Domestic gas leakage alarm. Encyclopedia of radio electronics and electrical engineering

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Constant monitoring of the concentration of explosive gases in the air of residential and working premises is the most effective means of preventing their ignition. Over time, the relevance of gas safety is only growing. The air environment surrounding us can contain not only the source of life - oxygen, but also dangerous substances, such as explosive gases. Gas detectors serve as technical safety devices. They are designed to detect excess concentrations of hazardous gases in the environment.

Natural gas - one of the most explosive - is widely used in everyday life as a cheap fuel for heating, water heating and cooking. As you know, the main component of natural gas is methane (CH₄), it contains from 70 to 98%. Therefore, to monitor the leakage of natural gas, it is necessary to use a sensor that responds to the concentration of methane in the air.

Concentration limits of flame propagation in a mixture of methane with air in volume percent: lower - 5, upper - 15 [1]. The lower concentration limit of flame propagation (LEL) or the lower explosive threshold (LEL) is the minimum proportion of a combustible substance in a homogeneous mixture with an oxidizing agent (air, oxygen), at which flame propagation through the mixture is possible at any distance from the ignition source (open external flame, spark rank, etc.). For methane 100% LEL (LEL) = 5 vol% = 50000 ppm = 33500 mg/m³ [2].

As a general rule, most commercially available gas leak detectors are set to one percent by volume. It is at this concentration that they give an alarm, turn on the ventilation system of the room and, using an electromagnetic valve, turn off the gas supply to the gas supply system of a residential building or apartment.

Semiconductor and thermal catalytic sensors of pellistor type are best suited for monitoring the leakage of combustible gases due to the simplicity of their design. Usually, semiconductor sensors are used only to signal the excess of the permissible concentration of methane in the air, and if there is a need to measure the gas concentration, then two-pellistor thermal catalytic sensors are used. They contain two pellistors - spirals of platinum wire, heated by a current passing through them to a temperature of about 400 ^оC. They form a conventional resistive voltage divider (Fig. 1).

Rice. 1. Resistive voltage divider

When there is no methane in the air, the resistances of both pellistors are equal, therefore, the output voltage of the divider is exactly half of the supply voltage U_Pete. Since the active pellistor is covered with a catalyst layer, on its surface, in the presence of methane, this gas is oxidized by air oxygen. The temperature of the active pellistor increases and the resistance increases. By including such a sensor in the measuring bridge, you can easily measure the unbalance voltage of the bridge:

∆U=U_O - OR_Pete/ 2,

and then determine the concentration of the gas.

Thermal catalytic sensors are inexpensive and easy to use, but they have a number of disadvantages. Firstly, at a high gas concentration, the sensor is oversaturated, and it fails. Secondly, the catalyst is depleted over time, which leads to a decrease in the sensitivity of the sensor. Thirdly, oxygen is needed for the chemical reaction taking place in the sensor, therefore, if it is deficient, the sensor greatly underestimates the readings. Finally, the thermal catalytic sensor can be "poisoned" by certain substances. It is sensitive, for example, to the attack of silicates or high concentrations of hydrogen sulfide (H₂S). In addition, thermal catalytic sensors require maintenance. Periodic checks are required to ensure they are working properly. The life time of the pellistor is a maximum of one to three years.

All these shortcomings are devoid of highly reliable and virtually trouble-free infrared combustible gas sensors (NDIR sensors). With the help of measuring and normalizing detectors, they evaluate the degree of absorption of infrared radiation by the gas. The detectors use infrared radiation of two wavelengths. One of them absorbs gas, and the second does not. The sensor measures the level of absorbed radiation and compares it with the level of the reference. This information is processed according to a rather complex algorithm, for which almost every sensor has a built-in microprocessor.

Digital NDIR sensors of the PrimePell series [3] can replace pellistor ones. At the heart of PrimePell sensors is a patented IR sensor and an ARM V7 microprocessor that controls the sensor, calculates the gas concentration and outputs the result in analog and digital form. The microcontroller also monitors the health of the sensor and registers power failures. Digital information can be received either through the serial port or through the interface I²C, which also changes the operating modes of the sensor.

With the correct supply voltage, the PrimePell sensor can replace the catalytic thermal sensor previously installed in the gas analyzer, and no modifications to the analyzer are required for this.

The layout of the domestic gas leak detector using the PrimePell infrared methane sensor is shown in fig. 2. The sensitivity of this sensor is 100 mV at 100% LEL. Internal "pellistors" of the sensor and resistors R1-R3 form a measuring bridge. The signal taken from the diagonal of the bridge amplifies the instrumental amplifier DA1 with a fixed gain of 10. Its output voltage of 1000 mV corresponds to 100% LEL. Therefore, the readings of the built-in digital voltmeter PV1 in tens of millivolts are numerically equal to the percentage of LEL.

Rice. Fig. 2. Schematic diagram of a domestic gas leak detector using a PrimePell infrared methane sensor (click to enlarge)

In the UK (the country of manufacture of the sensor), 100% LEL is 4,4% v/v, not 5% v/v in our country. Based on this, the response threshold of the comparator DA2, corresponding to one volume percent, is set to 227 mV by a selection of resistors R5 and R6. High threshold setting accuracy is not required. When the comparator is triggered, the HA1 piezo emitter with a built-in generator gives an alarm signal. In addition, the signal from the comparator through the U1 optocoupler goes to the actuators - the ventilation system control unit (fanlight openers, exhaust fans, etc.) and to the electromagnetic gas valve that turns off the gas supply.

The adjustment of the device is reduced to setting zero readings of the digital voltmeter PV1 using a variable resistor R2 in the absence of methane in the air. Guaranteed service life of the sensor is at least five years. Often it is not necessary to check its performance.

Literature

1. State standard of the USSR GOST 5542-87 "Combustible natural gases for industrial and domestic purposes. Specifications". - URL: ukrgazkom.com. ua/assets/files/normative/5542-87.pdf.
2. Table of concentration ratios for the most common combustible and toxic gases (normal conditions). - URL: centros.ru/tablica-gazy.
3. PrimePell Pellistor Replacement Infrared Gas Sensor. - URL: terraelectronica.ru/%2Fpdf%2FCLAIRAIR%2FPrimePell-datasheet-DES10-Iss4.pdf.

Author: A. Kornev

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