UA70166A - Moisture meter with thermocoulpes - Google Patents

Abstract

The proposed moisture meter with thermocouples contains two thermocouples with a common junction, two automatic switches, two differential amplifiers, a phase-sensitive rectifier, a low-pass filter, and a digital voltmeter, which are connected in series, and additionally, a high-frequency direct voltage source, a milliammeter, a variable resistor, a switch, and controlled direct-voltage source.

Description

Description of the invention

The invention belongs to the field of analysis of the composition of gaseous substances and can be used to control the humidity of air and other gaseous media using electrothermal means (heating thermocouple).

In some cases, there is a need to control the unevenness of the humidity field in heterogeneous materials and products. For this purpose, humidity measurement is carried out at different points of the controlled volume, followed by calculation of the gradient of the humidity field. Such problems arise when studying the distribution of moisture in special clothes made of multilayer fabrics, in multilayer electrical insulation, in composite materials, as well as when analyzing the structure of food products. The large dimensions of the existing humidity sensors do not allow measuring humidity in small volumes, and even more so, in individual points of space.

It is most convenient in such cases to use the working end of a heated thermocouple, which consists of two wire electrodes made of different thermoelectric materials, as a sensitive element of the sensor.

The small dimensions of the sensitive element of the thermocouple and the combination of not only the heat receiver, but also the electric heater in one element, allows you to control humidity in small volumes and hard-to-reach places. The working end is heated by electric current, which is passed through the thermocouple electrodes using the Peltier electrothermal effect in the working junction and the Joule electrothermal effect in the thermoelectrodes (see Korotkov P.A., London G.E. Dynamic contact measurements of thermal quantities - L. Mashinostroenie. 1974. - P.163-164).

At the same power of heating or cooling of the working end of the thermocouple, its thermoelectromotive force (thermoEMF) functionally depends on the heat transfer to the controlled environment, and therefore on the humidity of this environment. The temperature of the monitored environment is determined by the thermoEgERS thermocouple in the absence of additional heating or cooling, and the humidity is determined with additional heating or cooling according to the calibration curve that was taken for the measured temperature (see Thermoelectric devices 29 control / V.P. Hondyul, D. B. Golovko, Yu. O. Skrypnyk, G. I. Khimicheva, L. O. Glazkov. - Kyiv: Lybid, 1994. - P. 66-68). "

The well-known thermocouple hygrometer (see High-precision measurements with multifunctional thermosensors: a study guide / D.B. Golovko, V.O. Dubrovnyi, Yu.O. Skrypnyk, G.I. Khimicheva. - Lybid, 2000, - P.235-237) , in which two thermocouples are used for electrical separation of the heater and thermocouple circuits, while the splice heater and splice meter are either located next to each other or welded to each other. o 30 However, a thermocouple hygrometer, in which the working ends (junctions) are located next to each other, has a low sensitivity Ge) due to the large thermal resistance of air or electrical insulation. When welding the working ends (joints) to each other, the sensitivity of the hygrometer becomes high, but due to the finite value of the resistance of the common working end -- (joint) the electric current from the thermocouple-heater penetrates into the circuit of the measuring thermocouple-receiver, which causes an error due to displacement characteristics of a measuring thermocouple with an additional voltage. 325 A thermocouple hygrometer is also known (see RF Patent M 2014590 MPK 501 M25/56, 1991, Byul. Mo 11, 1994), which contains two electrodes made of different thermoelectric materials, connected by their middles, which form the common working end of the first and second thermocouples, two automatic switches, two differential amplifiers, a phase-sensitive rectifier and a low-pass filter connected in series, a digital voltmeter.

In addition, the circuit of the known hygrometer includes two voltage dividers, a third differential amplifier, C 50 a second low-pass filter and an adjustable broadband amplifier, the input of which is connected to the output of the first differential amplifier, the output of which is connected to one of the inputs of the second differential amplifier, the output of which is connected through the second low-pass filter to the control input of the broadband amplifier, and the second inputs of the second and third differential amplifiers are connected to the constant voltage source through voltage dividers. 45 Due to the need for alternating heating and cooling of the common working end of two thermocouples, period 7 of switching the current direction through the common working end is chosen to be greater than the thermal constant of the thermocouple, av | which causes the low speed of the hygrometer (switching frequency of the Hz unit). The presence of a galvanic connection of two thermocouples through the common working end causes a parasitic passage of an alternating current - heating-cooling at the input of the first differential amplifier, which distorts the amplitude of the variable component b 20 of the voltage, which is proportional to the humidity of the monitored medium. Thus, the well-known thermocouple hygrometer has low speed and low accuracy in point control of humidity and analysis of unevenness of humidity fields.

