RU2010191C1 - Method of determination of errors of thermoelectric thermometers - Google Patents

Abstract

FIELD: measurement technology. SUBSTANCE: reading α₁ of thermometer is registered, working junction of thermal converter is cooled for time Δt which is less than thermal constant of junction time. Reading α₂ is registered, cooling is stooped and achievement of starting reading α₁ is fixed. Working junction of thermal converter is heated by passing of constant current 1 through thermocouple wires for the coarse of time Δt₀ lesser than thermal constant of conversion time. Reading α₃ of thermometer is registered. After this working junction is cooled again for original time Δt and reading α₄ of thermometer is registered. Value Δ of error of thermometer is found by formula D=α₁-(α₁-α₂)/(α₃+α₂-α₁-α₄)KJΔt₀, where K is thermal coefficient of thermal converter determined in process of calibration. EFFECT: increased accuracy of determination of errors of thermoelectric thermometers under conditions of their continuous use, reduced time of testing. 2 dwg

Description

 The invention relates to temperature measurements by electrical methods and is intended for checking thermoelectric thermometers in the conditions of their operation at existing facilities without dismantling and replacement.

-1

, где t_к - значение температуры контрольного преобразователя;
Δ Е и Δ Е_к - величины приращения термоЭДС дифференциальной термопары и контрольного преобразователя соответственно.A known method of checking TEC [1], in which first connect the electrodes of the control and verified TEC, forming a differential thermocouple, then the control TEC is heated or cooled to a temperature at which the thermoelectric power of the differential thermocouple is zero, and, having determined the temperature of the control transducer, through the differential thermocouple circuit was passed a current pulse, the temperature was raised at its junctions 10-20 ^{° C} and measured at the same time the increment of the differential thermocouple and thermoelectric converter control, and the error converter under test is determined by the formula
Δt = t

-1

where t _to - the temperature value of the control transducer;
Δ E and Δ E _k are the increments of the thermopower of the differential thermocouple and the control transducer, respectively.

The disadvantage of this method is the need for high-precision TEP of the same type, which should work in a remote controlled thermostat in conditions close to the controlled object. In reality, the heat transfer conditions of the working junctions of the verified and control TECs are different, therefore the same pulse flowing through the working junctions of these TECs additionally heat the junctions not by the same value Δ t (10-20 ^о С), but by different values depending on the actual values of the heat transfer coefficients of their working junctions placed in different environments, in addition, high temperatures in the working junction area of the control TEC can also lead to noticeable changes in its calibration characteristics.

, где ε - заданное относительное изменение температуры рабочего спая термопреобразователя [2] .The closest to the proposed technical essence and the achieved effect is a method for determining the errors of thermoelectric thermometers, which consists in forcing the temperature of the working junction of the thermocouple at a constant controlled temperature and comparing the thermometer readings with the actual value of the controlled temperature, while first changing to a steady reading α _{1 of the} thermometer the set value Δ T the temperature of the free ends of the thermocouple and register thermometer reading α ₂ corresponding to the new temperature difference between the working junction and the free ends of the thermocouple, then the temperature of the free ends of the thermocouple is changed to the initial value and then the temperature of the working junction of the thermocouple is forcibly changed and the steady reading of the thermometer α _{3 is} recorded, and the error value Δ of the thermometer is determined by the formula
Δ = α ₁ -

where ε is the specified relative change in the temperature of the working junction of the thermal converter [2].

 The known method allows to determine the error of the thermoelectric thermometer without using the same type of control thermometer with high accuracy.

 However, the known method does not provide high accuracy in determining the error of the thermoelectric converter included in the circuit of the electric thermometer. This is because the coefficient ε is introduced into the calculation formula, which determines the convective heat loss during forced cooling of the working junction by air flow. This coefficient depends both on the parameters of the cooling channel (diameter, wall roughness, etc.) and on the volumetric velocity of the air flow supplied to the working junction. Since these parameters also change during the operation of the thermometer, and the volumetric flow rate is difficult to stabilize for the entire period of operation of the thermometer, errors arise in the value of the coefficient ε, which are difficult to take into account.

 In addition, a change in the temperature of the free ends of the thermocouple by Δ Т leads to various changes in the readings of the thermometer depending on the temperature of the working junctions, i.e., an additional methodical error of verification occurs, and it takes a long time to verify the thermometer using this method, since a steady-state value is recorded temperature of the working junction with additional thermophysical influences.

