JP3199293B2 - Gas detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

2. Description of the Related Art In a gas detector, it is necessary to detect and discriminate a very small amount of a combustion gas or an organic compound gas generated by a fire or the like with high sensitivity. Also, it must always be working properly. Conventionally, as a gas detector for such applications, in addition to a smoke detector that detects smoke by light, a semiconductor gas sensor that detects carbon monoxide, and an adsorption film with a wide selectivity is applied to the crystal oscillator,
Some devices detect gas from the change in resonance frequency. Smoke detectors that sense smoke by light sense smoke by measuring the attenuation of light in the smoke. A semiconductor gas sensor uses an oxidation-reduction reaction with a gas to detect an oxidizing gas such as carbon monoxide. In a device using a crystal oscillator, a saturated adsorption amount and a time constant are extracted from the response, and gas is detected by a pattern recognition technique.

[0002]

The above-mentioned conventional gas detector has the following disadvantages. That is, in the smoke detector, since the object to be detected is smoke, it cannot be detected unless the fire sufficiently proceeds. Also, smoke is not always generated during a fire. That is, the sensitivity to fire is low. In a gas detector using a semiconductor gas sensor, the object to be sensed is limited to an oxidizing gas, so that the sensitivity to fire is low and the number of false alarms increases. Although a sensor using a crystal oscillator has high sensitivity to gas, it is difficult to extract a time constant necessary for gas discrimination in an environment where the gas concentration changes during measurement. In addition, since these smoke-type, semiconductor-type, and quartz-crystal-type sensors do not have a function to notify the decrease in the sensitivity of the sensor itself, it is necessary to periodically use smoke or target gas to check whether the sensor is operating properly. Although an inspection is required, there is a disadvantage that the cost increases as the number of sensors increases.

The object of the present invention, high sensitivity gas or organic compound gas generated from abnormality such as a fire, reliably detect and determine the <br/> gas type, and number of the sensitivity test of the gas adsorption film An object of the present invention is to provide a small and inexpensive gas detector that does not require human labor.

[0004]

The gas detector of the present invention comprises:
It has two crystal oscillators provided with gas adsorption films having different adsorption speeds, detects a detection target gas by extracting a time constant from the response of the two crystal oscillators, and determines the type of the target gas. I do.

[0005] The gas detector includes one that extracts a time constant from the response of two crystal oscillators using a linear filter algorithm and determines the type of a target gas.

[0006]

[0007]

Since the time constant is extracted from the responses of the two crystal oscillators, gas detection and type determination can be performed with high sensitivity even if the concentration changes.

Since the dispersion of the response due to the disturbance of the crystal oscillator is used, the sensitivity inspection of the gas adsorbing film can be carried out without much manual operation.

[0009]

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing a configuration of an embodiment of a gas detector according to the present invention. FIG. 2 is a flowchart showing a procedure of a method for inspecting the sensitivity of an adsorption film by a computer 13 in FIG.
FIG. 4 is a flowchart showing a procedure of a gas discriminating method by the computer 13 in FIG. 1, FIG. 4 is a diagram showing a response of a quartz oscillator coated with a brand-new adsorption film using polychlorotrifluoroethylene in a normal environment, and FIG. FIG. 6 is a diagram showing the response of the crystal oscillator with reduced sensitivity in the environment of FIG.
Fig. 3 shows the response a of a quartz oscillator coated with a brand-new adsorption film using polychlorotrifluoroethylene to the gas generated from the vinyl cable coating, and the response b of the quartz oscillator coated with an adsorption film with reduced sensitivity. 7 shows the response of a quartz oscillator using a mixture of polyethylene and polytetrafluoroethylene as an adsorption film to 580 ppm of acetone, and FIG. 8 shows a mixture of polyethylene and polytetrafluoroethylene alternately exposed to 1220 ppm of acetone and air. FIG. 6 is a diagram showing a response c of a quartz oscillator 3 using as an adsorption film and a response d of a quartz oscillator 5 using polyethylene as an adsorption film.

The present invention has been made as a result of a detailed study of the adsorption of gas molecules to be detected on an adsorption film. Here, the findings obtained by the present inventors will be described.

Plasma-derived polymer adsorption film on a quartz oscillator by high-frequency sputtering of fluorine-containing polymer or hydrocarbon-based polymer such as polyethylene, polychlorotrifluoroethylene, or a mixture of polyethylene and polytetrafluoroethylene. Were formed, and the change in the resonance frequency of the quartz oscillator due to the adsorption of the gas molecules to be detected on these adsorption films was examined. It is known that the resonance frequency of a crystal oscillator is proportional to a change in the mass of the adsorption film, that is, the mass of the gas to be detected.

