JPH02206742A - Gas sensor - Google Patents

Abstract

PURPOSE:To obtain the gas sensor which is less affected by noises, facilitates amplification and is suitable for detection of a slight change by providing a crystal resonating plate inserted between electrodes and a means for detecting the change in the frequency of the crystal resonating plate on the sensor. CONSTITUTION:The crystal resonating plate 1 of the gas sensor body 3 for gas adsorption generates the oscillation of the frequency fx corresponding to the adsorbed amt. of the gas. On the other hand, the crystal resonating plate 1 of the gas sensor body 4 for reference generates the oscillation having the intrinsic reference frequency f0. The two oscillations are inputted to a frequency converting circuit 5 where the signal of the frequency (f0-fx) to be the difference in the frequencies of the two oscillations is outputted. The outputted signal is amplified in an amplifier circuit 5. The (f0-fx) which is the difference in the frequencies of the oscillations of the gas sensor bodies 3, 4 is proportional to the adsorbed amt. of the gas in the body 3 and, therefore, the adsorbed amt. of the gas is detected from the value of (f0-fx).

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a gas sensor used for gas detection.

[Problems to be solved by conventional techniques and inventions] Conventionally,
Catalytic combustion gas sensors and semiconductor gas sensors are known.

A catalytic combustion gas sensor consists of a bridge circuit between a platinum wire heater whose surface is coated with an active catalyst and sintered, and a resistor with a known resistance value.When flammable gas touches the catalyst, it burns and the platinum wire heater As the temperature of the platinum wire increases, the resistance value of the platinum wire increases. The increase in resistance of the platinum wire is determined by the output current of the bridge circuit, and the gas concentration, which is proportional to the increase in resistance, is detected. In the case of the above-mentioned catalytic combustion type gas sensor, there is a problem that the operating temperature as a sensor is high because detection is performed by burning gas.

On the other hand, a semiconductor type gas sensor uses a sintered body of a metal oxide semiconductor, and utilizes the fact that the resistance value of the sintered body changes depending on the gas concentration around the sintered body. In such semiconductor-type gas sensors, crystal growth of metal oxide occurs even when operating at relatively low temperatures, so the element changes significantly over time, causing a problem in that sensor sensitivity decreases as the sensor changes over time. .

The present invention has been made to solve these problems, and aims to provide a gas sensor that uses a crystal diaphragm sandwiched between metal electrodes, operates at low temperatures, and exhibits little change over time. .

[Means to solve the problem]

The gas sensor of the present invention uses a quartz diaphragm sandwiched between metal electrodes, and is characterized in that gas is detected by measuring the wave number of the oscillating surface of the quartz diaphragm.

[Effect]

In the gas sensor of the present invention, when gas is adsorbed to the crystal diaphragm, the oscillation frequency of the crystal diaphragm changes. This frequency change is interpreted as a signal change and analyzed to detect the amount of gas adsorbed.

〔Example〕

Hereinafter, the present invention will be explained based on drawings showing embodiments thereof.

First, the principle of changing the oscillation frequency of a crystal diaphragm will be explained. Crystal vibrations include longitudinal vibration, thickness shear vibration, bending vibration, etc., and the gas sensor of the present invention utilizes thickness shear vibration of a crystal diaphragm. FIG. 3 shows a schematic perspective view of a quartz diaphragm, in which 1 is a quartz diaphragm, and metal electrodes are mounted on two planes perpendicular to the direction of thickness tQ so as to sandwich the quartz diaphragm 1. It will be done. That is, the direction of the electric field is the direction of the thickness tQ. The thickness shear vibration of the crystal diaphragm 1 is a vibration having a standing transverse wave in which the two planes to which the metal electrodes are bonded are antinodes, that is, in the direction of the arrow in the figure. When the frequency of the thickness shear vibration is f, the relationship shown in equation (1) generally holds true.

2 LQ Lq However, V: propagation velocity of standing wave N: Frequency constant (By transforming 112 as follows, equation (2) is obtained.

f −t9=N df−t, +f・dt, =Q df=−f−dtQ/1Q =−f2・dt,/N (2) Now, gas is adsorbed on the surface of the crystal plate l. If the thickness is dt, then
Within a certain slight change range, based on the change in the apparent thickness tq of the crystal diaphragm 1 including the gas adsorption layer changed by the gas adsorbed on the plane of the crystal diaphragm 1, as shown in equation (2). The oscillation frequency of the crystal diaphragm 1 changes accordingly. The thickness when gas adsorption ff1dt is converted into crystal is expressed by equation (3).

