RU2506574C1 - Method to determine moisture content in gases and device for its realisation - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: invention relates to the field of measurement of gas moisture content. The method consists in the fact that gas is exposed to compression in a closed measurement chamber, in which an equal-arm yoke is installed, equipped with a measuring float and a counterweight, until pressure, at which gas density becomes equal to density of the measurement float, which is determined by float surfacing and horizontal position of the yoke, the values of temperature and pressure are recorded in the closed measurement chamber at the moment of float surfacing, and using the measured values, they determine the value of moisture content of the investigated gas according to the following ratios: d(g/kg dry air)=[m_steam (g)]÷[m_dry×10^-3 (kg)]=(A×E)÷{[m_float÷(P_lab+ΔP_exc)]-(A×E)×10^-3} (l), where A=(ρ_steam×10³)÷(ρ_dry-ρ_steam)=1638.8 - constant (2) ρ_steam - water steam density, ρ_steam=0.803 g/litre ρ_dry - dry air density, ρ_dry=1.293 g/litre E={[ρ_dry×V_float×T₀]÷[P₀×(T₀+t_lab)]-m_float÷(P_lab+ΔP_exc)} (3), where V_float - volume of the float (in litres), m_float - weight of the float with account of counterweight (in g), T₀=273°C, t_lab - temperature of the investigated air, °C, P₀ - normal atmospheric pressure, P₀=760 mm of mercury column, P_lab - pressure in the laboratory, mm of mercury column, ΔP_exc - value of excessive pressure ΔP_exc=(P_chamber-P_lab), mm of mercury column. P_chamber - pressure in the measurement chamber at the moment of float surfacing, mm of mercury column.

EFFECT: reduced operating costs and higher safety of measurements.

Description

The invention relates to the field of measuring the moisture content of gases. The moisture content of the gas, i.e. the mass of water vapor in a gas is expressed in (g / kg dry air) and is important in many fields of science and technology. Knowing the value of the moisture content of the gas, you can calculate any hygrometric parameters of the gas, including absolute and relative humidity, at any temperature.

There are various methods (methods) for measuring and reproducing gas humidity values presented in RMG 75-2004 (Recommendations on interstate standardization. GSI. Measurement of humidity of substances. Terms and definitions). Typically, the moisture content, which depends on the temperature of the gas, pressure and relative humidity, is determined by the dew point temperature, which is achieved either by cooling the gas or by changing its pressure. The most significant and costly operation is to change the temperature of the gas, because it requires either the consumption of expensive nitrogen, or complex and expensive equipment (thermostats, isothermal pressure chambers, etc.).

The well-known "Method and device for measuring the dew point", patent No. 2186375, 2000, patent holder of the Federal State Unitary Enterprise "Design Office of General Engineering named after VP Barmin", which measures the dew point by measuring the amount of condensate and temperature on the moisture-sensing element with one measuring tool.This method is expensive and time-consuming for determining the humidity of gases, and for its implementation a bulky device is used.In addition, when measuring annym way necessary to dismantle the hygrometer with the operation of the facility and deliver it in a calibration laboratory.

Closest to the claimed, taken as a prototype, is the "Method for determining the moisture content of gases and a device for its implementation" according to patent No. 2450262, application No. 2010150710 with priority 07.12.2010, patent holder "Closed Joint Stock Company" MERA "". In the known method, the following operations are performed: a sample is taken from a test gas with unknown moisture at measured actual temperature and pressure, and a gas sample in a chamber of known volume is compressed to a pressure (state of water vapor) at which dew (condensate) , the pressure and temperature are measured at which the condensate has also precipitated according to empirical dependencies describing the nomogram in the coordinate system [kg of water / kg of gas - pressure (for various temperatures}, for The values of pressure and temperature at which condensate precipitated determine the value of the mass ratio of gas moisture d [kg / kg].

The known device for determining the humidity of gases contains a closed measuring chamber in which temperature and pressure sensors and a condensation alarm are installed, the terminals of which are connected to a processing and control device, in addition, it contains a gas compression device and external temperature and pressure sensors, the outputs of which also connected to a processing and control device.

This method is reliable and accurate, but requires the use of a large membrane compressor, since you need to compress the air strongly, sometimes up to tens of atmospheres, until dew drops. This is potentially dangerous, like any other high pressure job. The use of a large membrane compressor does not allow the use of such an installation in a mobile system for calibrating hygrometers designed to calibrate hygrometers at their workplaces without the possibility of dismantling them during the verification procedure.