 The aim of the invention is to increase the accuracy of determining the errors of thermoelectric thermometers with real (non-linear) calibration characteristics in the conditions of their continuous operation and reducing the time spent on verification.

KJΔt_o, где К - термический коэффициент термоэлектрического преобразователя, определяемый в процессе калибровки.The goal is achieved by the fact that in the known method for determining the errors of thermoelectric thermometers, which consists in conducting operations of recording thermometer readings corresponding to the temperature difference of the working junction and the free ends of thermo-formation, followed by a change in the temperature of the working junction of the thermocouple, and determining errors by the results of comparison, according to the invention, additional operations performed in the following sequence: after registration of the thermometer readings α ₁ working the junction of the thermocouple is cooled for a time Δ T less than the thermal constant of the junction time and the readings of the thermometer α ₂ are recorded; stop cooling and record the achievement of the initial reading α ₁ ; then the working junction of the thermocouple is heated by passing direct current I through the thermoelectrodes for a calibrated time Δ t _o shorter than the thermal constant of the conversion of the thermocouple, and read the thermometer α ₃ ; after that, the working junction of the converter is again cooled during the initial time Δ t and the readings of the thermometer α ₄ are recorded; Δ - the error of the thermometer is determined by the formula
Δ = α ₁ -

KJΔt _o , where K is the thermal coefficient of the thermoelectric converter determined during the calibration process.

 In FIG. 1 shows a structural diagram of a device that allows the implementation of the method; in FIG. 2 - dependence of the temperature of the working junction of the thermoelectric converter from additional thermophysical effects.

 The device comprises a thermoelectric converter 1 with an isolated working junction 2, a protective cover 3 and an air channel 4, thermoelectrodes 5, a communication line 6, a bipolar switch 7, a secondary device 8 (millivoltmeter), an adjustable current source 9, a milliammeter 10, a compressor 11, a switch 12 air flow, connecting tubes 13 and stopwatch 14.

 The method is as follows.

Thermoelectric power of an ideal thermoelectric converter (TEC), consisting of homogeneous thermoelectrodes, in accordance with the thermoelectric law is determined by the expression E = S (T _pc - T _ck ), (1) where E is the thermoelectric power of TEP;
S is the sensitivity of the TEC (coefficient of thermopower);
T _RS - the temperature of the working junction;
T _ok - the temperature of the free ends.

 In the ideal TEC equation, the sensitivity value S is the same over the entire length of the TEC thermoelectrodes, which potentially provides a linear calibration characteristic.

During the operation of the TEC, irreversible physicochemical changes occur in its thermoelectrodes, changing the S value along the length of the thermoelectrodes. Moreover, these changes occur in the place of the thermoelectrodes where the destabilizing factor, for example temperature, manifests itself in the greatest way. This place is most often the zone of the working end of the TEC (hot junction zone). On the rest of the thermoelectrodes, the value remains unchanged. For this case, the TEC equation (1) can be represented as E = S _nest T _pc - S T _ck , (2) where S _nst is the sensitivity of the TEC in the area of the working ends (working junction).

- относительное изменение чувствительности рабочего спая ТЭП от дестабилизирующих факторов (погрешность чувствительности).The sensitivity of the TEC in the zone of the working junction can be represented as S _un = S + Δ S = S (1 + j), (3) where j =

- the relative change in the sensitivity of the working junction of the TEC from destabilizing factors (sensitivity error).

The relative change in sensitivity j depends on the value of the measured temperature T _pc . The heterogeneity of thermoelectrodes that appears during operation causes a zero offset of the thermometer, which also depends on the temperature T _pc . Therefore, we write equation (2) of the working TEC taking into account (3) in the form E = S [T _pc (1 + j) - T _ck + δ], (4) where δ is the absolute error of the TEC from the zero offset.

In equation (4), the values of the conversion errors γ and δ correspond to the measured temperature T _pc . Taking into account the errors introduced by the communication line and the secondary device, the thermometer reading can be represented as
α = KS [T _pc (1 + j _α ) -T _ck + δ _α , (5) where K is the coefficient of conversion of thermopower to the thermometer reading, deg / V;
γ _α and δ _α are the total relative multiplicative and absolute additive errors of the thermometer.