This adsorption film is also sensitive to the movement of molecules in the air, and the molecules in the air are constantly moving, so that there is a quartz oscillator response due to disturbance (noise) of these molecules even under a normal environment. I know. Since the response of the crystal oscillator due to the disturbance becomes smaller as the sensitivity of the attraction film decreases, the decrease in the sensitivity of the attraction film can be detected by monitoring the dispersion of the response of the crystal oscillator due to the disturbance as follows.

[0014]

(Equation 1) Here, k is an integer, m [k] is the response of the crystal oscillator at time k · Δt when Δt is a sampling time,..., Ts is the number of predetermined data,

[0015]

Is the average of Ts data. This dispersion of the crystal oscillator response is considered to represent the sensitivity of the adsorption film. That is, when the sensitivity of the adsorption film is reduced, the response of the crystal oscillator to not only the gas to be detected but also the disturbance is reduced. Therefore, it is possible to detect a decrease in the sensitivity of the adsorption film due to a decrease in the dispersion of the crystal oscillator response. Further, the response of the crystal oscillator to the disturbance is much smaller than the response of the crystal oscillator to the gas to be detected, so that the distinction is easy.

Furthermore, these adsorption films adsorb combustion gas and organic compound gas with high sensitivity, but do not adsorb only a limited type of gas, but have a wide selectivity. Therefore, in order to determine the adsorption gas, the saturated adsorption amount and the time constant must be extracted from the response of the crystal oscillator.
In particular, it is known that the time constant is an effective amount for gas discrimination.

When the gas concentration is constant, the response of the crystal oscillator is expressed using the following exponential function, so that it is easy to extract the time constant.

[0018]

M [k] = a {1−exp (−k · Δt / τ)} where a is a saturated adsorption amount of gas to the adsorption film, and τ is a time constant. However, when the gas concentration changes, the response of the crystal oscillator behaves in a complicated manner, and it is difficult to extract the time constant from the response of one crystal oscillator. Here, if a crystal oscillator coated with adsorption films having different gas adsorption speeds is separately prepared, and the response of the crystal oscillator is u [k], both m [k] and u [k] are functions of the gas concentration. If b _i and c _j are determined so as to satisfy the following equation, the term of the gas concentration can be eliminated, and even if the gas concentration changes, the time constant relating to m [k] can be extracted.

[0019]

(Equation 3) Here, i, j, L and H are integers, and b _i and c _j are real numbers. Therefore, it is possible to determine the b _i by using the algorithm of linear filter from the response two crystal oscillator. b _i is not the time constant for m [k] itself, but the time constant for m [k] can be obtained from b _i . Therefore, it is possible to extract the time constant even in an environment where the concentration changes.

From this knowledge, it is possible to inspect the sensitivity of the adsorption film without monitoring the target gas by monitoring the response of the crystal oscillator due to disturbance. Further, even in an environment where the concentration changes, gas can be detected and determined with high sensitivity by extracting a time constant from the responses of the two crystal oscillators.

The gas detector shown in FIG. 1 has a sensor cell 1, crystal oscillators 3, 5, 6, oscillating circuits 7, 8, 9, a multiplexer 10, a multiplication & sampling circuit 11, and a counter 1.
2, a computer 13 and a display device 14.

The sensor cell 1 exposed to the atmosphere is provided with a crystal oscillator 3 having a gas adsorption film 2 on the surface and a gas adsorption film 4 having a different gas adsorption speed from the gas adsorption film 2 by changing the material. A crystal oscillator 5 and a crystal oscillator 6 not covered with an adsorption film are provided. The crystal oscillator 6 generates a reference frequency for counting the frequencies of the crystal oscillators 3 and 5. Each of the crystal oscillators 3, 5, and 6 is provided with an oscillation circuit 7, 8, and 9, respectively. The oscillation circuits 7, 8, and 9 oscillate at the resonance frequencies of the corresponding crystal oscillators 3, 5, and 6. Has become. The multiplexer 10 includes the oscillation circuit 7,
Either one of 8 and multiplication & sampling circuit 1
1 outputs a signal whose cycle is the difference between the cycle of the output of the oscillation circuit 9 and the selected one of the oscillation circuits 7 and 8. The counter 12 measures the period of this signal,
Send to The computer 13 calculates the frequency by taking the reciprocal of the period, and displays the reduction in the sensitivity of the adsorption film and the determination result of the gas to be detected on a display device 14 such as an LED or CRT.

