RU2039977C1 - Method of measurement of concentration of mercury vapors - Google Patents

Abstract

FIELD: nondestructive check of gaseous substances. SUBSTANCE: in the method of measurement of concentration of mercury vapors based on measurement of the dependence of natural frequency of crystal vibrator on the additional connected mass at a constant flow rate of the mixture of mercury and air vapor, the time interval, during which the vibrator natural frequency varies by a preset value, is additionally measured. EFFECT: simplified measurement. 1 dwg

Description

 The invention relates to analytical instrumentation, non-destructive testing of gaseous substances and can be used in the chemical industry for environmental monitoring.

A known method of measuring the concentration of gaseous substances, based on measuring the dependence of the natural frequency of the piezoelectric resonator on the additional mass attached by the sorption coating to the resonator electrodes [1]
The disadvantage of this method is the need for accurate dosing of the test gas or steam.

Closest to the proposed method for measuring the concentration of mercury vapor in the air is the method [2]
When measuring the concentration of mercury vapor in air in a known manner, a known volume of the analyzed mixture is pumped through the measuring chamber in which the piezoelectric resonator is located. A gold coating is applied to the resonator electrode (s), which acts as a sorption coating. The amalgam formed during the interaction of mercury with gold increases the mass of the resonator electrodes and reduces its natural frequency. The natural frequency of the resonator is measured before pumping a known volume and after pumping this volume. The concentration of mercury vapor is estimated by the difference of the indicated natural frequencies. The known volume of the analytical mixture is set either by pumping it from a previously filled container of known volume, or by measuring the flow rate of the mixture and the duration of pumping the mixture through the measuring chamber.

 The disadvantage of this method is the need for sampling a certain amount of the analyzed gas mixture, which complicates the implementation of the method.

 The aim of the invention is to simplify the method.

 This is achieved by the fact that in the proposed method for measuring concentrations of mercury vapor in air, based on measuring the dependence of the natural frequency of the piezoelectric resonator on the additional mass attached to the electrodes of the resonator at a fixed flow rate of the analyzed mixture, measure the time interval during which the natural frequency of the resonator will change in advance a given value, and the concentration is judged by the ratio of the magnitude of the frequency change to the magnitude of the mentioned time interval.

 The method is as follows. The analyzed mixture of mercury vapor in the air is pumped at a fixed flow rate of the mixture through a chamber with a piezoelectric resonator, on the electrodes of which a sorption coating is applied in the form of a thin layer of gold, the time interval is measured, during which the natural frequency of the resonator changes by a predetermined value, the concentration of mercury vapor is estimated by the ratio of the change in the frequency of the resonator to the value of the time interval.

 The drawing shows a block diagram of one of the possible variants of a device that implements the proposed method.

 The device contains a measuring chamber 1 with a similar and outlet openings 2, 3. Inside the chamber there is a piezoelectric resonator 4, on the electrodes of which a thin layer of gold is deposited, which is a sorbent for mercury vapor. The resonator 4 is included in the circuit of the high-frequency generator 5. The outlet 3 of the chamber is connected to a pump 6, which provides a fixed (constant) flow rate of the analyzed mixture of mercury vapor and air. The output of the generator 5 is connected to the input of the frequency meter 7, the output of which is connected to one of the inputs of the code comparator 8, the second input of which is connected to the comparison code task unit 9. The output of the comparator 8 codes is connected to the inputs "Stop" of the chronometer 10 and the pump 6.

 Prior to the measurement, the frequency of the generator 5 is constant, its value is periodically measured by a frequency meter 7 and, in the form of a digital code, is supplied to one of the inputs of the code comparator 8. The second input of the comparator 8 codes receives a signal in the form of a digital code from block 9 of the task of the comparison code. The digital equivalent of the comparison code corresponds to a frequency that is less than the initial frequency of the generator 5 by a predetermined value. As a result of this, there is no Stop signal at the output of comparator 8.

 The measurement begins with an external Start command, which turns on the pump 6 and puts the chronometer 10 in the time interval measurement mode. The analyzed mixture is pumped through the inlet 2 of the measuring chamber along the electrodes of the resonator 4 and then through the outlet 3 and is pumped into the atmosphere by the pump 6.

 Due to the interaction of mercury vapor with the gold coating of the electrodes of the resonator 4, the natural frequency of the latter begins to decrease. The rate of change of the natural frequency of the resonator 4 is proportional to the concentration of mercury vapor in the air. When the natural frequency of the resonator changes by a predetermined value, the digital signals (codes) at both inputs of the code comparator 8 are made identical and the Stop signal appears at its output, turning off the pump 6 and blocking the measurement of the time interval by the chronometer 10.

