RU2298781C2 - Device for measuring gas moisture - Google Patents

Abstract

FIELD: measurement technology.

SUBSTANCE: device has sensitive element quartz piezo-element covered with silver sulfide layer and sorption coating, like capron having thermodynamically stable crystal structure, covering electrodes. Quartz piezo-element frequency F and sorbent film thickness h are selected with the relation hF=13-15 mcm*MHz kept in view. Quartz piezo-element frequency is within the limits of 9-25 MHz.

EFFECT: reduced sensitive element response time; increased stability of its characteristics.

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Description

The proposed device relates to the field of analytical instrumentation, in particular to instruments for measuring the relative humidity of gases, and is an improvement of the known device according to ed. testimonial. No. 463901, class G01N 25/58.

The main invention describes a device for measuring the humidity of gases, containing a sensitive element made in the form of a quartz piezoelectric element with silver electrodes, coated with a layer of a polymer sorbent, for example capron, moreover, the silver electrodes are coated with a protective layer of silver sulfide, and the sorbent film deposited on the piezoelectric element has a thermodynamically stable crystalline structure. A disadvantage of the known sensitive element is the large inertia. So, its time constant reaches 50 s, and the transition process time is 15 minutes.

The inertial properties of such a sensitive element are determined by the thickness of the sorbent film and the diffusion coefficient of water vapor in the sorption film.

The time constant of the sensing element with a film of a polymer sorbent (kapron) with an abrupt change in humidity is determined from the expression

where τ is the time constant;

h is the sorbent film thickness;

D is the diffusion coefficient of water vapor in the sorbent film;

k is the coefficient of proportionality.

The thickness of the sorbent film on one side of the quartz piezoelectric element is determined by the formula

where F is the resonant frequency of the oscillations of the piezoelectric element;

N is the frequency coefficient;

ρ _k , ρ _c - respectively, the density of quartz and the density of the sorbent;

ΔF _c is the change in the frequency of the quartz piezoelectric element due to the sorbent film of mass m _c deposited on both sides of it.

As follows from expression (1), to reduce the inertia of the sensitive element, it is necessary to reduce the thickness of the sorbent film.

To reduce the thickness of the sorbent film, it is necessary to reduce the mass of the sorbent applied to the piezoelectric element, which is proportional to ΔF _c , or to increase the oscillation frequency of the quartz piezoelectric element F.

With an increase in the oscillation frequency of quartz resonators, although their relative instability ΔF _n / F remains constant, the absolute change in the frequency caused by the instability ΔF _n increases. This leads to an increase in the error of sensitive elements due to instability and, consequently, to a deterioration in their accuracy.

The aim of the invention is to reduce the inertia of sensitive elements while maintaining stability and accuracy.

This goal is achieved by the fact that in a device for measuring gas humidity according to ed. testimonial. No. 463901, class G01N 25/58 the frequency of the quartz piezoelectric element F and the film thickness of the sorbent h are selected in accordance with the expression h · F = 13 ÷ 15 μm · MHz. The frequencies of quartz resonators are in the range from 5 to 25 MHz. Quartz resonators with lower or higher frequencies were not used for the manufacture of sensitive elements.

This solution allows to achieve a significant reduction in the inertia of the sensitive elements of hygrometers, and their stability and accuracy do not deteriorate.

The drawing shows the proposed device.

The sensitive element consists of a quartz piezoelectric element (quartz plate 1 with silver electrodes 2), a protective layer of silver sulphide 3 located on the electrodes 2. A moisture-sensitive film of polycaproamide (capron) is deposited on plate 1 and electrodes 2 with a protective layer of silver sulphide 4. Polycaproamide film has a thermodynamically stable crystalline structure.

The main frequency of a quartz piezoelectric element without a sorbent film is selected in the range from 9 to 25 MHz, and the ratio between the polycaproamide film thickness h and the frequency of the quartz piezoelectric element F is chosen to be h · F = 13 ÷ 15 μm · MHz.