First, the switch 7 is set to position 1-1 and the secondary device 8 records the reading α _{1 of the} thermometer being verified, measuring the temperature T _x = T _pc1 in the active (hot) object (Fig. 2)
α ₁ = KS [T _pc1 (1 + γ _α1 ) - -T _cк + δ _α1 ], (6)
Then, the compressor 11 is turned on for a time Δ t ₁ and the TEC working junction is forcedly cooled by the air flow entering junction 2 through channels 4. The time interval Δ t _{1 is} chosen less than the thermal time constant τ from the condition Δ t ₁ = (0.5 - 0 , 7) τ. (7)
The volumetric speed of the air flow is set using the adjuster 12 of the air flow so that the thermometer at the end of the interval Δ t _{1 is} reduced by 3-5 sensitivity thresholds. The duration of the compressor 11 is controlled by the stopwatch 14, after which the reading of the α ₂ thermometer is recorded (Fig. 2, curve 15) α ₂ = KS [T _pc2 (1 + γ _α2 ) -T _ck + δ _α2 ] = KS [(T _pc1 -ΔT ₁ ) (1 + γ _α2 ) -T _ck + δ _α2 ], (8) where Δ T ₁ is the cooling temperature of the working junction relative to the measured temperature T _pc1 ;
γ _α2 and δ _α2 are _TEC conversion errors at temperature T _pc2 .

=

, (9) где Q_к - конвективные потери тепла;
α (v) - коэффициент теплоотдачи спая, определяемый скоростью потока V;
F - поверхность охлаждения спая;
Т_в - температура воздушного потока.The forced cooling process proceeds exponentially. However, when condition (7) is satisfied, the cooling process corresponding to the initial section of the exponent practically proceeds according to the proportional law. Therefore, the cooling temperature of the working junction, taking into account its mass m and specific heat capacity c, can be determined from the expression
ΔT ₁ =

=

, (9) where Q _к are convective heat losses;
α (v) is the heat transfer coefficient of the junction, determined by the flow rate V;
F - junction cooling surface;
T _in - air flow temperature.

T

1-

1+

-T_ck+

(10)
При высокотемпературных измерениях, когда T_pc1 >> Т_в, можно считать, что
α₂= KS[T_pc(1-εΔt₁)x(1+γ_α2)-T_ск+δ_α2] , (11) где ε =

- коэффициент конвективных потерь, пропорциональный скорости (расходу) охлаждающего воздуха.In view of expression (9), the temperature indication α _{2 can be} represented as
α ₂ = KS

T

1-

1+

-T _ck +

(10)
In high-temperature measurements, when T _pc1 >> T _in , we can assume that
α ₂ = KS [T _pc (1-εΔt ₁ ) x (1 + γ _α2 ) -T _ck + δ _α2 ], (11) where ε =

- convective loss coefficient proportional to the speed (flow rate) of cooling air.

After registration, the thermometer α ₂ readings turn off the compressor 11 and the temperature of the working junction of the TEC (Fig. 2, curve 16) after the time interval Δ t ₂ again reaches the initial value, which is recorded by the secondary device 8 of the thermometer.

Next, the switch 7 is set to position 2-2, and a heating current pulse from the source 9 is passed through the thermoelectrodes 5 of the TEC. The pulse duration Δ t ₃ is also chosen from condition (7), and the pulse amplitude is set such that by the end of the pulse the thermometer reading increases by the value of 3-5 thresholds of its sensitivity. The calibrated duration of the heating pulse Δ t ₃ = Δ t _{o is} controlled using a stopwatch 14.

At the end of the pulse, switch 7 is again set to position 1-1 and the reading of α _{3 of the} thermometer is recorded (Fig. 2, curve 17) α ₃ = KS [T _pc4 (1 + γ _α3 ) -T _ck + δ _α3 )] = KS [ (T _pc1 + ΔT ₂ ) (1 + γ _α3 ) -T _ck + δ _α3 ], (12) where T ₄ is the temperature of the overheating of the working junction of the TEC relative to the measured temperature T _pc1 .

, (13) где К - коэффициент Пельтье;
I - ток, протекающий через рабочий спай ТЭП.The process of additional heating of the TEC from the flowing current also occurs exponentially. However, at the selected heating time Δ t _{o the} process is almost linear. In this case, the working junction of 2 TEC is heated mainly due to Peltier heat released directly in force, and the heating of thermoelectrodes is negligible. The junction superheat temperature, by analogy with expression (9), will have the form
ΔT ₂ =

, (13) where K is the Peltier coefficient;
I is the current flowing through the working junction of the TEC.

 The magnitude of the current I is measured by a milliammeter 10.

- термический коэффициент рабочего спая.The reading of the thermometer (12) taking into account the value of the superheat temperature (13) can be represented as:
α ₃ = KS [(T _pc1 + KIΔt _o ) x (1 + γ _α3 ) -T _ck + δ _α3 ], (14) where K =

- thermal coefficient of the working junction.

After registration, the readings of the thermometer α ₃ again turn on the compressor 11 and forcibly cool the working junction during the time interval Δ t ₄ (Fig. 2, curve 18). At the end of the cooling pulse, the α ₄ thermometer is recorded. In accordance with expression (11), the thermometer can be represented as α ₄ = KS [T _pc5 (1 + γ _α4 ) -T _ck + δ _α4 ] = KS [T _pc4 -ΔT ₃ ) (1 + γ _α4 ) -T _ck + δ _α4 = = KS [(T _pc1 + KIΔt _o ) (1-εΔt ₄ ) (1 + γ _α4 ) -T _ck + δ _α4 ], ⁽¹⁵⁾ where γ _α4 and δ _α4 are TEC conversion errors at temperature T _pc5 .

Решение системы уравнений (16) относительно температуры рабочего спая T_pc1 термопреобразователя дает
T

=

KJΔt_o. (17)
При определении измеряемой температуры Т_х = T_pc1 в соответствии с выражением (17) исключается влияние как погрешностей термоэлектрического преобразователя ( γ и δ ), так и непостоянства коэффициента конвективных потерь ε (в выражение (17) коэффициент ε не входит). Последнее позволяет с повышенной точностью определить действительное значение температуры рабочего спая, а следовательно, и измеряемой температуры. Термический коэффициент К, входящий в расчетную формулу (17), определяется электрическими и теплофизическими свойствами термоэлектродов (постоянная Пельтье π и удельная теплоемкость С), а также геометрическими размерами рабочего спая (масса m) и не зависит в переходных режимах от условий теплообмена ТЭП в месте его установки на контролируемом объекте. Поэтому термический коэффициент ТЭП постоянный и его определяют в процессе калибровки. Для этого ТЭП помещают в среду с известной температурой Т_о и выполняют все операции рассмотренного способа. Далее из выражения (17) по известной температуре Т_о определяют значение термического коэффициента
K =

. (18)
В процессе поверки ТЭП достаточно установить требуемое значение нагревающего тока I, длительность импульса Δ t_o и по калиброванному значению коэффициента К определить действительную температуру рабочего спая Т_рс из выражения (17).Since the changes in the temperature of the working junction of the TEC do not exceed several sensitivity thresholds, we can assume that the conversion errors in the vicinity of the working point (T _x = T _pc1 ) are equal, that is, γ _α1 = γ _α2 = γ _α3 = γ _α4 ; δ _α1 = δ _α2 = δ _α3 = δ _α4 . If the duration of the cooling pulses Δt ₁ and Δt _{4 are} chosen equal (Δt ₁ = Δt ₄ ), then the thermometer readings can be represented as a system of equations

The solution of the system of equations (16) regarding the temperature of the working junction T _{pc1 of the} thermal converter gives
T

=

KJΔt _o . (17)
When determining the measured temperature T _x = T _pc1 in accordance with expression (17), the influence of both errors of the thermoelectric transducer (γ and δ) and inconstancy of the convective loss coefficient ε are excluded (coefficient ε is not included in expression (17)). The latter allows one to determine with high accuracy the actual value of the temperature of the working junction, and hence the measured temperature. The thermal coefficient K included in the calculation formula (17) is determined by the electrical and thermophysical properties of the thermoelectrodes (Peltier constant π and specific heat C), as well as by the geometric dimensions of the working junction (mass m) and does not depend on the conditions of heat transfer of the TEC in place in transient conditions its installation on a controlled facility. Therefore, the thermal coefficient of the TEC is constant and it is determined during the calibration process. For this, the TEC is placed in an environment with a known temperature T _o and all the operations of the considered method are performed. Next, from the expression (17) from the known temperature T _about determine the value of the thermal coefficient
K =

. (eighteen)
In the process of checking the TEC, it is enough to establish the required value of the heating current I, the pulse duration Δ t _o, and from the calibrated value of the coefficient K determine the actual temperature of the working junction T _pc from expression (17).

