the measurement object.

 The combination of these essential features is necessary and sufficient to achieve the specified technical result: an increase in the accuracy and productivity of measuring the performance of an object. Namely, in the proposed method, the sensor (coil) with the measurement object has a direct electromagnetic connection, due to which the active resistance of the coil changes (increases). This new coil resistance changes with the temperature of the object. In this case, thermal contact (as far as possible) should be excluded.

The principle of the proposed method is illustrated in figure 1. In this figure, the numbers indicate:
1. Inductor (inductor);
2. The object of study;
3. Power supply with alternating current with frequency ω;
4. δ is the gap between the coil and the surface of the investigated object.

From the point of view of electrical engineering, an inductor located close to the conductive surface of an object can be represented by an equivalent equivalent circuit as an “air” (that is, without a closed magnetic circuit) transformer (Fig. 1b), the primary winding of which is characterized by electrical parameters:
R ₁ is the active resistance of the coil;
L ₁ is the inductance.

The secondary winding of this transformer is a loop of induced current, which is closed to itself. This round is characterized by:
R ₂ - active resistance;
L ₂ - inductance.

 M is the coefficient of mutual induction of the inductor and the induced ("virtual") coil with current.

- активное сопротивление индуктора.Considering the equivalent equivalent circuit (Fig. 1b) as a two-terminal device with respect to the input terminals a and b, we obtain:

- active resistance of the inductor.

где μ₀ и μ - соответственно абсолютная и относительная магнитная проницаемость.In the theory of induction heating, based on the solution of Maxwell's equations, it is shown [L. 2, p. 13-31], that if an alternating (sinusoidal) current flows through the inductor, then the penetration depth of the electromagnetic wave into the metal is determined by the formula:

where μ ₀ and μ - respectively, the absolute and relative magnetic permeability.

.The relative magnetic permeability, with the exception of ferromagnets, does not depend on the magnitude of the induced current and is very accurate to 1. It is also shown there that in conductive media the magnetic field H lags in phase from the electric field E by an angle π / 4, i.e. . specific resistivity is expressed by the ratio:
z = (1 + j) • ρ / h,
Where

.

Если индуктор представляет собой многовитковую катушку и ее ширина существенно больше зазора δ, то в уравнении (4') R₁ оказывается существенно меньшим сопротивления виртуального витка "приведенного" к зажимам индуктора, т.е.

Например, для "нагруженного" индуктора, геометрические размеры которого показаны на фиг.2 и имеющего число витков w=60 при частоте тока f=80 кГц и удельном сопротивлении металла трубки ρ₀=4,4•10^-6 Ом•см, экспериментальное значение R_инд=5,63 Ом при R₁=0,73 Ом. Заметим, что R₂=4,86•10^-3 Ом (расчетное значение). Коэффициент приведения активного сопротивления виртуального витка с током ко входу индуктора в данном случае составляет 4,90: 4,86^-3= 1008. Из этого следует, что многовитковая катушка (индуктор) представляет собой своеобразный "микроскоп", позволяющий проанализировать (с точки зрения изменения сопротивления) область объекта, находящегося под индуктором. Это и является физической предпосылкой предлагаемого метода измерения температуры.Passing from resistivities to integral ones, it can be argued that z ₂ = R ₂ + jωL ₂ and R ₂ = ωL ₂ , i.e. the active resistance of the considered area of the object is equal to its inductive resistance. Given this position, equation (4) is simplified:

If the inductor is a multi-turn coil and its width is significantly greater than the gap δ, then in equation (4 ') R ₁ turns out to be significantly less than the resistance of the virtual coil "reduced" to the terminals of the inductor, i.e.

For example, for a “loaded” inductor, the geometrical dimensions of which are shown in FIG. 2 and having the number of turns w = 60 at a current frequency of f = 80 kHz and a specific resistance of the tube metal ρ ₀ = 4.4 • 10 ^-6 Ohm • cm, experimental the value of R _ind = 5.63 Ohms with R ₁ = 0.73 Ohms. Note that R ₂ = 4.86 • 10 ^-3 Ohms (calculated value). The coefficient of reduction of the active resistance of a virtual coil with current to the input of the inductor in this case is 4.90: 4.86 ^-3 = 1008. It follows from this that the multi-turn coil (inductor) is a kind of "microscope" that allows analyzing (from the point of view of resistance changes) the area of the object under the inductor. This is the physical premise of the proposed method of measuring temperature.

с учетом (1) и (5) температурная зависимость выразится соотношением

где

эквивалентное сопротивление виртуального витка при температуре t₀ ^o (опорная точка).If the length of the virtual turn L, its width m, and the depth h, then

taking into account (1) and (5), the temperature dependence is expressed by the relation

Where

equivalent resistance of a virtual coil at a temperature of t ₀ ^o (reference point).