The device operates as follows.

When a sensitive element is included in the electronic generator circuit, it performs mechanical shear vibrations along the thickness of the quartz plate with a resonant frequency depending on the mass of the substance deposited on the surface of the piezoelectric element.

When a sensitive element is placed in the analyzed gas medium, the polycaproamide film selectively sorb water vapor from the gas, changing the mass of the element and, consequently, the vibration frequency of the sensitive element. The change in the oscillation frequency serves as a measure of the humidity of the analyzed medium.

The manufacture of the sensing element can be carried out in a known manner by ed. testimonial. No. 438913, class G01N 25/56.

The measurement of the thickness of the sorbent film in the manufacture is carried out using dependence (2) on the change in the resonant frequency of the oscillations of the piezoelectric element during the deposition of the sorbent film.

To determine the effectiveness of the invention, it was tested experimentally. Four experimental batches of the proposed sensitive elements with a moisture-sensitive polycaproamide film were made using quartz resonators III G-14BT frequency B1 (GOST 6503-67) with a fundamental oscillation frequency of 9 to 25 MHz. Comparative tests of these sensitive elements were carried out to determine their inertia (time constant τ and transient time T _P ) and parameter stability for one year. At the same time, tests were carried out on sensitive elements made on the basis of the invention by ed. testimonial. No. 463901 with a fundamental oscillation frequency of 2 MHz (sorbent film thickness h = 2.8 μm), taken as the base object.

The table shows the main parameters of the sensitive elements: the frequency of the quartz resonator F, the thickness of the sorbent film h, the change in the oscillation frequency of the resonator due to the mass of the deposited sorbent film ΔF _C , the integral sensitivity of the element ΔF _0-100 , equal to the change in the frequency of the element when the relative humidity changes from 0 to one hundred%.

The main test results are also given here: time constant τ, transient process time Т _П and error of sensitive elements caused by instability δ _Н.

The time constant τ and the time of the transient process T _{P were} determined with an abrupt change in relative humidity from 20 to 70%. The error caused by instability was determined by the formula:

where ΔF _N is the monthly average change in the oscillation frequency of the sensing element at a relative humidity of 0%, Hz;

ΔF _0-100 is the integrated sensitivity of the element, Hz.

Table Parameter Resonator frequency, F, Hz 5 · 10 ⁶ 9 · 10 ⁶ 14.110 ⁶ 18.110 ⁶ 25 · 10 ⁶ h, μm 2,8 1.55 ± 0.11 1.0 ± 0.07 0.78 ± 0.055 0.56 ± 0.04 ΔF _C , Hz 36.510 ³ 6610 ³ 10210 ³ 13110 ³ 181 · 10 ³ ΔF _0-100 , Hz 2900 5250 8100 10400 14400 τ, s 42 ± 5 12 ± 3 6 ± 2 3,5 ± 2 2 ± 1 T _P min 5 ± 0.2 1.2 ± 0.25 0.8 ± 0.25 0.57 ± 0.25 0.38 ± 0.2 δ,% one 0.95 0.98 0.9 0.95

The test results indicate that with the same stability, and therefore accuracy, compared with the base object, the proposed device has significantly better dynamic characteristics.

Thus, the present invention gives a positive effect when used, which consists in reducing the inertia of the sensitive elements while maintaining their accuracy and stability.

Claims (1)

A device for measuring the humidity of gases containing a sensing element made in the form of a quartz piezoelectric element with silver electrodes coated with a silver sulphide layer coated with sorption coatings, for example, a capron, having a thermodynamically stable crystalline structure, characterized in that the frequency F of the quartz piezoelectric element and thickness h the sorbent films are selected in accordance with the expression h · F = 13 ÷ 15 μm · MHz, and the frequency of the quartz piezoelectric element is selected in the range from 9 to 25 MHz.