The error of the thermoelectric thermometer is defined as the difference between the thermometer reading α ₁ and the actual value of the temperature of the working junction of the TEC, which is determined from expression (17), i.e.

KJΔt_o (19)
При изменении температуры контролируемой среды до значения Т_х = T_pc2 осуществляют последовательно изложенные операции по дополнительным теплофизическим воздействиям и определяют погрешность термометра Δ₂ и т. д. в различных точках рабочего диапазона температур.Δ ₁ = α ₁ =

KJΔt _o (19)
When the temperature of the controlled medium is changed to a value of T _x = T _pc2 , the operations described above are carried out sequentially for additional thermophysical influences and the error of the thermometer Δ ₂ , etc., is determined at various points in the operating temperature range.

As an example, according to the proposed method, the error of a thermoelectric thermometer with a chromel-kopel thermocouple type ТХК-0179, having a time constant τ = 90 s, is determined. The value of the heating current during the tests was set at 0.6 A, the duration of the heating pulse was 45 s, the working junction of the TEC was cooled by washing it with an air flow at a constant volume velocity for a time of 50 s. Thus for cooling at 3 ° ^C Air flow rate was 50 l / h, a calibrated value of the thermal coefficient determined during calibration at a temperature T = 200 ^{° C} and was 4,339 deg / A ˙s. Reading thermometer placed in the melt between the solid and liquid phases of lead (lead solidification point 327.50 ° ^C), was α ₁ = 331.305 ° ^C. After it cooled thermometer reading ₂ α = 328.104 ° ^C. After achieving the initial value of the thermometer readings α ₁ and additional heating of the working junction of the TEC by the flowing current, the indication of α ₃ was α ₃ = 334,602 ^о С, and after repeated cooling, α ₄ = 330,203 ^о С.

×
Действительное значение составляет
Δ_g= 331,305-327,502=
= 3,803^oC= α₁-T_o
Способ позволил определить погрешность термометра с относительной погрешностью γ
γ =

100 % =

= 5.2 %
Полученное значение относительной погрешности определения вполне допустимо, так как характеризует показания определения погрешности.The error of the thermoelectric thermometer was determined from expression (19) from the readings α ₁ , α ₂ , α ₃ , α ₄ .

×
The actual value is
Δ _g = 331.305-327.502 =
= 3.803 ^o C = α ₁ -T _o
The method allowed to determine the error of the thermometer with a relative error of γ
γ =

100% =

= 5.2%
The obtained value of the relative error of determination is quite acceptable, since it characterizes the readings of the determination of the error.

 The method does not change the temperature of the free ends of the thermocouple during the test, which eliminates the error of verification from the nonlinearity of the calibration characteristics of real TEC. Taking readings of the thermometer in transient conditions reduces the verification time, since it is not necessary to register the steady-state thermometer readings. (56) 1. USSR author's certificate N 1173206, cl. G 01 K 15/00, 1985.

 2. Copyright certificate of the USSR N 1397752, cl. G 01 K 15/00, 1986.

Claims (1)

METHOD FOR DETERMINING THE ERRORS thermoelectric thermometer comprising the result of our opepatsy pegistpatsii indications thermometry corresponding paznosti Temperature pabochih junction and the free ends tepmoppeobpazovatelya, with subsequent change of temperature pabochih junction tepmoppeobpazovatelya and of determination of reading error of the result of comparison, characterized in that after pegistpatsii readings thermometry α ₁ pabochih junction the thermocouple is cooled for a time Δt less than the thermal constant of the junction time, and the temperature is recorded pmometpa α ₂ ppekpaschayut cooling and fiksipuyut achievement pepvonachalnogo readings α _1, then nagpevayut pabochih junction tepmoppeobpazovatelya by ppopuskaniya chepez tepmoelektpody DC I for kalibpovannogo vpemeni Δt _o, a smaller thermal time constant vpemeni the converting tepmoppeobpazovatelya and pegistpipuyut reading thermometry α _3, whereupon pabochih the converter junction is again cooled during the initial time Δt and the thermometer α _{4 is} recorded, and the error value Δ of the thermometer is determined by the formula
Δ = α ₁ -

K · I · Δt _o ,
where K is the thermal coefficient of the thermoelectric converter determined during the calibration process.

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