RU2548061C1 - Method to detect moisture content of gases - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: measuring vessel with a capacitance sensor installed in the form of a multi-electrode flat-parallel air capacitor, to which a measurement capacitor of available capacity is connected, filled with the investigated gas, and the value of temperature and pressure are recorded. High DC voltage is supplied to the capacitance sensor installed in the measurement vessel and to the measurement capacitor of available capacity. The outlet voltage is measured on the measurement capacitor, and moisture content of the investigated gas is determined according to the temperature moisture characteristic of the sensor.

EFFECT: increased sensitivity.

2 cl, 6 dwg

Description

The invention relates to the field of measuring equipment and can be used to measure the moisture content of various gases, as well as oil and oil products.

There are various methods for determining the moisture content of gases. Most of them allow you to determine the moisture content by indirect measurements of the characteristics of the test gas (air) when its state changes before dew occurs due to lower temperatures or increased pressure (RF patent No. 2450262, 05/10/2012).

A known optical method for determining the moisture content of gases (patent for a utility model of the Russian Federation No. 37416, 12/29/2003, patent holder of LLC NLP "TKA"). The method is based on the absorption of vacuum ultraviolet by water vapor at a wavelength of 121.6 nm. Water vapor at this length has an absorption coefficient of more than 200 cm ^-1 , while for oxygen this coefficient is 0.5 cm ^-1 . First, the signal from the sensor is measured by blowing dry air I ₀ , then the signal I _λ with a known content of water vapor is measured. The dependence of the proportion of energy absorption on water concentration is determined by the Lambert-Bouguer-Beer law:

where σ _λ is the absorption coefficient of water vapor at a wavelength of 121.6 nm;

N is the concentration of water molecules;

l is the distance from the lamp window to the photodetector window.

Studies of an optical hygrometer showed linearity of the scale, which is important when calibrating the device, and speed. But this method of measuring gas humidity has a significant drawback - a short lamp life and a constant decrease in the intensity of its radiation, which requires calibration of the device before each measurement.

The closest way to determine the moisture content of gases, taken as a prototype, is the dielcometric method of indirect measurement of the humidity of substances and gases (Berlinger MA Measurement of humidity. - M .: Energy, 1973, p. 55-63) depending on the dielectric constant of these substances. The method is based on the use of a capacitive sensor in the form of a multi-electrode plane-parallel air condenser placed in a shielded dielectric blown housing with a tight cover with leads.

The measurement of the sensor capacitance with this method is carried out on alternating current, for example, according to a bridge circuit. The main disadvantage of this method is the low sensitivity due to the small change in the dielectric constant from the moisture content of this gas.

The objective of the claimed solution is to reduce operating and time costs, increase the accuracy and reliability of measurements.

This goal is achieved due to the fact that in the known method for determining the moisture content of gases, a measuring vessel with a capacitive sensor installed in it in the form of a multielectrode plane-parallel air condenser is filled with the test gas and the temperature and pressure are recorded, according to the claimed solution, a measuring capacitor of a known type is connected in series with the capacitive sensor capacitance, they supply a high constant voltage to a capacitive sensor installed in the measuring vessel and to the itelnogo capacitor of known capacity, measure the output voltage at the measuring capacitor and determine the moisture content of the test gas by the temperature and humidity characteristics of the sensor.

The technical result obtained by implementing the proposed solution is to increase the sensitivity of the claimed method, which is achieved due to the fact that the capacitive sensor located in the measuring vessel is connected to a high voltage constant voltage source, and the sensor capacitance is determined by the voltage taken from the constant measuring capacitor connected in series with a capacitive sensor.

The essence of the proposed method is as follows. Water vapor in the air is in the form of dipoles, uniformly but randomly in orientation, distributed throughout the volume. The measuring cell with the test gas (air) is a plane-parallel capacitor with a random arrangement of dipoles in it in the absence of an external electric field (Figure 1). The dielectric constant of such air differs little from unity (dry air). If a constant high voltage is applied to the plates of the capacitor, then the water vapor dipoles will be oriented in the direction of the applied electric field (Figure 2). The capacity of such a capacitor will increase the more, the greater the humidity of the air between its plates. The mass ratio of moisture (mass moisture content) d can be calculated taking into account the difference in the capacitance of the capacitor in the absence of an external field and in the presence of an external field.

where C ₁ is the sensor capacity in the absence of an external field;

- емкость в присутствии внешнего поля; $${C^{\prime }}_{\mathrm{one}}$$

- capacity in the presence of an external field;

K is the coefficient of proportionality, taking into account the geometric dimensions of the sensor, as well as the temperature and pressure of the test air.

