JPS6350658B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for accurately detecting the degree of contamination of the atmosphere of a smoking room or the like, that is, contamination due to extremely low concentration of gas.

Recently, metal oxide semiconductor elements such as SnO ₂ N-type semiconductors have come to be widely used as sensing elements for detecting reducing gases such as carbon monoxide and alcohol. However, when this element is used as an element to detect pollution in a smoking room, pollution inside a car compared to the outside, pollution inside a garage due to CO in engine exhaust gas, etc., it is necessary to detect extremely low concentration gases, and the element itself has to be detected. is affected by changes in indoor humidity, etc., and for this reason, it is not possible to detect with sufficient accuracy using a method that directly measures the indoor atmosphere using an element.

The present invention has been made in view of these points, and includes a metal oxide semiconductor gas sensor that is brought into contact with the atmosphere of the room to be measured and the outside atmosphere alternately, so that the sensor can interact with the atmosphere of the room to be measured and the atmosphere. Low-concentration gas in a room atmosphere is characterized in that the gas concentration in the atmosphere to be measured is detected by determining the difference in the output of This is a detection method.

In general, the resistance value (Rs) of the above element changes in almost inverse proportion to the gas concentration (C) in the atmosphere or its power root, and as shown in Figure 1, the logarithm of both, logRs,
The relationship of logC is a linear inversely proportional relationship. Therefore, the gas concentration in the atmosphere can be determined by the change in the resistance value of the element, but the characteristics of the relationship between the resistance value and the gas concentration change over time. In other words, when the device is energized, as time passes, the line A showing the above characteristics decreases while maintaining a constant slope (line A ₁ ,
A ₂ ), and when left in a non-energized state, the line showing the above characteristics gradually rises. Therefore, in actual usage conditions, the resistance value of the element corresponding to the gas concentration changes depending on the interval and cycle of repeated energization and de-energization, resulting in a lack of measurement accuracy, especially in low concentrations such as smoking rooms. This method could not be applied to detecting the degree of contamination in an atmosphere containing many gases and humidity changes.

The line (A, A ₁ , A ₂ ) indicating the above characteristics varies depending on the thermal history given to the element, but the slope of the line (A, A ₁ , A ₂ ) also changes due to these variations. It has been experimentally confirmed that there is almost no change. Therefore, this fact, that is, the above line (A,
Gas concentration can be measured accurately by utilizing the characteristic that the slope of A ₁ , A ₂ ) does not change due to aging or thermal history of the element.

Generally, the resistance of a detection element is expressed by the following formula.

Rs=K/C〓 K and α are constants determined by the gas and the type of element, and α is 0.5 to 0.8 for normal elements. Therefore, the resistance of the element in the atmosphere (Rss) and the resistance in the atmosphere to be measured are The resistance of the element (Rsx) is Rss=K/Cs〓 (Cs is the gas concentration in the atmosphere) Rsx=K/Cx〓 (Cx is the gas concentration in the atmosphere to be measured) Cx=Cs (Rss/Rsx) ^1/ 〓 Above formula Therefore, since Cs is known, the gas concentration (Cx) of the atmosphere to be measured can be found by finding the value of Rss/Rsx.

Embodiments of the present invention will be described below with reference to the drawings.
1 is a constant voltage power supply; 2 is a gas sensor made of a metal oxide semiconductor; an impedance converter 4 or an impedance converter 5 and an inverter 6 are switchably connected to the sensor 2 via a switch 3; 8. Connect a parallel circuit consisting of a switch 10, a capacitor 11, and an integrator 12 via a resistor 9, and connect this to a zero detector 13.
It is connected to the controller 14 through. This controller 14 controls the switches 3, 8 and 10, and also controls the gas detection section 30 in which the sensor 2 is arranged. Switches 3 and 8 are constructed from analog switches such as relays or FETs.

The gas detection unit 30 is disposed in a through hole in a wall 40 that separates an indoor atmosphere 60 of a smoking room or the like to be measured from an outdoor atmosphere (atmosphere) 50, and the sensor 2 is held by a holding plate 31 having an L-shaped cross section. There is. This holding plate 31 is rotated by a control means (not shown) and is alternately displaced to the position shown by the solid line and the position shown by the two-dot chain line, thereby placing the sensor 2 in the measurement atmosphere 60 and the outdoor atmosphere 50. Let me touch you.

Next, a method of gas detection using this device will be explained. First, switch 10 is turned on to shoot out the residual voltage in capacitor 11, and with the output voltage of integrator 12 set to zero, switch 10 is turned off.