The objective of the claimed solution is to reduce the complexity and operating costs and increase the safety of measurements.

This goal is achieved due to the fact that in the known method for determining the moisture content of gases, including gas compression in a closed measuring chamber with built-in temperature and pressure sensors, the gas is compressed to a pressure at which the gas density becomes equal to the density of the measuring float equipped with a counterweight, which is determined on the ascent of the float and the horizontal position of the rocker arm, fix the temperature and pressure in a closed measuring chamber at the time of the ascent of the float, using measured values, determine the value of the moisture content of the test gas according to the following relationships:

Where

ρ _steam is the density of water vapor, ρ _steam = 0.803 g / liter

ρ _dry - dry air density, ρ _dry = 1.293 g / liter

where V _popl - the volume of the float (in liters),

m _float - weight of the float, taking into account the counterweight (in grams),

T ₀ = 273 ° C,

t _lab - temperature of the test air, ° C,

P ₀ - normal atmospheric pressure, P ₀ = 760 mm Hg,

P _lab - pressure in the laboratory, mm Hg,

ΔP _huts - excess pressure, ΔP _huts = (P _chamber -P _lab ), mm Hg

P _chamber - pressure in the measuring chamber at the time of float surfacing, mmHg

This goal is achieved due to the fact that the known device for determining the moisture content of gases, containing a closed measuring chamber with built-in temperature and pressure sensors, is additionally equipped with an equal-arm beam mounted in the measuring chamber, while on one side of the beam there is a measuring hollow hermetic float of known weight and volume and a counterweight is located on the other arm of the rocker arm, in addition, the measuring chamber is equipped with a valve for discharging a gas sample and a valve for For injection of a sample of the test gas into the measuring chamber. And also due to the fact that scales with an arrow are installed on the equal-arm beam. In addition, the device may include a microprocessor, which is designed for digital processing of measurement results and calculation of moisture content in the test gas sample, and electronic scales connected to the microprocessor.

The technical result of the claimed method and device is to reduce the complexity and operating costs and increase the safety of measurements, due to the fact that in the proposed method there is no need to obtain a high pressure at which condensate precipitates. The float used in the claimed solution is made hollow inside, as light as possible, adapted to the studied gas medium, which has a low density. The gas is compressed only until the float emerges, i.e. to a pressure at which the gas density becomes equal to the density of the measuring float. The use of known (experimentally determined) parameters of the float and the temperature and pressure values recorded in the chamber at the time of float surfacing, makes it possible to determine the moisture content by the proposed algorithm

The advantages of the proposed method and device:

a) to implement the method does not require high pressure and the use of expensive equipment, which increases safety during work. A powerful compressor can be replaced with a micro compressor;

b) the implementation of all operations of the proposed method takes several minutes;

c) the possibility of automating the process of obtaining the moisture content of gas, if instead of visually fixing the equality of the weight of the float and the counterweight, use electronic scales placed in the measuring chamber and connected to the microprocessor.

d) the proposed solution allows you to create a device for periodic calibration of instruments for measuring the humidity of gases, without dismantling them from the facility, in operating conditions;

d) the error of the proposed method (method) depends only on the errors of the used measuring instruments (pressure and temperature) and can have extremely small values.

The claimed solution is illustrated by the drawing, which shows a device that implements the proposed method.

A device for determining the moisture content of gases consists of a measuring chamber 4, in which there are installed inertialess sensors for temperature 1, pressure 2 and an equal-arm rocker 3, in the center of which there is an arrow of the balance 9, on one side of the rocker 3 there is a measuring hollow sealed float 7, and on the other shoulder rocker arm 3 is placed a lead counterweight 8. Valve 6 is used to release a sample of gas. The valve 5 is used to inject samples of the test gas into the measuring chamber 4. The device may also include a microprocessor, which is designed to digitally process the measurement results and calculate the moisture content in the test gas sample. The measuring float 7 is sealed and hollow inside, as light as possible, adapted to the studied gas environment. The weight and volume of the float 7 is determined experimentally, and the weight of the counterweight 8 is selected close to the weight of the float.

