Method to measure relative air humidity

FIELD: measurement equipment.

SUBSTANCE: in the method to measure relative air humidity based on measurement of difference of oscillation frequencies of resonators - a working and a reference ones with subsequent amplification of an analytic signal and regeneration of film coatings with an inert gas, three piezoquartz resonators are used with internal oscillation frequencies of 13-16 MHz, two of which are working ones with different hydrophilic coatings, properties of which are optimised for operation in a certain range of temperatures, and one is a reference resonator without a film coating, which supports the permanent frequency of oscillations, at the same time the device is equipped with a switch of working resonators.

EFFECT: measurement of relative air humidity in a wide range of temperatures, also in the negative range, higher accuracy of measurements, reduced time of regeneration of film coatings of resonator electrodes.

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The invention relates to measuring technique and can be used to measure relative humidity from 0 to 100% in the temperature range (- 20÷50)°C.

Known methods of measuring the relative humidity (see Moscow VA, stack I.A., Fisenko A.N. Methods and means of hydro-meteorological measurements. - Voronezh: VAIO, 2009. - P.77 - 100) are characterized by high precision measurements of relative humidity above 40%. The measurement accuracy is significantly reduced at low temperatures, less than (-3)°C (phase transition).

The relative humidity in a wide range of temperatures measured with the aid of piezoresonance sensors, which are a kind of selective converters sorption type and is made on the basis of piezoquartz resonators (CRP) with the selective moisture-sensitive coatings. The amount of adsorbed moisture depends at a given temperature, the ratio of partial pressure of water vapor ppthe vapour pressure pnat this temperature. Thus, CRP converts the ratio Rp/pn(or relative humidity) in frequency, and analytical signal method does not depend on the aggregate state of water.

There is a method of measuring the relative humidity based on the application of p is tortonese sensor, contains one piezoquartz resonator at-cut with a semiconductor substrate and a hydrophilic film coating (see EN 2141639, IPC G01N 5/02). Relative humidity is determined by the absolute change of the frequency of oscillations of CRP in the sorption of water vapor on the film surface. The sensor is characterized by high sensitivity, operates in a wide temperature range. The disadvantage of a narrow interval of the measured relative humidity due to the low sorption capacity of the film coating of the electrodes CRP, which leads to the disruption of the oscillation frequency when the relative humidity is 0.22%.

Closest to the claimed technical solution is a way of measuring the relative humidity of the air, based on the measurement of the difference frequencies of the two resonators with frequencies of oscillation 9 MHz - working and reference (see VV Malov. Piezoelectric sensors. - M.: Energoatomizdat, 1989. - S - 189). The electrodes of the resonators modified various film coating is hydrophilic and hydrophobic, respectively. It is possible to measure relative humidity from 0 to 100% in the temperature range 5÷50°C with the main relative error of 2.5 to 5.0%. The measurement error increases with repeated acts of sorption-desorption of vapors of the odes. Desorption is carried out using an inert gas pumped through the cell detection. The probability of the sensor to 0.92 for 2000 hours.

The method is implemented using the sensor "Wave" (figure 1), consisting of 1 - working piezoquartz resonator with a hydrophilic film coating electrodes; 2 - basic piezoquartz resonator with a hydrophobic film coating of electrodes; 3 - cell detection with a single input; 4 - generator self-oscillations of the resonators; 5 - shaper of the differential oscillation frequency of resonators; 6 - schematic of the frequency detection; 7 - amp analytical signal.

The sensor works as follows: from the generator self-oscillations of the resonators 4 pin 1 and reference 2 resonators signal with frequency ωcustomequal to the natural frequency of oscillation of the resonators. In the cell detection 3 use the dispenser enter the air sample. Due to the absorption of moisture from the hydrophilic film coating of the electrodes, the oscillation frequency of the resonator 1 is proportional to the concentration of water vapor in the sample and is Δω1customω1The frequency of the reference resonator 2 is changed slightly due to related sorption of nonpolar compounds, pressure drops in the system when the input samples and is Δω2customω2. The signal is s from the work 1 and the support 2 resonators come in shaper difference frequency 5, which generates a harmonic signal of the oscillation of the two resonators2And0cosφ01-φ022sin(ωwith aaboutbwith atin-Δω)t,where a0the amplitude of the oscillations of the resonators, φ01and φ02- initial phase, Δω=Δω1-Δω2- the difference between the frequency of oscillation of the working 1 and reference 2 resonators, t - time. The signal at the circuit of frequency of detection of 6, which gives Δω, is proportional to the mass of adsorbed water vapor, and then to the amplifier frequency oscillations 7. Relative humidity is defined by the ratio:f=Δmm0100%where Δm and m0- mass adsorbed and saturated water vapor at a known temperature of measurements, respectively.