Outdoor atmosphere 50 where sensor 2 becomes the standard atmosphere
switch 3 is terminal 2 when it is in contact with
6 side, switch 8 is connected to the terminal 28 side. When this state is maintained for a certain period of time (Ts), the integrator 1
The output voltage of sensor 2 gradually increases, and the integrator output shows the voltage (Vss) of sensor 2 in the standard gas.
A voltage proportional to xTs appears. Next, holding plate 3
1 is rotated so that the sensor 2 comes into contact with the atmosphere to be measured 60 (solid line position in FIG. 3), and the switch 3 is connected to the terminal 27 and the switch 8 is connected to the terminal 29. In this case, a negative voltage appears at terminal 29 by inverter 6. Therefore, the integrator 12 is discharged by this signal, and the integrator output becomes zero after a certain time (Tx).
In other words, Vss×Ts−Vsx×Tx=0 (Vsx: voltage in the atmosphere of the room being measured) Tx=Ts・Vss/Vsx=Ts・Rss/Rsx, and by measuring this discharge time (Tx),
An output proportional to Rss/Rsx is obtained, and Rss/Rsx is calculated. When the integrator output becomes zero, the controller 14 operates the switches 3 and 8 to connect them to the terminals 26 and 28, respectively, in response to a signal from the zero detector 13 that detects this, and the gas detection section 30 A signal is sent to rotate the holding plate 31, and the sensor 2 is moved to the position indicated by the two-dot chain line, and brought into contact with the outdoor atmosphere. By repeating the same operation at regular intervals, the discharging time for the above charging time (Ts) is determined.
Find Rss/Rsx. Once this value is determined, the figure 1
The concentration of gases such as carbon monoxide in the atmosphere to be measured can be determined from the relationship log C-log Rs. In this case, the characteristics of the element (relationship between log C and log Rs)
changes over time, for example, as shown in the line A → A ₁ → A ₂ , but since the slope is constant, the concentration of the gas to be measured is not affected by changes over time, etc., and can always be accurately measured. will be done.

In other words, since the resistance value ratio etc. of the same element are detected every short time, if the characteristic of the element at the time of detection is, for example, line A in Fig. 1, two points on this line A are in the room to be measured. This corresponds to the sensor output for the atmosphere and atmosphere, and there is almost no change over time in the short time required to detect these two points. Furthermore, if detection is performed again after a relatively long period of time has passed and the characteristics of the element change to, for example, line A ₁ , the two points on line A ₁ will be different from the atmosphere in the room to be measured and the atmosphere. This corresponds to the sensor output, but since there is no difference in slope between line A and line _A1 , the above resistance value ratio etc. can be obtained in the same way as when calculated on line A. In this way, changes in characteristics over time are canceled out, and by determining the difference in output between the atmosphere in the room to be measured and the atmosphere, common environmental fluctuations in the atmosphere in the room to be measured and the atmosphere, such as temperature, humidity, air pollution, etc. etc. are also canceled out. Therefore, the influence of these common variables is removed and
The degree of contamination of the atmosphere in the measurement room compared to the atmosphere can be accurately detected.

When the atmosphere in the room to be measured reaches a predetermined degree of contamination, the measurement signal may be used to activate an alarm, or a ventilation fan may be activated to ventilate the room.

In addition, in the above embodiment, an example was shown in which one element is used and exposed to the indoor and outdoor atmosphere alternately, but the element may be alternately moved indoors and outdoors, or the indoor atmosphere may be exposed to the indoor atmosphere using piping. The element may be brought into contact with the outdoor atmosphere and the outdoor atmosphere alternately. Furthermore, various methods other than those described above can be used for determining the ratio of the resistance values of the elements.

As explained above, the present invention eliminates the influence of environmental changes and determines the gas concentration in the atmosphere to be measured by comparing the output of the metal oxide semiconductor gas sensor between the atmosphere in the chamber to be measured and the atmosphere. This makes it possible to measure the degree of contamination in rooms such as smoking rooms with low-concentration gas atmospheres, which was previously thought to be impossible.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the relationship between gas concentration and resistance value in a metal oxide semiconductor element, Fig. 2 is a circuit diagram showing an example of an apparatus for carrying out the method of the present invention, and Fig. 3 is a cross-sectional explanation of the gas detection section. It is a diagram. 2... Sensor, 3, 8... Switch, 4, 5... Impedance converter, 6... Inverter, 12... Integrator, 14... Controller, 30... Gas detection section,
50...Outdoor atmosphere, 60...Indoor atmosphere.

Claims (1)

[Claims] 1. A metal oxide semiconductor gas sensor is brought into contact with the atmosphere of the room to be measured and the atmosphere outside the room, and the difference in the output of the sensor between the atmosphere of the room to be measured and the atmosphere is determined. A method for detecting a low concentration gas in an indoor atmosphere, characterized by detecting a gas concentration in an atmosphere to be measured while removing the influence of environmental changes common to both.