The principle of operation of the proposed device is as follows. The gas to be determined by its moisture content is pumped by a small-capacity membrane compressor into a hermetically sealed measuring chamber 4 with integrated inertia-free analog-digital sensors of temperature 1 and pressure 2. When the pressure of the test gas increases, its density increases and when the weight of the float 7 and the counterweight 8 of the float are reached "Pops up." The fact that the weight of the measuring float 7 and the counterweight 8 is equal is fixed by the horizontal position of the rocker arm 3 (the arrow of the balance 9 is at “0”), the rocker arm is in a horizontal position. At this moment, the temperature and pressure are fixed, which are used to calculate the moisture content and other parameters of the gas, or are digitally transmitted to the microprocessor to process the measurement results and the corresponding calculations. And when using electronic scales, it becomes possible to automate the entire measurement process.

In the proposed method, the following operations are performed:

1) the test gas (for example, laboratory air) with unknown humidity at known (measured) temperature and pressure values is taken into the measuring chamber, while the temperature t _lab and pressure P _lab in the laboratory are recorded;

2) the gas sample in the measuring chamber is compressed until the float emerges, i.e. to a pressure at which the gas density becomes equal to the density of the measuring float equipped with a counterweight;

3) the pressure P _chamber and the temperature at which this equality is achieved (at the time the float emerges) are measured and the overpressure ΔP _log = P _chamber -P _{lab is} determined;

4) the measured values of t _lab , P _lab , ΔP _houses are transferred to a microprocessor-based data processing device, and the moisture content of the test air d [g / kg dry air] is determined by known ratios:

d (g / kg dry air) = [m _steam (g)] - [m _dry · 10 ^-3 (kg)] =

= (A · E) ÷ {[m _pop - (P _lab + ΔP _log )] - (A · E) · 10 ^-3 },

where A = (ρ _pair · 10 ³ ) - (ρ _dry -ρ _pair ) = 1638.8 - constant,

ρ _steam is the density of water vapor, ρ _steam = 0.803 g / liter,

ρ _dry - density of dry air, ρ _dry = 1.293 g / liter,

E = {[ρ _dry · V _popl · T ₀ ] ÷ [P ₀ · (T ₀ + t _lab )] - m _pop ÷ (P _lab + ΔP _log )}

- functional that determines the influence of the parameters used

float on the sensitivity of the proposed method,

where V _popl - the volume of the float (in liters), is determined experimentally,

m _float - weight of the float (in grams), determined experimentally,

T ₀ = 273 ° C, is the transition temperature from the Celsius scale to the absolute temperature scale according to Kelvin,

t _lab - temperature of the test air, ° C,

P ₀ - normal atmospheric pressure, P ₀ = 760 mm Hg,

P _lab - pressure in the laboratory, mm Hg,

ΔP _huts - excess pressure, ΔP _huts = (P _chamber -P _lab ), mm Hg

P _chamber - pressure in the measuring chamber at the time of float surfacing, mmHg

A check of the correctness of the relationships given in the algorithm can be carried out using the "i-d" - wet air condition diagram

To explain the principle of operation of the proposed device, we consider practical examples.

Example 1. The laboratory pressure is 752 mm Hg, the laboratory temperature is 23 ° C. In this case, an exemplary thermohygrometer shows a relative humidity φ in the laboratory of 48%. For further calculations, we use the generally accepted “i-d” diagram of the state of moist air (gas), which graphically displays the processes of humidification, heating, drying, gas cooling [1]. From this diagram it follows that at the indicated values of pressure, temperature and humidity, the moisture content d is 5.8 (g / kg air dry).

With a membrane compressor, this air is pumped into the measuring chamber 4, in which an equal-arm balance 3 with a float 7 and a counterweight 8 is installed. It has been experimentally established that the volume of the float is V _pop = 0.1675 liter = 167.5 ml.

The weight of the float, taking into account the counterweight, m _popl = 0.220 g

When the overpressure ΔP _gages in the chamber 4 is reached _. = 88 mmHg we observe that the float 7 “floated”. The weight of the air with a volume equal to the volume of the float 7, took a value of 0.220 g, respectively.

From the well-known gas equation of state

can calculate the molecular weight of air

The mass of this air in laboratory conditions (without excess pressure) is:

The weight of this air is the sum of the weight of dry air and the weight of water vapor, taking into account their volumes:

Air volume reduced to normal conditions:

Using equation (4), we calculate the volume of steam:

Then we get:

The moisture content of this air is defined as:

The moisture content value obtained in this way is correlated with the “i-d" diagram of the state of moist air and we obtain a relative humidity value of φ = 48% at an air temperature of 23 ° C, this value corresponds to the initial conditions of the problem, which was to be proved.