The basis of the invention is the task of creating such a thermocouple hygrometer, in which the introduction of new elements and connections would ensure an increase in speed and accuracy against humidity.

The problem is solved by the fact that in a thermocouple hygrometer, which contains two electrodes from different of thermoelectric materials connected by their middles, which form the common working end of the first and second thermocouples, two automatic switches, two differential amplifiers, a phase-sensitive rectifier and a low-pass filter connected in series, a digital voltmeter, according to the invention, an alternating voltage generator of increased frequency are introduced , a milliammeter, a rheostat and a key, two copper blocks and a source of regulated 60 direct current voltage, while the free ends of the first thermocouple are connected through a milliammeter, a rheostat, and a key to the paraphase outputs of the high-frequency alternating voltage generator, the free ends of the second thermocouple are placed on copper blocks that are connected to the inputs of automatic switches of different names, the inputs of the same name are connected to each other, the outputs of the automatic switches are connected to the inputs of the first differential amplifier, the output of which is connected to the input of the phase-sensitive rectifier, the control inputs of which are connected to because the paraphase outputs of the alternating voltage generator are increased th frequency, each of which is connected to the control input of the first and second automatic switches, the output of the low-pass filter is connected to one input of the second differential amplifier, the second input of which is connected to a source of regulated DC voltage, and the output is connected with a digital voltmeter input.

The introduction of a high-frequency alternating voltage generator, a milliammeter, a rheostat, a key, two copper pads and a source of regulated direct current voltage into the circuit of a thermocouple hygrometer, included with other elements in the specified manner, provides additional heating of the common working end of two thermocouples by the alternating current of the first thermocouple and the formation of a constant voltage current, proportional to humidity, in the second thermocouple on the background of parasitic AC voltage. Periodic switching of voltages at the output of the second thermocouple with /o frequency of the high-frequency generator by automatic switches turns the constant information voltage into an alternating one, and the parasitic alternating voltage into a constant one. The following amplification and phase-sensitive rectification ensure the selection of the information component of the voltage in the form of a constant voltage, which is measured by a voltmeter, and the parasitic voltage in the form of an alternating voltage is suppressed by a low-pass filter. As a result, it is possible to significantly increase the frequency of changing the direction of the current through the common working end and exclude the influence of /5 galvanic connection of two thermocouples on the informational component of the alternating voltage, which ensures high speed of the thermocouple hygrometer and the accuracy of humidity control.

The drawing shows the electrical circuit of a thermocouple hygrometer.

The hygrometer circuit contains a generator 1 of variable voltage of increased frequency, to the paraphase outputs of which, through a milliammeter 2, a rheostat З and a key 4, wire electrodes 5 and b thermocouples of various 2o thermoelectric materials are connected. Electrodes 5 and 6 are connected by their middles, forming a common working end 7 of two thermocouples (5, 6, 7 and 8, 9, 7). The free ends of the first thermocouple are connected to generator 1, and the free ends of the second thermocouple are connected to copper blocks 8 and 9. One pair of the same-named inputs of the automatic switches 11 is connected to the copper block 8, the second pair of the same-named inputs is connected to the copper block 9. The outputs of the automatic switches 10 and 11 are connected to the inputs of the differential amplifier 12, to the output of which the phase-sensitive rectifier 13 and the low-pass filter 14 are connected in series. One input of the differential amplifier 15 is connected to the output of the low-pass filter 14, the second input is connected to the source 16 of regulated DC voltage. A digital voltmeter 17 is connected to the output of the differential amplifier 15. The control inputs of the automatic switches 10 and 11 are connected to the paraphase outputs of the high-frequency generator 1, and the control inputs of the phase-sensitive rectifier 13 are connected to both outputs of this generator. Ge

З The experimental environment is marked with item 18.

A thermocouple hygrometer works like this. X,

The common working end 7 of two thermocouples is placed in one of the points of the test medium 18. Through -/- electrodes 5 and b of the first thermocouple and the common working end 7, an alternating current passes from the generator 1 of alternating voltage of increased frequency. In one half-cycle (DC-1/27) of alternating current through the working end of 7 o

Зз5 current flows in the direction in which it is cooled due to the absorption of Peltier heat. what

At the same time, Joule heat is released in the electrodes of the first thermocouple and its working end, which reduces the cooling temperature. Taking this into account, the temperature of the working end of the thermocouple, which is determined by the temperature T of the test medium, decreases only to the value: "voi os) g Plakta Z s s where: I is the strength of the current flowing through the thermocouple; "» p - Peltier coefficient of thermoelectric materials of electrodes 5 and 6; " K - total resistance of electrodes 5 and 6 of the thermocouple and its working end 7;

