SU1723511A1 - Method of measuring relative humidity - Google Patents

Abstract

The invention relates to a measurement technique, in particular to a remote measurement of the humidity of air or gases. The essence of the invention lies in the fact that air is sampled by a fan, heated in a recuperative heat exchanger, and the psychrometric temperature difference in the heated air sample that blows the psychrometer is determined. The barometric pressure of air and the partial pressure of water vapor are determined before and after heating the sample. The psychrometric temperature difference in a heated or cooled air sample is measured at a dry thermometer temperature of 40 ° C and above and a positive excess air pressure at the psychrometer, which measures and determines the relative humidity p of the air from the following relationship: p P1n - A ( B + Pc) (tcH - tM) / P2H, where PIH is the partial pressure of saturated water vapor at temperature IM; A is a constant psychrometric coefficient; B is barometric pressure; Рс - excess pressure in the measuring chamber; ten, tM is the air temperature in the measuring chamber along the dry and wet perimeter; Р2н is the pressure of saturated water vapor at a temperature tc in the object under study. 3 il. WITH S

Description

The method relates to a measurement technique, in particular to remote measurement of air humidity or gases using a psychrometer.

In a measurement technique for measuring air humidity, a psychrometric method is known for determining the relative humidity of atmospheric air at barometric pressure. A psychrometer containing dry and wet thermometers installed in the investigated volume of air, which washes the sensitive elements of the thermometers due to natural or forced convection, is used by the psychrometric method to measure the relative humidity of the air. The wet bulb of a wet thermometer is usually wetted with a cambric wick from a tank of water (continuously) or with a pipette (periodically) before each measurement of the psychrometric temperature difference. The water tank is installed in the air volume itself, and the water level in it is regulated by a level control, moved 1-1.5 m to a convenient place for maintenance.

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The disadvantages of this method of measuring the relative humidity of the air are: a limited range of air temperatures at which water is in the liquid phase; The measurement accuracy is unsatisfactory at air temperatures close to 0 ° C below, due to small values of the psychrometric temperature difference between dry and wet thermometers, equal to several tenths of a degree, which are commensurate with the measurement error of the thermometer and, due to small partial pressures of water vapor, commensurate with the measurement error of the thermometer, and, due to small partial pressures of water vapor, commensurate with the measurement error of the barometric air pressure lnymi barometers equal to 0.3 mbar (or 0.225 mm Hg, st.3,06 mm VS, 30.02 Pa); the inability to measure the relative humidity of air at temperatures above 10b ° C, as the water in the tank of the wet thermometer boils; the inability to accurately remotely measure the relative humidity of the air due to the complexity of the devices for adjusting the water level in the wet thermometer tank, and humidifying the air with water overflowing from this tank (especially in mobile refrigeration or drying chambers) supplied to a dry thermometer with circulating convection currents.

The closest to the technical essence of the solution is a method of indirect determination of the relative humidity of air having a negative temperature, psychrometric method.

In this method of measuring the relative humidity of the air in the test volume, a sample is taken from it, which is sucked by the fan through the heater and measuring chamber. It measures the psychrometric temperature difference with a dry and wet thermometer. The air pressure in the heat exchanger and in the measuring chamber is lower than atmospheric by the amount of vacuum created by the fan, which is comparable with the vapor pressure at negative temperatures. Dry and wet thermometers are washed by a common stream of air passing through the measuring chamber. Therefore, in the latter, the transfer from the water reservoir at the wet thermometer to the dry thermometer occurs, causing an underestimation of the readings of the dry thermometer, and consequently, creates an additional error of measurement of the relative humidity

The measurement of the psychrometric temperature difference in the measuring chamber is carried out at any given value of the temperature of the wet thermometer. but

the value of this value is not indicated, and therefore the measurement errors of the relative humidity of the air at each new setpoint temperature of the wet thermometer will have their own, different from the other value. Moreover, the lower the set value of the temperature of the wet thermometer, the higher the measurement error of the relative humidity of the air (FIG. 3).

Using this method, the relative humidity of the air in the test volume (duct) having a negative temperature is determined by the formula

Ј PlH-AB (tc.H.-tM) / P2H,

where p is the relative humidity of the air in the investigated volume;

PtH is the elasticity of saturated water vapor at the measured value of m in a measuring chamber;

A is a psychrometric coefficient having a constant value at a given speed of blowing thermometers with air; .