Example 2. The pressure in the laboratory is 750 mm Hg, the temperature in the laboratory is 25 ° C. In this case, an exemplary thermohygrometer shows a relative humidity φ in the laboratory of 80%. From the “i-d” diagram it follows that at the indicated values of pressure, temperature and humidity, the moisture content d is 16.5 (g / kg dry air).

The volume of the float V _popl = 0.1675 liter = 167.5 ml.

The weight of the float, taking into account the counterweight, m _popl = 0.220 g

When overpressure ΔP _huts = 101.1 mm Hg is reached in chamber 4 we observe that the float 7 “floated”. The weight of the air with a volume equal to the volume of the float 7, took a value of 0.220 g, respectively. From the known gas equation of state (1), we calculate the molecular weight of this compressed air

The mass of this air in laboratory conditions (without excess pressure) is:

Air volume reduced to normal conditions:

The weight of this air is the sum of the weight of dry air and the weight of water vapor, taking into account their volumes:

whence follows:

Then we get:

The moisture content of this air is defined as:

The moisture content value obtained in this way is correlated with the “i-d" diagram of the state of moist air and we obtain a relative humidity value of φ = 80% at an air temperature of 25 ° C, this value corresponds to the initial conditions of the problem, which was to be proved.

Example 3. The pressure in the laboratory is 750 mm Hg, the temperature in the laboratory is 25 ° C. In this case, an exemplary thermohygrometer shows a value of relative humidity φ in the laboratory of 20%. From the “i-d” diagram it follows that at the indicated values of pressure, temperature and humidity, the moisture content d is 4.0 (g / kg dry air).

The volume of the float V _popl = 0.1675 liter = 167.5 ml.

The weight of the float, taking into account the counterweight, m _popl = 0.220 g

When the overpressure ΔP _gages in the chamber 4 is reached _. = 94.8 mmHg we observe that the float 7 “floated”. The weight of the air with a volume equal to the volume of the float 7, took a value of 0.220 g, respectively.

From the well-known gas equation of state (1), we calculate the molecular weight of this compressed air

The mass of this air in laboratory conditions (without excess pressure) is:

Air volume reduced to normal conditions:

The weight of this air is the sum of the weight of dry air and the weight of water vapor, taking into account their volumes:

From where it follows:

Then we get:

The moisture content of this air is defined as:

The moisture content value obtained in this way is correlated with the “i-d" diagram of the state of moist air and we obtain a relative humidity value of φ = 20% at an air temperature of 25 ° С, this value corresponds to the initial conditions of the problem, which was to be proved.

Thus, the conducted experimental-theoretical verification for three different states of the air under study showed the full applicability of the proposed method and device.

Claims (5)

1. The method of determining the moisture content of gases, including gas compression in a closed measuring chamber with built-in temperature and pressure sensors, characterized in that the gas is compressed to a pressure at which the gas density becomes equal to the density of the measuring float equipped with a counterweight, which is determined by the float and the horizontal position of the rocker arm, fix the temperature and pressure in a closed measuring chamber at the time of float surfacing and using the measured values determine ix moisture content of the test gas in the following ratios:

Where

ρ _steam is the density of water vapor, ρ _steam = 0.803 g / liter
ρ _dry - dry air density, ρ _dry = 1.293 g / liter

where V _popl - the volume of the float (in liters),
m _float - weight of the float, taking into account the counterweight (in grams),
T ₀ = 273 ° C,
t _lab - temperature of the test air, ° C,
P ₀ - normal atmospheric pressure, P ₀ = 760 mm Hg,
P _lab - pressure in the laboratory, mm Hg,
ΔP _huts - the value of excess pressure, ΔP _huts = (P _chamber -P _lab ) mm Hg
P _chamber - pressure in the measuring chamber at the time of float surfacing, mmHg 2. A device for determining the moisture content of gases, containing a closed measuring chamber with built-in temperature and pressure sensors, characterized in that the measuring chamber has an equal-arm rocker, on one side of which there is a hollow measuring float of known weight and volume, and on the other arm of the rocker counterweight, in addition, the measuring chamber is equipped with a valve for injecting samples of the test gas into the measuring chamber and a valve for discharging the gas sample. 3. A device for determining the moisture content of gases according to claim 2, characterized in that a balance with an arrow is installed on the equal-arm beam. 4. A device for determining the moisture content of gases according to claim 2, characterized in that it contains a microprocessor that is designed to digitally process the measurement results and calculate the moisture content in the test gas sample. 5. A device for determining the moisture content of gases according to claim 2, characterized in that it contains electronic scales connected to a microprocessor.