The novelty of the claimed solution lies in the fact that by applying a constant high voltage to the capacitive sensor installed in the medium of the test gas, the moisture content of the test gas is determined by the voltage taken from the measuring capacitor connected in series with the capacitive sensor, using the temperature and humidity characteristics of the sensor. The proposed method for determining the moisture content of gases is a fundamentally new method for measuring the humidity of substances.

The claimed solution is illustrated by graphic materials, where:

Figure 1 shows the position of the dipoles in the capacitor in the absence of an external electric field;

Figure 2 shows the position of the dipoles in the capacitor when applying a constant high voltage;

Figure 3 shows an electrical circuit for measuring the capacitance of a multi-electrode plane-parallel air capacitor;

Figure 4 shows an electrical circuit with an additional circuit;

Figure 5 shows a graph of the dependence of d on U _o ;

Figure 6 shows the installation for implementing the method.

To implement the claimed method for determining the moisture content of gases, a measuring vessel with a capacitive sensor installed in it in the form of a multi-electrode plane-parallel air condenser, to which a measuring capacitor of known capacity is connected in series, is filled with the gas under study and the temperature and pressure are recorded, then a high constant voltage is applied to the capacitive sensor installed in the measuring vessel and on the measuring capacitor of known capacity, measure the output voltage measurement on the measuring capacitor and determine the moisture content of the test gas according to the temperature and humidity characteristics of the sensor.

Installation for implementing the method (Fig.6) contains a capacitive sensor made in the form of a multielectrode plane-parallel air capacitor, consisting of alternately located plane-parallel electrodes 6, interconnected by conductors 7. The capacitive sensor is placed in a shielded dielectric blown housing 5 with a cover 4, on which conclusions 3 are made from the positive and negative groups of electrodes 6, while the positive terminal of the high-voltage source 1 is connected to the positive group and capacitors C ₃ and C ₄ , and a measuring capacitor C _{2 of} known capacitance, which forms a measuring bridge, is connected to the negative group. The dielectric blown housing 5 is equipped with a fitting 2 for supplying the test air and a fitting 8 for discharge.

For a specific sensor, the temperature and humidity characteristics are taken, i.e. the dependence of U _o on humidity at various temperatures and are recorded in the passport.

It is known that the permissible voltage U _add on the plates of an air condenser (for dry air), forming the corresponding electric field strength (EP), should be three times less than the breakdown voltage U _pr Thus, when U _CR = 30 kV / cm, U _ext = 10 kV / cm.

For the safe operation of the condenser with moist air, the operating voltage U of the _{slave of} such a capacitor must be further reduced by a factor of two in comparison with U _add , i.e. U _slave ≤5 kV / cm.

An electrical circuit for measuring the capacitance of such a capacitor can be implemented as follows.

In series with the measuring cell C ₁ include an additional capacitor C ₂ (figure 3) and measure the voltage on it with an electrostatic voltmeter. From the condition of constant charge when the capacitors are switched on sequentially, the equality C ₁ U ₁ = C ₂ U ₂ implies, from where the capacitance of the measuring cell can be calculated:

To increase the sensitivity, you can use a bridge circuit (figure 4): parallel to the circuit shown in figure 3, include an additional circuit C ₃ and C ₄ , and the output voltage U _o , depending only on changes in C ₁ , to remove from points A and B . In dry air (moisture content d = 0) in the measuring cell C _{1, the} output voltage is set to zero using a capacitor C ₀ . An electrostatic voltmeter measuring U _o is selected with a measurement limit of up to 10 V, which significantly increases the resolution of the voltmeter and increases the measurement accuracy.

For each design of the capacitive sensor, the temperature and humidity characteristic is preliminarily taken (Fig. 5):

where f (Gs, T, P) is a function that takes into account the geometric dimensions of the measuring sensor Gs, temperature T, and pressure P of the test gas.

