JPS6237338B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a gas detection method using a gas sensing element made of a metal oxide semiconductor.

In general, gas-sensitive elements made of metal oxide semiconductors have different sensitivity and selectivity to various gases depending on temperature, voltage, etc.
In addition to manufacturing conditions, temperature and
I use it by changing the voltage. However, the speed at which the gas sensing element output follows changes in gas concentration tends to be slow at low temperatures and low voltages. For example, to explain the response characteristics when used at 150 to 250℃ and 20V, as shown in Figure 1, when the gas concentration suddenly increases, the gas sensing element S
Although the output appears relatively quickly (for example, within 20 seconds), the output voltage V decreases very gradually when the gas concentration suddenly decreases. Therefore, Figure 1a
Between A and B, even if there is a gas whose concentration is less than the gas concentration corresponding to the output voltage shown by the solid line, or even if the gas concentration is zero, there will not be much change in the output. It's a bad thing. This means that changes caused by gas adsorption, etc. that occur in the metal oxide semiconductor to a degree corresponding to the gas concentration between O and A in Figure 1a, are restored by gas desorption in response to gas removal or a decrease in gas concentration. This is thought to be due to the amount of time it takes. (Figure 1b is a measurement circuit diagram.) This desorption of gas is promoted by increasing the temperature of the gas sensing element. (1) In normal use, only one of the two electrodes is used as a heater. If used, both electrodes are used as heaters to increase the temperature of the gas sensitive element.

(2) Temporarily increase the heater voltage.

(3) Increase the voltage between both electrodes.

(4) Heated with an external heater.

There are other means such as Of course, these means may be used in combination. For example, according to the gas sensing element explained in FIG. For devices in contact with gas, it may take several tens of seconds, and this operation of actively desorbing gas is called a reset.

In this way, when a reset is performed, the temperature of the gas sensing element immediately after the reset is higher than that under normal use, and the surface condition of the element has also changed, so as shown in Figures 2a and b. Normally, the output voltage will overshoot and it will take some time (for example, 10 seconds to 10 minutes) for the gas sensitivity characteristics to return to normal. This is called the stability time To. FIG. 2b is a graph of the output voltage with and without reset when the gas concentration suddenly drops to zero.
T _R indicates the reset time. Generally, a reset is performed when the gas-sensitive element is first used, or when it is restarted after being out of use for a long time due to a power outage. The main purpose is to take off the clothes you are wearing.

As explained above, when conventional gas alarms issue an alarm due to a high concentration of gas, the alarm does not stop even if the gas is reduced or removed, and in the case of gas concentration measuring devices, the alarm remains high even if the gas is reduced or removed. In various application devices using gas-sensitive elements, such as those used to provide instructions, there has been a problem in that the element's ability to follow changes in gas concentration is slow.

In the present invention, by repeating the reset and stabilization time to bring the gas sensitive characteristics to a normal state, and the applied operations of the gas sensitive element such as measurement and detection, according to a fixed time schedule that is adapted to the characteristics of the gas sensitive element,
This is a gas detection method using a gas sensing element that exhibits a function that accurately reflects the gas concentration, that is, it has a reset mechanism, and the reset mechanism and output extraction mechanism are operated on a predetermined time schedule. After a heating reset for a certain period of time based on the heating, the normal output voltage overshoots and the gas sensitivity characteristic recovers to normal after a predetermined stabilization time has elapsed, gas measurement is started, and the gas concentration is detected after a predetermined time. This is a gas detection method using a gas sensing element made of a metal oxide semiconductor.

In the gas detection method according to the present invention, a metal oxide semiconductor gas sensing element is used, a reset mechanism,
A time control mechanism that operates methods such as output, alarm, measurement, and control according to a fixed time schedule is connected as appropriate.

FIG. 3 is a block diagram showing one example of the gas detection method according to the present invention. The metal oxide semiconductor gas sensing element is heated to a predetermined temperature by power supplied from the power supply device, and the change in resistance thereof is extracted as an output voltage by the output device.
The output of the gas sensing element is transmitted to the measurement and alarm device of the terminal, and the output is transmitted to the output on/off device, memory, and output controlled by the time control device based on a predetermined time schedule, as necessary. The information may be transmitted to a measurement and alarm device via a selection device. Further, the gas sensitive element can be reset by a reset device, and this reset operation is also controlled according to a time schedule by a time control device. It goes without saying that power is also supplied to the time control device, output device, etc. from the power supply device.

In the device according to the present invention, the time schedule is as follows: (1) Resetting for a certain period of time (T _R seconds) (2) Returning to a normal circuit state, and a certain time measuring circuit does not operate the measuring device. (The stabilization time is To seconds.) (3) After the stabilization time To seconds has elapsed, operate the measuring device for Tm seconds.

(4) Reset again for a certain period of time.

That is, one cycle of this time schedule is ( _TR + To + Tm) seconds.

Next, an example in which the apparatus according to the present invention is used will be described.

Figure 4 shows a case where the gas concentration is constant and only reset is performed at regular time intervals, and the meter will indicate an output corresponding to the form ABHIJDEKLMG unless the meter operations other than reset are time-controlled. However, if you perform time control, ACIJDFLMG
Indicates the output corresponding to the form. HI and KL indicate the overshoot due to reset.

Figure 5 shows the case where the gas concentration decreases. Assuming that the gas concentration changes like ABCD, if measurements are made continuously without resetting the meter for the second time, the meter's indication (output voltage) will be A. ′B′E, and the concentration between B′E indicates a much higher concentration than the actual concentration CD. However, if only the reset is time-controlled, the instrument's instructions will be
DFGHB'JKLMPQ becomes DFGHB'JKLMPQ, and if the instrument is time-controlled, it becomes DIHB'JNMPQ and the overshoot is removed. In FIG. 5, if each point between H-B' and MP is recorded using a dot-type recorder as a measuring instrument, a substantially continuous recording chart can be obtained.
It is also possible to use a memory circuit in the measurement circuit to instruct or record information such as HB'RMP. It is also possible to fix the indication between B'R and B' using an indicator. In these cases, (reset time T _R + stabilization time To)
Although this will cause a delay in the instructions, the blank time for instructions can be eliminated. Of course, this indicating instrument may be a digital display, or a continuous recording instrument may be used.

In the above explanation, an example has been explained in the case of measuring and recording gas concentration, but the circuit output corresponding to the instruction of the measuring device can be used for various devices such as alarms and ventilation fan control.

As explained above, the gas detection method according to the present invention has an extremely excellent ability to follow the gas concentration.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between gas concentration, output and time, and its measurement circuit diagram. Figure 2 is the gas concentration,
A graph showing the relationship between output and time; FIG. 3 is a block diagram of the device according to the present invention; FIG.
FIG. 5 is a graph showing the relationship between output and time when using the device according to the present invention.

Claims (1)

[Claims] 1. In a gas detection method using a gas-sensitive element made of a metal oxide semiconductor, the gas-sensitive element is heated and reset for a certain period of time, and then the output voltage of the element is overshot by heating, and the gas-sensitive characteristics are restored to a normal state. A gas detection method comprising: starting gas measurement after a predetermined stabilization time has elapsed, and detecting the gas concentration after the predetermined time.