Regeneration films sorbents spend purge cell detection inert gases, such as hydrogen. The completeness of the regeneration of film coverings controlled by R is snasti frequency of oscillation of the working and reference resonators. Featured manufacturer of the desorption time is 16 minutes

The magnitude of the analytical signal Δω, accuracy, measurement range relative humidity, the range of operating temperatures depend on the characteristics of film coating of supporting and working resonators.

The disadvantage of this method is the inability of the measurements at low temperatures (<5°C) due to changes in characteristics of thin film coatings of the electrodes of the resonators. Another drawback of this method is the impossibility of accurately controlling the regeneration of film coating resonators, which greatly affects the accuracy of repeated measurements of relative humidity. This is because the installation device "on zero" is the difference of the frequencies of oscillation of the working and reference resonators, with a hydrophobic film coating of the electrodes of the reference resonator minor sorption of gases and even water vapor (see Kochetov JU Determination of volatile organic compounds in the gas phase using piezoscanner sensors based on synthetic and natural polymers: thesis(candidate of chemical Sciences: 02.00.02.- Saratov, 2002. - 143: ill. RSL OD 61 03-2/229-4).

The disadvantage is a long time desorption of water vapor from the amount of film coating. At Eisenia coating thickness leads to a decrease of the interval defined relative humidity: lower limit is limited by the sensitivity of film coating, the top is their low sorption capacity (see Kuzmenko T.A. application of the method of piezoquartz microinvasive in analytical chemistry. Voronezh: Publishing house of Voronezh state technology. Acad., 2001. - Pp.28-34).

The technical result of the invention is the measurement of relative humidity in a wide range of temperatures, including in the negative range, increased accuracy, reduced time of regeneration of film coating of the electrodes of the resonators.

The technical result is achieved by the fact that in the previously known method of measuring the relative humidity of the air, based on the measurement of the difference frequencies of the two resonators with frequencies of oscillation 9 MHz - working and reference electrodes are modified by various film coating is hydrophilic and hydrophobic, respectively, with further strengthening of the analytical signal and the regeneration of film coating with an inert gas, what's new is that there are three piezoquartz resonator with its own vibration frequencies 13-16 MHz, two of which are working with different hydrophilic film coatings, the properties of which are optimized to work in a certain temperature range, and one reference resonator without film coating that maintains a constant oscillation frequency; the device is equipped with a re what suchtelen working resonators.

The invention consists in that for the measurement of relative humidity from 0 to 100% in the temperature range (-20÷50°C applied three piezoquartz of the resonator natural frequency 13-16 MHz reference with a constant oscillation frequency, independent of changes in temperature and humidity, and two business SIC with different hydrophilic film coatings, sorption characteristics which are optimized to work in different temperature intervals.

The method can be implemented using the device (figure 2), including: 1a and 1B workers piezoquartz resonators with a hydrophilic film coating of the electrodes, sorption characteristics which are optimized for operation in temperature ranges from -20 to -3°C and from -3 to +50°C, respectively; 2 - basic piezoquartz resonator, sealed in the hermetic casing; 3 - cell detection with two inputs to each work resonator; 4 - generator self-oscillations of the resonators; 5 - driver differential oscillation frequency of resonators; 6 diagram of the frequency detection; 7 - power of the analytical signal; 8 - switch working resonators.