, (1) где C_κ- концентрация паров ртути;
а коэффициент пропорциональности (определяется по смесям с известной концентрацией паров ртути);
f₁, f₂ частота генератора 5 (собственные частоты резонатора 4) в моменты соответственно начала и окончания измерения;
T_κ- показания хронометра 10 (временной интервал, на протяжении которого частота генератора 5 изменяется от f₁ до f₂).The concentration of mercury vapor in the analyzed mixture is determined by the ratio:
C _κ a

, (1) where C _κ is the concentration of mercury vapor;
and the coefficient of proportionality (determined by mixtures with a known concentration of mercury vapor);
f ₁ , f _{2 the} frequency of the generator 5 (natural frequencies of the resonator 4) at the moments of the beginning and end of the measurement, respectively;
T _κ - readings of the chronometer 10 (the time interval during which the frequency of the generator 5 varies from f ₁ to f ₂ ).

 The proportionality coefficient a in the ratio of 1 is determined by the parameters of a particular device that implements the proposed method. The most important of these parameters are the natural frequency of the piezoelectric resonator, coefficient a increases in proportion to the second power of the natural frequency; the consumption of the analyzed mixture, coefficient a first increases in proportion to the increase in consumption, then increases more slowly than the increase in consumption, then it does not change with increasing consumption, and with an even greater increase in consumption it begins to decrease.

 The coefficient a is measured experimentally at a fixed value of the flow rate of the analyzed mixture, which (flow) is determined by the design of the measuring chamber 1, the parameters (power, speed) of the pump 6, as well as the cross-section of the piping of the device and remains unchanged during operation of the device.

 The coefficient a is measured as follows.

An open vessel with liquid mercury is placed in a container of a sufficiently large volume, which, partially evaporating, forms a mixture of mercury vapor and air in the vessel. The capacity is connected to a device that implements the proposed method, through an optical cuvette, which is visible through the radiation of a mercury lamp. Then, the “Start” command turns on the device and simultaneously measures the optical absorption of the radiation from a mercury lamp passing through an optical cell through which the analyzed mixture is pumped by the pump of the device. The concentration of mercury vapor C _{κ is} calculated by known formulas, based on the optical absorption of saturated mercury vapor. By measuring the time interval T _κ during which the natural frequency of the resonator changes by a predetermined value f ₁ -f ₂ , coefficient a is calculated from relation (1). Since the flow rate of the analyzed mixture is determined only by the design features of a particular device that implements the proposed method, it remains unchanged (fixed for this device) throughout the life of the device. Owing to the same circumstance, there is no need to measure the flow rate.

 PRI me R. An AT-cut pieoelectric resonator (frequency ≈15 MHz) was used, on the electrodes of which a thin (0.01 μm) gold coating was applied. The concentration of mercury vapor in a process vessel was measured.

2·10

7,6·10^-8 г/л
При конкретной реализации способа в качестве измерителя частоты можно использовать серийный электронно-счетный частотомер, в качестве компаратора кодов микросхемы типа К561ИП2 или К555СП1, в качестве задатчика кода сравнения систему переключателей.The set value of the change in the natural frequency of the resonator was set equal to f ₁ -f ₂ 4 Hz. The value of the coefficient of proportionality a in relation (1) was a 2˙10 ^-8 (g / L Hz / min). When measured in accordance with the proposed method, the time interval T _κ , during which the natural frequency of the resonator changed by 4 Hz, was T _κ = 63 with 1.05 min. From here, the determined concentration was:
C _κ = a

2 · 10

7.6 · 10 ^-8 g / l
For a specific implementation of the method, a serial electron-counting frequency meter can be used as a frequency meter, as a comparator of chip codes of type K561IP2 or K555SP1, as a switch for a comparison code, a switch system.

Claims (1)

 METHOD FOR MEASURING THE CONCENTRATION OF MERCURY VAPORS, which consists in applying a coating of a material with predetermined physicochemical properties to the piezoelectric resonator electrode located in the measuring chamber, filling the measuring chamber with the mixture of mercury and air vapors being studied, fixing the flow rate of the mixture, and measuring the natural frequency of the resonator and its change is used to judge the concentration of vapors, characterized in that, in order to simplify the method, they additionally measure the time interval during which the natural frequency and the resonator changes by a predetermined value, and the concentration is judged by the ratio of the change in the natural frequency of the resonator to the value of the measured time interval.

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