(6) в (4'), находим температурную зависимость активного сопротивления индуктора:

где

- приведенное к зажимам индуктора активное сопротивление виртуального витка при температуре в опорной точке.Substituting Values

(6) in (4 '), we find the temperature dependence of the active resistance of the inductor:

Where

- the resistance of the virtual coil reduced to the terminals of the inductor at a temperature at the reference point.

При этом условии активное сопротивление нагруженного индуктора при температуре в опорной точке
R₀ = R₁ + R'₂₀, (9)
то же сопротивление при искомой температуре t^o
t^o = t₀ ^o + Δt^o, (10)

Из решения уравнений (9), (10) и (11) и следует приведенная выше формула (3) для вычисления искомой температуры.To measure the active resistance of a loaded inductor, it is advisable to turn on a capacitor of such a value of capacitance C _comp in order to compensate for the inductance of the loaded inductor, i.e. it is necessary to tune this system to resonance:

Under this condition, the active resistance of the loaded inductor at a temperature at the reference point
R ₀ = R ₁ + R _'20, (9)
the same resistance at the desired temperature t ^o
t ^o = t ₀ ^o + Δt ^o , (10)

From the solution of equations (9), (10) and (11), the above formula (3) follows to calculate the desired temperature.

As a specific example of the implementation of the proposed method, in FIG. Figure 2a schematically shows an experimental setup for measuring the temperature of a tube with an outer diameter = 14 mm and with a wall thickness Δ = 1.0 mm. The specific resistance of the tube material at a temperature of 20 ^o C ρ ₀ = 4.4 • 10 ^-6 Ohm • see A coil with a width of 8.0 mm and 60 turns was used as an inductor. The inductance of this coil (measured) is 65.4 μH. From formula (5) it follows that the frequency should be at least 27.3 • 10 ^-3 Hz, at t ₀ ^o = 350 ^o C. In this case, the frequency is 80 kHz. The gap between the inductor and the tube is δ = 1.0 mm. The wall temperature was measured independently by a thermocouple temperature sensor 5, the hot junction of which is tightly pressed against the inside of the tube wall.

The electrical circuit of the experimental setup is shown in FIG. 2b. The value of the capacitor 6 is determined by the relation (8); as a recording device, a C8-9 digital storage oscilloscope (item 7) is used, which has 2 recording channels. Information on the voltage U _{ind was} recorded on the first channel, and the current through the inductor I _ind , taken using the current transformer 8, was recorded on the channel 8. The resistance of the inductor R _{t was} determined as the ratio of U _ind to I _ind at the moment t _{0 of} the current amplitude value (Fig. 2c) . In FIG. 3 shows the dependence of the active resistance of the inductor on temperature in the range from 20 to 300 ^o C, which can serve as a calibration curve for determining the temperature of the installation shown in FIG. 2. In the presented case, the error of temperature measurement is not worse than 2% at the end of the measurement range.

It should be noted that information about the temperature of an object arrives in a time equal to one period of current oscillation. In this case, for 12.5 μs, therefore, the proposed measurement method can be considered practically inertialess. For comparison, in FIG. Figure 4 shows the experimentally measured temperature dependence of the active resistance of the same coil from a copper wire, that is, the well-known thermoresistive method for measuring temperature is reproduced. Coil resistance was measured with an E7-8 type instrument. The dotted line in the same figure shows the dependence R _cat = f (t ⁰ ), constructed under the assumption that the sensor ideally perceives the temperature of the object, i.e. the thermal resistance between the tube and the sensor is zero. As can be seen from the graphs, the measurement error can reach 250% at the end of the measurement range.

 Based on the foregoing, we can conclude that the implementation of the proposed method for measuring temperature provides the specified technical result, namely: improving the accuracy and productivity of measuring the temperature of the object.

 Comparative analysis with an analogue and prototype showed that the inventive method does not have identical in combination of features that are key to the new "Method of thermoresistive temperature measurement", and meets the criteria of the invention of novelty. Comparison of the proposed method not only with the prototype, but also with other known methods of measuring temperature allows us to conclude that the set of features is sufficient to solve a scientific and technical problem in the framework of the goal.

Literature:
1. S.A. Spectrum. "Electrical measurements of physical quantities. Measurement methods." L .: Energoatomizdat, 1987.

 2. V.S. Nemkov, V.B. Demidovich. "Theory and calculation of induction heating devices." L .: Energoatomizdat, 1988.

Claims (1)

The method of thermoresistive measurement of the temperature of the object, which consists in measuring the active resistance of a multi-turn coil, which is placed next to the measurement object, characterized in that an alternating current is supplied to the coil with a frequency that penetrates the electromagnetic wave into the metal wall of the object to a depth equal to or less wall thickness of the object, measure the active resistance of the coil at a known temperature of the object, and then the same resistance at the desired temperature and calculate the desired rate formula

where α is the temperature coefficient of resistance of the material of the measurement object;
t ⁰ ₀ is the known temperature of the object;
R ₀ is the active resistance of the coil at a known temperature;

- active resistance of the coil at the desired temperature;
R ₁ - the resistance of the coil before placing it next to the measurement object.

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