The method is implemented as follows: from the generator self-oscillations of the resonators 4 on one of the working cavities (1a or 1B depending on the ambient temperature, in the future 1 and reference 2 resonators signal with frequency ω customequal to the natural frequency of oscillation of the resonators. The choice of the working resonator in accordance with the specified temperature range is performed with the switch 8. The use of two ASMS with different properties of film coatings can measure relative humidity from 0 to 100% in a wide temperature range, including negative. Properties of thin film coatings (chemical composition, method of application, mass) is optimized for operation in temperature ranges from -20 to - 3°C for the resonator 1a; from 3 to 50°C resonator 1B.

The use of piezoquartz resonators with self-oscillation frequency 13-16 MHz increases the resolving power of the sensor in comparison with traditionally used resonators with the natural frequency of 9 MHz 6 times, according to the equation Sauerbrey, and 4 times in accordance with the experimental data (see Kuzmenko T.A., Kochetov GU, Silina J.E. and others, "Determination of trace hydrogen sulfide in the gas stream using phasedetector". Journal of analytical chemistry. - 2007. - So 62, No. 8.- S-874). This approach allows to reduce the weight (thickness) of film coating without reducing the sensitivity of microinvasive. The reduction of weight of film coating of the electrodes leads to a significant reduction in the time of desorption of water vapor from the volume of the sorbent and,therefore, the total time to determine relative humidity. Piezoquartz resonators with their own frequencies above 15 MHz to use is impractical because of the high sensitivity leads to uncompensated drift of the zero signal, and the determination of small concentrations of water vapor (0-5% relative) presents significant difficulties.

Through the entrance of the cell detection 3 to the appropriate worker resonator 1 via the dosing unit is the air sample. Due to the absorption of moisture from the hydrophilic film coating of the working electrodes of the resonator 1, its oscillation frequency is proportional to the concentration of water vapor in the sample and is Δω1customω1. The frequency of the reference resonator 2 remains constant. Signals from desktop 1 and reference 2 resonators come in shaper difference frequency 5, which generates a harmonic signal of the oscillation of the two resonators similar to the prototype. The difference between the frequency of oscillation of the working 1 and reference 2 resonators can be written: Δω=Δω1-Δω2. Since the reference resonator is sealed in an air-tight casing, then its frequency will change slightly within the line of drift CRP: Δω2custom±3 Hz, then the analytic signal Δω=Δω1±3 Hz. The use of the reference resonator with a constant frequency of colemani which allows to reduce the error detection during regeneration the film coating of the electrodes and to a more accurate sensor "zero". The signal at the circuit of frequency of detection of 6, then the amplifier frequency oscillations 7. The definition of relative humidity and regeneration of film coating of the electrodes is conducted similarly to the prototype.

Comparison of some characteristics of the proposed technical solutions and the closest analogue is presented in the table.

Comparative characteristics of technical solutions and prototype
Comparison parametersTechnical solutionSimilar
The interval measured relativity humidity, % attributes.0÷1000÷100
Range of working temperatures, °C1) -20÷35÷50
2) -3÷50
The natural frequency of oscillation of the resonator, MHz159
Sensitivity microinvasive water vapor, Hz/ % attributes.1)3,12,5
2) 2,5
The relative error of measurement, %1) 1,8-2,02,5÷5
2) 1,4÷1,8
The installation of the device "on 0" (regeneration time of film coating of sorbent), min1) 1216
2)7

The proposed method of measuring humidity using three piezoquartz resonators with frequencies of oscillations 13-16 MHz, two of which are working CRP with different hydrophilic film coatings, and one reference CRP without film coating that maintains a constant oscillation frequency, allows you to:

- to expand the range of operating temperatures (-20÷50°C) measuring the relative humidity in the range from 0 to 100%;

to increase the sensitivity of microinvasive water vapor at positive temperatures;

to reduce the relative error of detection;

- reduce the time of regeneration of the sorbent and, therefore, the total time to determine the relative humidity of the air.

The method of measuring the relative humidity of the air, based on the measurement of the difference frequencies of the resonators is working and reference, followed by the rise analiticheskoj the signal and regeneration of film coating with an inert gas, characterized in that there are three piezoquartz resonator own frequencies 13-16 MHz, two of which are working with different hydrophilic film coatings, the properties of which are optimized to work in a certain temperature range, and one reference resonator without film coating that maintains a constant oscillation frequency, the device is equipped with a switch working resonators.



 

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