Next, the operation of the gas detector will be described. When each of the oscillation circuits 7, 8, and 9 is put into an operating state, each of the crystal oscillators 3, 5, and 6 starts oscillating. The crystal oscillator 6 always oscillates at the same resonance frequency regardless of the presence or absence of the detection gas. When the detection gas is generated, adsorption of the gas molecules to be detected on the gas adsorption films 2 and 4 on the crystal oscillators 3 and 5 starts, and the resonance frequency of the crystal oscillators 3 and 5 shifts according to the amount of adsorption to oscillate. The oscillation frequency of the circuits 7 and 8 shifts. The shift of the oscillation frequency is proportional to the mass of the gas to be detected adsorbed on the gas adsorption films 2 and 4. The counter 12 outputs the difference between the outputs of the oscillation circuits 7 and 8 and the oscillation circuit 9, that is, the mass of the gas to be detected adsorbed on the gas adsorption films 2 and 4. Further, since the temperatures of the crystal oscillators 3, 5, and 6 are the same, the change in the resonance frequency due to the temperature change is canceled out, and the compensation for the temperature change is performed at the same time. The multiplexer 10 selects the oscillating circuits 7 and 8 at regular time intervals. The period of the oscillating circuits 7 and 8 is 1 second or less, which is sufficiently shorter than the gas measurement time. It can be considered that the output has been obtained.

The computer 13 performs a sensitivity test on the adsorption film according to the following procedure.

That is, as shown in FIG. 2, the sensitivity of the adsorption film can be inspected by obtaining the response of the crystal oscillator (step 21) after obtaining the response of the crystal oscillator (step 21) (step 21). 22).

[0026]

(Equation 4) Then, it is compared with a predetermined threshold value (step 23). When it becomes smaller than the threshold value, it is considered that the sensitivity of the adsorption film is lowered, and a warning display of the sensor sensitivity lowering is performed (step 24). If it is larger than the threshold value, the process returns to step 21.

The computer 13 determines the gas type according to the following procedure.

That is, as shown in FIG. 3, first, a gas must be detected (step 31), and a time constant must be extracted from the crystal oscillator response in order to determine the type of the gas. The time constant is extracted using such an algorithm. That is, p is an integer, and the response vector M of the crystal oscillator 3 from time k · Δt to time (k−p) · Δt

[0029]

M = (m [k], m [k−1],..., M [k−p]) ^T
(T represents transposition) and time (k−L−) retroactive from time (k−1) · Δt
p) Response m of crystal oscillator 3 up to Δt and time k · Δt
(P + 1) × (L + H + 1) matrix Φ composed of the response u of the crystal oscillator 5 from time till (k−H−p) · Δt

[0030]

(Equation 6) Calculate B = {Φ ^T Φ ⁻¹ Φ ^T M using
_{1, b 2, ..., b} L, c 0, c 1, ..., obtaining c _H} (step 32). Of the obtained B, ｛b ₁ , b ₂ ,
…, B _L方程式, the equation for x

[0031]

(Equation 7) Is solved (step 33). This equation has L solutions, where r is a positive real solution, and the time constant τ is

[0032]

Τ = Δt / log (r) (step 34). Where L,
How to select the value of H depends on the number of time constants included in the crystal oscillator response.
Even when the value of H is changed, the value of the time constant obtained is almost the same. The target gas can be determined using the time constant obtained by the above algorithm (step 35).

Next, the result of examining the effectiveness of this embodiment will be described.

First, the result of using the dispersion of the crystal oscillator response for detecting the decrease in the sensitivity of the adsorption film will be described. FIG. 4 shows the response of a crystal resonator coated with a brand new adsorption film in a normal environment. The sampling time Δt is 4 (seconds),
Ts is 150, that is, the measurement time is 600 seconds, and the adsorption film is made of a quartz oscillator (9 MHz, AT one cut, thickness 0.1) by sputtering of polychlorotrifluoroethylene.
mm, diameter 85 mm) formed on the surface. The sputtering power was 1 W / cm ² and the sputtering time was 2 hours. The dispersion of the crystal oscillator response is as large as 0.168. In other words, it is considered that the crystal oscillator responds to disturbances other than the detected gas to some extent, and the sensitivity of the adsorption film is still high. FIG. 5 shows the response of a quartz oscillator coated with an adsorption film whose sensitivity has been reduced by performing a high-temperature acceleration test in a normal environment. The dispersion of the crystal oscillator response is as small as 0.051. Thus, the gas detector is not sensitive to only one gas to be detected, but is also sensitive to negligible changes in the normal environment, i.e., disturbances, thus monitoring the crystal oscillator response to disturbances. By doing so, the sensitivity can be checked.

Next, the response of the crystal oscillator to the gas to be detected will be described. The gas to be detected is 0.012 g of gas generated by heating a vinyl cable coating used as a communication cable coating in a 30 liter container with a heater at about 150 ° C. for 1 minute. FIG. 6 shows the response of the crystal oscillator using polychlorotrifluoroethylene to the gas. a is the response of the crystal oscillator using the brand new adsorption film, and b is the response of the crystal oscillator using the adsorption film with reduced sensitivity. It can be seen that the sensitivity of the quartz oscillator having a small variance of the response due to disturbance under a normal environment to the detected gas is also lowered. As is clear from the responses shown in FIGS. 4 and 5, the response of the crystal oscillator to the gas to be detected is sufficiently large as compared with the response due to the disturbance, so that the distinction is easy.