B is the observed barometric pressure;

tc.H., tM is the air temperature measured by dry and wet thermometers in the measuring chamber;

Ran - the elasticity of saturated water

steam at the air temperature measured by a dry thermometer in the investigated volume (duct).

The device for measuring the relative humidity of the air contains two dry

thermometer. One wet thermometer, sampling duct, electric heater for heating the air sample, fan, measuring chamber and tank with water for wetting the wick on a wet thermometer, installed inside the measuring chamber.

The disadvantage of this method is that it does not provide high accuracy of remote measurement

relative humidity of air (especially at temperatures close to 0 ° C) and does not allow measuring air humidity at temperatures of 100 ° C and higher. In addition, the measurement error of small and large

The relative humidity values of the air in this method are not the same. They can differ by almost an order of magnitude and increase with decreasing air temperature, at which the psychrometric difference is measured by the psychrometer. This is because, firstly, the formula for calculating the relative humidity of air is not the vacuum in the measuring chamber in which the psychrometer is installed, although the magnitude of this vacuum is comparable to the elasticity of water vapor in the air at positive temperatures, close to 0 ° C; second, because the air supplied to the dry thermometer is moistened with water vapor from the water tank and the wick transported with convective circulation currents, resulting in a decrease in the temperature of the dry thermometer itself and the psychrometric temperature difference, and therefore, the measured value of the relative humidity of the air is overestimated and, thirdly, because the known method of measuring the relative humidity of the air allows the measurement of the psychrometric temperature difference in the measuring chamber at any positive (specified) temperature values air through the wet thermometer i. with different measurement errors that, at temperatures close to 0 ° C and low relative air humidity, reach ± 7% or more.

The purpose of the invention is to improve the accuracy of remote measurement of the relative humidity of the air both at negative temperatures and at temperatures above 100 ° C, and at a pressure different from atmospheric.

This goal is achieved by the fact that in the method of measuring the relative humidity of the air, which includes taking a sample of air by a fan, heating it in a recuperative heat exchanger, measuring the psychrometric temperature difference between a dry and a wet thermometer in a measuring chamber, the temperature of a dry thermometer in a test volume, barometric pressure of atmospheric air and partial pressures of water vapor before and after heating the sample; in the measuring chamber, the fan maintains a positive overpressure PC, which The gauge is measured and entered into the formula to calculate the relative humidity of the air as a correction to barometric pressure (B + Pc), and the air temperature using a dry thermometer entering the measuring chamber is adjusted with a heater or cooler in the temperature range providing the specified error measuring relative air humidity. To determine the lower temperature of the dry thermometer,

in which a predetermined measurement error of relative air humidity is provided, preliminarily for a minimum measured value of relative humidity, the dependence of its measurement error on temperature (Fig. 3) is found, from which a lower temperature value of a dry thermometer is determined for a predetermined error value. The upper value of the temperature range of a dry thermometer is the boiling point of water at operating pressure (B + PC). The measurement error of the relative humidity of the air decreases to ± 0.5-2%. The temperatures are measured in the following order.

Air temperature is measured first.

in the studied volume, and after a period

enough time to pass

samples from it to the measuring chamber,

psychrometric difference measured

temperatures.

The relative humidity of the air in the investigated volume (duct), measured by the proposed method with regard to the static pressure in the measuring chamber, is calculated by the formula

(р Р1н - А (В + PC) (ten. - tM) / P2H, where (р- relative humidity of air in the studied volume;

PIH is the elasticity of saturated water vapor at the measured value of tM in the measuring chamber;

A is a psychrometric coefficient having a constant value at a given speed of blowing thermometers with air;

B is barometric pressure; PC - static overpressure of air in the measuring chamber, measured by a pressure gauge; ..

i.p. Dm - air temperature measured by dry and wet thermometers in the measuring chamber;

Р2н is the elasticity of saturated water vapor at the air temperature measured by a dry thermometer in the volume under study (duct).

The goal is achieved by the fact that in a device for measuring relative humidity of air, containing two dry and one wet thermometers, a fan, a heater and a measuring chamber with a water tank to wet the wick of a wet thermometer, a fan is installed in front of the measuring chamber and creates a positive The excess pressure, measured by a manometer, for cooling air samples having a temperature of 100 ° C and higher, additionally has an air cooler, and instead of a dry and wet thermometer, The Assman aspiration psychrometer is installed in the chamber, which provides for blowing in thermometers at a speed of 2 m / s and above. At the same time, a partition between the dry and wet thermometers is installed in the measuring chamber, a water tank for wetting the wet thermometer is removed from the measuring chamber, and a wick for wetting the wet thermometer ball is laid in a V-shaped tube connected at one end to the water tank.