Ki is a coefficient that takes into account the fraction of Joule's heat that flows from electrodes 5 and 6 to the working end 7; 9 - the heat transfer coefficient of the working end 7, determined by the humidity of the environment under investigation; - E is the plane of the heat transfer surface. o In the next half-period (di-1/23) of alternating current, when the direction of current flow changes to the opposite, as a result of the summation of Peltier and Joule heat, the temperature of the working end 7 rises - to the value:

F "7 t, one hundred CVE yo

If you choose a period of alternating current that is much less than the thermal time constant of their thermocouple (i1/), then the heating temperature of the working end is averaged: 2 tu 8 A (third t B 9 v ho o i.e. will be determined only by Joule's heat.

As we can see from expression (3), the constant overheating of the working end 7 relative to the temperature of the experimental environment will be determined by the heat transfer coefficient 52, i.e. controlled humidity. Because the measurement of humidity is carried out in two stages.

Initially, the key 4 is open and the common working end 7 is not heated. Since the copper blocks 8 and 9 are located at the temperature of the surrounding medium, then the thermoEMF on the blocks of the free ends of the second thermocouple:

ET st -To3, (8 b5 where there is - the thermoEMF coefficient 5 and 6 from different thermoelectric materials (Seebeck coefficient).

Automatic switches 10 and 11 are switched with the frequency of generator 1 and periodically change the polarity of the thermoelectric power source (4) relative to the inputs of the differential amplifier 12. As a result, the differential amplifier 12 amplifies the alternating voltage of a rectangular shape: 500 ШЙ СТ - Toovidpvipoyai 5) where vdpvip 2diyi - envelope of alternating sign voltage .

The amplified voltage is supplied to the phase-sensitive rectifier 13, which is controlled by the variable voltage of the high-frequency generator 1. The rectified voltage is smoothed by the low-pass filter 14 and through the differential amplifier 15 is fed to the digital voltmeter 17. In the absence of a compensating voltage (0-0) at the second 70 input of the differential amplifier 15, the voltage measured by the voltmeter

This is Koka To, (6) where Ko is the common amplification factor of differential amplifiers 12 and 15;

Kz - the rectification coefficient of the phase-sensitive rectifier 13, taking into account the transmission coefficient of the filter 14 of lower frequencies.

From expression (6), we see that the measuring voltage 0" is proportional to the temperature T of the medium under study.

At the second stage, the key 4 is closed and the alternating current of the generator 1 of its frequency is used to overheat the common working end 7 of the first and second thermocouples with Joule's heat relative to the temperature T of the medium under study. ThermoEeeERS of the free ends of the second thermocouple, taking into account the temperature To of the free ends, which is stabilized by copper pads 8 and 9, takes the value: Kivi? (7)

Ez ee -- --.)- That's INK! 2 SE and where K' is the resistance of the common working end 7.

In expression (7), the second term represents an obstacle in the form of a voltage drop at the common working end from the alternating heating current:

Ivya tt zipizyai - ri, (8) where Id is the amplitude of the heating alternating current;

Ф is the phase shift caused by the thermal inertia of the thermocouple. (Se)

Automatic switches 10 and 11, working with the frequency Her, convert the constant component of the voltage (7) into the variable voltage of this frequency: no (8) -

Tsar -ee T--- - - - That | Izidizipoyaly z se p Tsya o

З The second component of voltage (7) takes the form: c. and g. (10 t zip (slj - fividp vp llj Psovosov-sov 2(2lj- fi) po)

From expressions 9 and 10, we see that the first and second components of the voltage (7) after the commutation transformation with the frequency It are spread over frequencies. The information component of the voltage (9) has a frequency of Her, and the interference (10) has a doubled frequency of 2Her. After phase-sensitive rectification, the information component (9) s is converted into a direct current voltage, which passes the low-pass filter 14 and is measured with a voltmeter 17. The interference voltage (10) is converted into an alternating voltage of the switching frequency and is attenuated by the low-pass filter 14. Thus, the constant voltage that is measured takes the form: 2 (11)

At the end of the first stage, after measuring the temperature T with a digital voltmeter 17, the output voltage of the differential amplifier 15 is compensated by the voltage of the source 16. Therefore, at the end of the second stage, the measured voltage, taking into account the compensation of the component voltage (b ) in voltage (11) becomes equal to:

Ge) in (12)

What is EC? Ka --

F o

To determine the humidity of the environment under investigation, the thermocouple hygrometer is subject to calibration.