Next, in order to determine the gas, (1) a mixture of polyethylene and polytetrafluoroethylene, (2) polyethylene as targets, and a sputtering power of 1 W / cm ² , The sputtering time was 2 hours, and those formed on the surfaces of the crystal oscillators 3 and 5 by high frequency sputtering were used.

First, FIG. 7 shows the response of a quartz oscillator 3 using a mixture of polyethylene and polytetrafluoroethylene to 580 ppm of acetone as an adsorption film. The time constant resulting from this response is about 140 seconds. The time constant is about 30 seconds for other gases, for example, methanol, and about 630 seconds for benzene, and the time constant is effective for gas discrimination.

Next, as an example of the response when the gas concentration is not always constant, FIG. 8 shows the response of two crystal oscillators alternately exposed to acetone and air at 1220 ppm. c is the response of the quartz oscillator 3 coated with an adsorption film using a mixture of polyethylene and polytetrafluoroethylene, and d is the response of the quartz oscillator 5 coated with an adsorption film using only polyethylene. Although it is difficult to extract the time constant of c from only the response of c as described above, p = 150 using not only the response of c but also the response of d, 2 ≦ L ≦ 7, 2 ≦ H ≦ 4. As a result of estimating the time constant of c in each case where H and L were changed such that H ≦ L, a time constant of about 150 seconds was estimated in each case, and it was confirmed that the target gas was acetone. I was able to determine.

Since the gas detector of this embodiment detects the target gas by the response of two quartz oscillators having adsorption films having different gas adsorption speeds, the gas detector can detect the gas with high sensitivity even in an environment where the gas concentration changes. Detection and gas type discrimination,
Since a decrease in the sensitivity of the adsorption film is detected based on the dispersion of the crystal oscillator response due to the disturbance, a large number of labors are not required for the sensitivity test.

[0040]

As described above, according to the present invention, the sensitivity of the adsorption film can be inspected without manual operation by using the dispersion of the crystal oscillator response due to the disturbance, so that the cost can be reduced. By extracting the time constant by the algorithm of a linear filter from the response of two crystal oscillators provided with adsorption films with different gas adsorption speeds on the surface, it is difficult to change the concentration during measurement, which was difficult in the past. This has the effect that the early detection of gas and the type determination can be performed accurately.

[Brief description of the drawings]

FIG. 1 is a configuration block diagram of an embodiment of a gas detector according to the present invention.

FIG. 2 is a flowchart showing a procedure of a method for inspecting the sensitivity of an adsorption film by a computer 13 in FIG. 1;

FIG. 3 is a flowchart showing a procedure of a gas discriminating method by a computer 13 of FIG. 1;

FIG. 4 is a diagram showing the response of a quartz oscillator coated with a brand-new adsorption film using polychlorotrifluoroethylene in a normal environment.

FIG. 5 is a diagram showing a response of a crystal oscillator having reduced sensitivity in a normal environment.

FIG. 6 shows the response a of a quartz oscillator coated with a brand-new adsorption film using polychlorotrifluoroethylene and the response b of a quartz oscillator coated with an adsorption film with reduced sensitivity to gas generated from a vinyl cable coating. FIG.

FIG. 7 is a diagram showing the response of a quartz oscillator using a mixture of polyethylene and polytetrafluoroethylene as an adsorption film to 580 ppm of acetone.

FIG. 8 shows a response c of the crystal oscillator 3 using a mixture of polyethylene and polytetrafluoroethylene alternately exposed to 1220 ppm of acetone and air as an adsorption film, and a response d of a crystal oscillator 5 using polyethylene as an adsorption film. FIG.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-231548 (JP, A) JP-A-60-201233 (JP, A) JP-A-63-168537 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) G01N 5/02 JICST file (JOIS)

Claims (2)

(57) [Claims] 1. Using a crystal oscillator having a gas adsorption film provided on its surface, determining a change in the resonance frequency of the crystal oscillator due to adsorption of a gas to be detected onto the gas adsorption film,
A gas detector for detecting the target gas and determining the type of the target gas, comprising: two crystal oscillators provided with gas adsorption films having different adsorption speeds; A gas detector, wherein a gas to be detected is detected by extracting a constant, and a type of the target gas is determined. 2. Using a linear filter algorithm, 2
2. The gas detector according to claim 1, wherein a time constant is extracted from the responses of the two crystal oscillators, and the type of the target gas is determined.