The proposed techniques for producing remote measurements of the relative humidity of air in the temperature range of a dry thermometer, providing a predetermined measurement error, and positive overpressure in a measuring chamber having a partition and Asman's psychrometer, provide, according to the method, a decrease in measurement error, and an additional air cooler expands confidence the limits of measurement of the psychrometric method and thus the attainment of ts. whether the invention.

The remote measurement of the relative humidity of the air by the proposed method is illustrated in the drawing shown in FIG. 1 and 2, and graphs of relative measurement errors f (tc.nj, A Pn / Pn f (tc.n.). D P2n / P2n f {tc) shown in FIG. 3. which are preliminarily determined to determine the desired temperature of the dry thermometer tc.n. at which the psychrometric temperature difference is measured and the specified maximum error of measurement of the minimum value of the relative humidity of the air is ensured.

The shaded area a-b-c-d characterizes possible errors in measuring the relative humidity of the proposed method at tc.n. 40 ° C (obtained in example 1 for a given value of 0.05 when measuring p - 10%), the e-f-b-c region characterizes the possible measurement errors at an uncertain but positive temperature value of the wet thermometer, adopted by the prototype. The measurement errors (Fig. 3) are related by

A p / p ДРп / Рп + ДР2н / Р2н, where Л is a relative measurement error of relative air humidity

(curve 1 for p 100%, curve 2 for p 10%).

And Рп / Рп - relative error of measuring the partial pressure of water

vapors in the studied volume using a psychrometer installed in the measuring chamber (curve 3 for d (p 100%, curve 4 for p 10%);

AP2n / P2n - relative error

measuring the partial pressure of saturated water vapor at a temperature tc in the investigated volume 9 (curve 5);

tc is the temperature according to the dry thermometer in the investigated volume 9.

A device for remotely measuring relative air humidity (Figs. 1 and 2) includes a dry thermometer 1, a wet thermometer 2 with a device 3 for humidifying the sensor. Dry and

wet thermometers are located in a sealed measuring chamber 4 having a longitudinal partition 5 connected by an air line to a fan 6 and heat exchangers 7 and 8, which

connected to the test volume 9 using a remote air duct 10. In the test volume 9, a temperature sensor 11 is installed, connected by remote wires 12 to the secondary

an instrument 13 mounted on a control panel 14. Air pressure is measured by a barometer t5, pressure c. measuring chamber with 4-gauge 16.

In the remote measurement of the relative humidity of the air contained in the test volume 9, an air sample is taken from it by the fan 6 and blown through the remote air duct 10, recuperative heat exchangers 7 and 8 and

a sealed measuring chamber 4, in which the sensors of dry 1 and wet 2 thermometers are located, separated from each other by a partition 5. The speed of blowing the sensors of dry and wet thermometers with air is 2 m / s and higher, provided by a fan 6 and an aspiration fan psychrometer 17.

The temperature of the air in the sealed chamber 4 is controlled by means of heat exchangers 7 and 8 and is maintained between the minimum value

determined previously for a given maximum measurement error, and the boiling point of water at pressure in chamber 4 (Fig. 3, region a-b-c-d).

According to the results of temperature and pressure measurements, the partial pressure of water vapor in the measuring chamber 4 is calculated, which is equal to that in the test chamber.

volume 9, and calculate the desired relative humidity of the air by the formula

p Pp / P2n P1n-A (B + + Pc) (tc.n. -tM) / P2H,

where p is the relative humidity of the air in the investigated volume 9;

Рп is the partial pressure of water vapor in the investigated volume 9 at tc, equal to the partial pressure of water vapor in the measuring chamber 4 at tc.n .;

Р2н is the pressure of saturated water vapor at a temperature tc in the investigated volume 9 ;.

Pin is the pressure of saturated water vapor at temperature tM;

A- constant psychrometric coefficient;

B - barometric pressure of atmospheric air;

Рс - static overpressure in chamber 4;

tc.n., tM is the air temperature in chamber 4 using dry and wet thermometers.

PRI me R. The method was implemented by measuring the relative humidity of a gas in an isothermal nitrogen-cooled car after preliminary testing under laboratory conditions.