The working end of the thermocouple 7 is placed in an air environment with 10095 humidity at a given temperature. Such

Humidity corresponds to the dew point, which is easy to control. Calibration and measurement of humidity are carried out at the same overheating power of the common working end, which is set by milliammeter 2 using » reohord 3. During calibration, the minimum reading of voltmeter 17 is fixed: 2, (13)

CI

Shk che eK Koka - - - b 1K2k3 se bo de "ad o - heat transfer coefficient of the working end 7 at 10095 relative humidity of the gas medium ( po? a).

To determine the relative humidity in the environment under investigation, at the same temperature, the ratio of voltages (12) and (13) is found: b5

By language (14)

Ov, where is the heat transfer coefficient of the medium under study, and She o . (15)

She

When controlling humidity, when the range of humidity changes is small, it can be assumed that the measured relative humidity is a fraction of 10095 humidity: 70 Uk 10096 si6)

M where MU is the relative humidity in 95, which is measured;

CC reading of the voltmeter during calibration;

Y - reading of the voltmeter during humidity control.

Calibration is carried out at different temperatures of the gas, in particular, air medium. In the process of humidity control, the temperature of the environment is first determined in accordance with (6), and then the corresponding value of the calibration voltage ОО у for this temperature is substituted into formula (16). When controlling medium and low humidity, calibration of the hygrometer is carried out at the averaged humidity values of the corresponding ranges.

As we can see from formula (16), the results of the humidity measurement are not affected by the parameters of the heated thermocouple and the parameters of the measuring circuit, which ensures high accuracy of monitoring the gradient of the humidity field.

This is also facilitated by the suppression of the obstacle from the passage of the heating current from the first thermocouple to the second due to the switching conversion of the thermoEMF of the working end into an alternating current voltage. Increasing the conversion frequency to hundreds of Hz significantly increases the speed of the thermocouple hygrometer.

Example. For the manufacture of two thermocouples with a common working end, copper and

Constantan wires with a diameter of 0.5 mm and a length of 100 mm, welded in their middles. The thermal EMF coefficient of the copper-constantan pair is -3.2 mMkV/K. Magnetic control contacts (reed switches) are used as automatic switches, the control windings of which are connected to an alternating voltage generator with a frequency of 250Hz. With the heating current

I-600mMA common working end is overheated by 3-12 9K depending on the humidity of the environment being tested (1006095). The common gain factor of two K-400 differential amplifiers, which (se) provided sensitivity to temperature 0.03-0.052K, and to relative humidity 0.1-0.2905. "co

The distribution of humidity and temperature in a package of three-layer fabric heated to a temperature of 372 (simulator of wet human skin) was investigated. At 10095 humidity in the gap between the water and the first layer of the fabric package (587, the diffusion of water vapor is clearly observed, due to which the humidity in the gap between the first and second layers o decreases to the value of 9296, and between the second and third - to the value of 8395 at the humidity of the surrounding environment

From 70-75965. The thermocouple hygrometer made it possible to detect the profile of the humidity field depending on the composition and density of the fabrics, as well as to compare the hygienic qualities of fabrics made of natural and synthetic fibers. « with 40 eh I - well she Y i rai v. -і u ra -i ry re ІІ svy 4 "shi g s ("in") ? g і -

FO 50 c

F M

5B Fig.

R

Claims (1)

The formula of the invention 60 Thermocouple hygrometer, containing two electrodes of different thermoelectric materials, connected by their middles, which form the common working end of the first and second thermocouples, two automatic switches, two differential amplifiers, a phase-sensitive rectifier and a low-pass filter connected in series, a digital voltmeter , which is distinguished by the fact that a high-frequency alternating voltage generator, a milliammeter, a rheostat and a key, two copper pads and a source of regulated direct current voltage are introduced into it, while 65 The free ends of the first thermocouple are connected through a milliammeter, a rheostat and a key to the paraphase outputs of the high-frequency alternating voltage generator, the free ends of the second thermocouple are placed on copper blocks that are connected to the different-name inputs of automatic switches, the inputs of the same name are connected to each other , the outputs of the automatic switches are connected to the inputs of the first differential amplifier, the output of which is connected to the input of the phase-sensitive rectifier, the control inputs of which are connected to the paraphase outputs of the high-frequency alternating voltage generator, each of which is connected to the control input of the first and second automatic switches, the output of the low-pass filter is connected to one input of the second differential amplifier, the second input of which is connected to a source of regulated DC voltage, and the output is connected to the input of a digital voltmeter. 70 Official Bulletin "Industrial Property". Book 1 "Inventions, useful models, topographies of integrated microcircuits", 2004, M 9, 15.09.2004. State Department of Intellectual Property of the Ministry of Education and Science of Ukraine. " (Se) Zo Te) "- "c) and - - and? -i ("c) - (o) 4) 60 b5