Gas sensor range introduction

May 12, 2023

The calibration of the gas sensor range can be quite easy or very complicated and expensive, depending on the type and concentration range of the gas. According to the principle, in order to achieve satisfactory accuracy, the equilibrium mixture of target gas and background ambient gas is the best calibration gas. However, although it can be done, the skill requirements for operators are higher than normal. In fact, most calibration gases are bought from chemical plants. The following chapters introduce several methods of range calibration.

A. Pre-mixed calibration cooking equipment, Gas stove, Cooking Equipment

The method of pre-mixing calibration gas is the first and most popular method of gas sensor calibration. The pre-mixed calibration gas can be compressed and stored in a gas cylinder under a certain pressure. The size of these bottles can be arbitrary, but in the field calibration, people like small and light cylinders. These small and portable gas cylinders can be divided into two categories: low-pressure and high-pressure gas equipment.

Low-pressure gas cylinders with thin walls and light weight are usually non-recyclable and disposable. The high-pressure gas cylinder is designed for the purification of dangerous goods. For calibration gases, these cylinders are usually very thick and can withstand a pressure of 2000 psi.

In order to calibrate the sensor and make the high-pressure gas flow out of the high-pressure gas cylinder, a pressure reducer is needed. It is composed of pressure controller, pressure gauge and flow restrictor. Flow restrictor is a kind of extremely small hole suitable for a certain amount of air flow under a given pressure.

During the calibration process, in order to get a proper reading, some sensors require moisture. The steps of this humidification process are the same as the sensor zero setting.

B. Infiltration equipment

The osmosis device is a sealed container filled with gas-liquid equilibrium chemicals. Gas molecules permeate through the edge or top of the permeation container. The penetration rate of gas molecules depends on the permeability and temperature of the substance. The permeability is stable over a long period. The constant calibration gas formed by mixing with the infiltrating chemical will know its permeability after the temperature is given. This requires constant temperature caliber measuring device and flow controller. However, the osmosis tube continuously delivers chemicals at a constant rate, with storage and safety issues arising. The permeability of a given gas may be too high or too low for the application. For example, a high vapor pressure gas permeates too quickly and a very low vapor pressure gas chemical has a permeability that is too low for any use.

Most infiltration equipment can be found in the laboratory, often used in analytical instruments. For gas monitoring, the concentration required for sensor calibration is typical of high permeability equipment. Therefore, its application is restricted.

C. Cross calibration

With the cross calibration method, each sensor is subject to interference from other gases. For example, to calibrate 100% LEL ethane gas, usually 50% ELE methane gas is used instead of the actual ethane gas. This is because ethane is liquid at room temperature and has a low vapor pressure. Therefore, it is difficult to use a precise gas mixture and maintain it under high pressure.

In other words, methane has a very high vapor pressure and is very stable. In addition, it can be mixed with air and kept under high pressure. Compared with ethane mixed gas, methane can be used in more calibration occasions, and it has a long life. 50% ethane mixture is easily available. Therefore, manufacturers of combustible gas alarms recommend using methane as a substitute for calibrating other gases.

There are two ways to complete methane as a substitute for calibrating other gases. The first method is to calibrate the combustible gas alarm with methane, and at the same time, multiply the obtained reading by the response factor in the manual to replace the reading of other gases. This is the case with the most commonly used catalytic sensors.

The catalytic sensor is a line output, so the response factor is used in accordance with the full-scale range. For example, when calibrating the sensor with methane, the pentane output is only half of methane. Therefore, the response factor of pentane is 0.5. So when the sensor actually detects pentane and calibrates with methane, the reading is multiplied by 0.5 to obtain the pentane reading.

The second method still uses methane as the calibration gas, but the calibration reading is double. For example, 50% LEL methane calibration gas is used to calibrate 100% LEL pentane. Although methane gas is used during calibration, after the instrument is calibrated, the reading is the concentration of pentane gas.

Many low-range hazardous gas sensors can use cross-gas calibration. Similarly, the infrared detector absorbs any gas at the same wavelength, and a cross calibration method can be used. The advantage of the cross calibration method is that it allows the calibration of the sensor to use a gas and its range is easy to obtain and handle.

However, there are some problems with the cross-calibration method. One is that the response factor of each sensor is different, because it is impossible to make each sensor the same when manufacturing the sensor. For example, in a catalytic sensor, the heater voltage is specified in the manual. In addition, the response factor cannot be used. The response characteristics will vary with the setting of the heater voltage. Therefore, using the actual target gas to calibrate the sensor for periodic detection is a good method.

Stable non-flammable and non-toxic gases of various concentrations can be obtained from suppliers. Please contact the instrument manufacturer for details.

D. Gas mixing

Not all calibration gases are available. Even if it is available, it is possible that the calibration gas is not available at a certain concentration or fixed background mixture. However, many gas mixtures can be calibrated for low concentration range gas monitors after dilution

Author:

Ms. eleven

E-mail:

puda@elecapoka.com

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+86 13903037996