The measuring chamber has a longitudinal partition and dimensions of 150 x 100 x 80 mm with an Assman aspiration psychrometer attached to it from above, the fan and heater were installed in the laboratory car and connected in series with a 26 mm inner diameter rubber hose. The fan fan is connected to a controlled isothermal car using the same rubber hose 24 m long,;

A water tank for wetting the wet thermometer was attached to the outer wall surface of the measuring chamber. A rubber Xf-shaped tube with a diameter of 6 mm was connected to the bottom of the tank, the other end was connected to a copper tube fixed in the wall of the measuring chamber, the other end of which was inside the latter and was 5 mm below the ball of the wet thermometer. The wick at one end was mounted on a bulb of a wet thermometer. The other end of the wick was located in the tube below the water level in the tank to wet the wet thermometer. The water level in the tank was 5 mm below the bulb of a wet thermometer.

The gas volume temperature in an isometric car was measured with a platinum resistance thermometer installed at the inlet end of the rubber hose.

Barometric pressure measured

control barometer. The pressure in the measuring chamber was measured with a water manometer. The partial pressures of saturated water vapor are determined by

tables of saturated water vapor. The calculation of the relative humidity of the gas in an isometric car made by the formula

  PlH-A (B + Pc) (tc.n, -tM) / P2H,

where p - relative humidity;

PIM is the elasticity of saturated water vapor at the temperature of a wet thermometer,

tM;

A 0 "000662 - constant psychrometric coefficient, taken on the passport of the Assman psychrometer;

 mmHg - barometric pressure measured by a control barometer with an accuracy of ± 0.225 mmHg;

RS 110 Pa - static overpressure in the measuring chamber, measured with a water pressure gauge with accuracy

± 10 Pa;

tc.n., tM is the temperature of the dry and wet thermometers measured by glass mercury thermometers with a divide value of 0.2 ° C, with a scale of 0-50 ° C, with an accuracy of + 0.1 ° C;

Р2н is the elasticity of saturated water vapor at a temperature tc in the investigated volume, which was measured during laboratory tests with an Assmann psychrometer with an accuracy of ± 0.1 ° C, and during testing of an isothermal car with a resistance thermometer TGE 0879-01 with an accuracy of ± 0.5 ° WITH.

The measurement of the psychrometric temperature difference in the measuring chamber was made at tc.n. 40 ° C, This value

the temperature of the dry thermometer was determined according to the graph of the dependence of the measurement error of relative humidity on temperature (Fig. 3), obtained previously in laboratory studies,

which corresponds to the specified maximum error of measurement of relative humidity D (0.05 for a measured value of p 10%.

The measurement error of the relative air humidity (p in an isothermal car with the air temperature in it is tc + 4. O and - 20 ° C, calculated using the well-known formula for the proposed method, not

exceeds the specified maximum value of 0.05 with p 10% and 0.02 at 100%, i.e. determined by the region a-b-c-d (Fig. 3), if the temperature measurement in the measuring chamber and in the isothermal car is made with an accuracy of ± 0.1 ° C.

Using the proposed method for measuring humid air can improve the measurement accuracy in the entire measured temperature range from negative values of temperatures to 100 ° C higher.

Forum and Measurement Method of measuring relative humidity 15, which consists in supplying a sample of air to a dry and humid temperature.

five

meters, determine the pressure at which temperatures are measured by thermometers, and using the values found, the relative humidity of the air, characterized in that, in order to improve the measurement accuracy, for the minimum measured value of relative humidity, the working error is found for the minimum measured value of relative humidity measurements from the air temperature, according to which the temperature value is determined for a given error value; e is from the value found to the corresponding boiling point of water at working pressure.

Claims (1)

Claim A method of measuring relative humidity 15, namely, that an air sample is supplied to dry and wet thermometers, the pressure is determined at which temperature is measured by thermometers, and relative humidity is calculated from the values found, characterized in that, in order to increase the accuracy of measurements , preliminary, for the minimum measured value of relative humidity, at operating pressure, the dependence of the measurement error on the air temperature is found, according to which for a given value the error The values determine the temperature value; during measurements, the air temperature is maintained in the range of values from the water found to the corresponding boiling point at operating pressure. Aa 4 FIG. I 1.6 / 4 At I P m \\ \\ g ' b / 4 • V \ d TT --- and \\ \ V - ·· \\ \ d. X \ X> \\ 1 \ V Ά ·· - i tc / 00 0.02 0.06 Ο, θ4 0.02 -20 About 2040 60 Temperature, ° C “C. FIG. J