According to the dependence of d on U _o (Fig. 5), a calculation equation can be drawn up, which for specific geometric dimensions, for example, will have the following form:

After calculating the mass ratio of moisture (mass moisture content) d and measuring the temperature and air pressure, the relative humidity φ (%) of the test air is determined from the Ramzin table (I-d).

The claimed solution is illustrated by the following example.

At the setting of the humidity setpoint, the measuring vessel is filled with the capacitive sensor installed in it, the relative humidity of which, measured by the control hygrometer, is φ = 72% at T = 18.9 ° C.

After turning on the high voltage (U _source = 2 kV), an electrostatic voltmeter in the diagonal of the differential circuit showed U _o = 1.48 V, which according to empirical formula (5) corresponds to the value d = 9.85 [g / kg _dry ]. At a temperature of T = 18.9 ° C and a pressure of 756 mm Hg. Art. this value of d according to formula (6) corresponds to a relative humidity of φ = 72.29%.

Thus, the error obtained by the proposed method, the measurement of relative humidity was:

The sensitivity of the electrical circuit of the proposed method is confirmed by the following experiment.

Without connecting a high voltage at a temperature T = 19.1 ° C, the values of the capacitance of the measuring device (vessel with condenser plates filled with air) were measured with the mass moisture ratio (mass moisture content) d from d ₁ = 0.5 [g / kg _dry ], which corresponds to a relative air humidity of φ = 4.8%, up to d ₂ = 10.5 [g / kg _dry ], which corresponds to φ = 70% at the same temperature.

The electric capacitance of the vessel at d = 0.5 g / kg turned out to be equal to C = 113.0 pF, and at d = 10.5 g / kg it was 113.05 pF.

When you turn on the external electric field (U _source = 2000 V), the voltage change across the reference capacitor C ₂ was 1.48 V at d = 10.5 [g / kg _dry ]. In dry air, the voltage U ₂ = 127.4 V.

In accordance with the expression (3):

113.0 × (2000-127.4) = C ₂ × 127.4 → C2 = 1661 pF.

At a moisture content of d = 10.5 [g / kg _dry ], the voltage on C ₂ was 127.4 + 1.48 = 128.88 (V), which is caused by a change in the capacitance of C ₁ .

Also, in accordance with the expression (3):

C ₁ × (2000-128.88) = 1661 × 128.88 → C ₁ = 114.41 pF.

Thus, the capacitance changed by 1.41 pF, i.e. the sensitivity of the electric circuit to a change in the capacitance of the measuring device (a vessel with capacitor plates filled with air) is more than 1 V / pF.

Given the nonlinearity of the output characteristic, it is advisable to use two measuring vessels with small gaps for low moisture content, as more sensitive, and with large gaps for high moisture content, when leakages are unavoidable with small gaps between the electrodes.

A feature of the proposed method is the need to use a high voltage source and an electrostatic voltmeter.

The obtained experimental measurement results (δ≅0.4%) make it possible to use the air humidity measures created according to the proposed solution as special standards of the upper link of the state calibration scheme (GPS).

The claimed solution can be used to create fairly cheap, simple and easy-to-use measures and means of measuring air humidity, easy to manufacture, not requiring expensive materials and having high sensitivity, in the range of relative humidity in accordance with the state calibration chart (GPS) GOST 8.547 -86, as well as working instruments for measuring the mass moisture ratio of various gases, as well as oil and petroleum products.

Claims (2)

1. A method for determining the moisture content of gases, according to which a measuring vessel with a capacitive sensor installed in it in the form of a multi-electrode plane-parallel air condenser is filled with the test gas and a temperature and pressure value are recorded, characterized in that a measuring capacitor of known capacity is connected in series with the capacitive sensor, a high constant voltage to the capacitive sensor installed in the measuring vessel, and to the measuring capacitor of known capacity, measure t the output voltage at the measuring capacitor and determine the moisture content of the test gas by the temperature and humidity characteristics of the sensor. 2. A method for determining the moisture content of gases according to claim 1, characterized in that a constant voltage from 1 kV to 5 kV is supplied to the capacitive sensor installed in the measuring vessel and to the measuring capacitor.

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