ρ9 ・A-dtQ = ρ ・A-titdtq = ρ
/ρ9・dt...(3) However, tq: Density of the crystal ρ: Density of the gas layer adsorbed on the crystal diaphragm A: Area of the gas adsorption layer, that is, the gas adsorption surface on the quartz diaphragm 1 When a gas of thickness dt is adsorbed, (21
, (3), the oscillation frequency of the crystal diaphragm is expressed by equation (4).

From equation (4), it can be seen that the resonance frequency change df of the thickness shear vibration of the crystal diaphragm 1 is proportional to the minute thickness change dt of the gas adsorption layer.

The above is the operating principle of the gas sensor of the present invention.

Next, an example of a gas sensor using the above operating principle will be described based on the drawings.

FIG. 1 is a sectional view of the gas sensor main body. In the figure 1
is a crystal diaphragm with platinum on both sides.

Electrodes 2, 2 to which noble metal such as palladium is attached are provided. When gas is adsorbed to the surface of the electrode 2 in the gas sensor main body consisting of the crystal diaphragm 1 sandwiched between the electrodes 2.2, the oscillation frequency changes as described above. FIG. 2 is a block diagram of a gas sensor using two gas sensor bodies 3.4.

One gas sensor body 3 is in a gas atmosphere and adsorbs gas, and the other gas sensor body 4 is isolated from the gas atmosphere and is used for reference to compare changes in oscillation frequency due to gas adsorption with the gas sensor body 3. There is. In the gas sensor of the present invention, vibrations generated from the two gas sensor bodies 3 and 4 are input to the frequency conversion circuit 5. The frequency conversion circuit 5 outputs a signal having a frequency corresponding to the difference in the vibration frequencies generated by the two gas sensor bodies 3 and 4, and this signal is amplified by the amplifier circuit 6.

Next, the operation of the gas sensor having the above configuration will be explained.

The crystal diaphragm 1 in the gas sensor main body 3 for gas adsorption generates vibrations at a frequency fx corresponding to the amount of gas adsorption. On the other hand, the reference gas sensor main body 4 has a reference frequency f specific to the crystal diaphragm l. Vibration occurs. The two vibrations are input to a frequency conversion circuit 5, which outputs a signal with a frequency of fo-fX, which is the difference between the frequencies of the two vibrations, and the output signal is amplified by an amplifier circuit 6. . Since fo-f, which is the difference in frequency of vibration between the gas sensor body 3 and the reference gas sensor body 4, is proportional to the amount of gas adsorbed in the gas sensor body 3, the amount of gas adsorption can be detected from the value of fo-fx.

Although noble metals are used as electrode materials in the above embodiments, other metals or other conductive materials may be used to provide selectivity to the gas to be detected.

〔Effect of the invention〕

As explained above, since the gas sensor of the present invention uses a quartz diaphragm, there is little change over time, and since changes in the amount of gas adsorption are detected as changes in the oscillation frequency of the quartz diaphragm, it can operate at low temperatures.

Furthermore, since changes in the oscillation frequency of the crystal diaphragm can be taken directly as changes in the frequency of the signal, the influence of noise in signal processing is reduced compared to the conventional sensor that detects the resistance value of the sensor itself as a current value and converts it into a signal. It is easy to amplify and is suitable for detecting minute changes.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a gas sensor body according to the present invention, FIG. 2 is a block diagram showing the configuration of a gas sensor using the gas sensor body of FIG. 1, and FIG. 3 is a crystal diaphragm used in the gas sensor body of the present invention. FIG. ■...Crystal diaphragm 2...Electrode 3...Gas sensor body 4...Reference gas sensor body 5...Frequency conversion circuit 6...Amplification circuit Note that in the figures, the same symbols are the same or A considerable portion is shown. Agent Masuo Oiwa

Claims (1)

[Claims] (1) A crystal diaphragm sandwiched between electrodes, Detects changes in the oscillation frequency of the crystal diaphragm. means to A gas sensor characterized by: