Detailed introduction to calibration methods for gas detectors
The calibration of gas detector depends on the type and concentration range of gas. In order to achieve satisfactory accuracy, the mixture of target gas and background ambient gas is a good calibration gas. In fact, most calibration gases are purchased from Chemical plant.
A. Pre mixed calibration gas
The method of pre mixing calibration gases is the preferred and most popular method for gas sensor calibration. Pre mixed calibration gases can be compressed and stored in gas cylinders at a certain pressure. These bottles can be of any size, but during on-site calibration, people prefer small and lightweight gas 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. High pressure gas cylinders are designed for pure chemical hazardous materials. For calibration gases, these cylinders typically have thick walls and can withstand a pressure of 2000 psi.
In order to calibrate the sensor and allow high-pressure gas to flow out of the high-pressure cylinder, a pressure reducer is required. It is composed of a pressure controller, a pressure gauge, and a flow limiting hole. A flow limiting hole is a type of extremely small line hole that allows a certain amount of air flow under a given pressure.
During the calibration process, some sensors require moisture and humidity to obtain appropriate readings. This humidification process is the same as the sensor zero setting.
B. Penetration equipment
The permeation device is a sealed container containing gas-liquid equilibrium chemicals. Gas molecules penetrate through the edges or top cover of the container. The permeation rate of gas molecules depends on the permeability and temperature of the substance. Permeability is stable over a long period of time. A constant calibration gas formed by mixing with penetrating chemicals, its permeability is known after the temperature is given. This requires a constant temperature caliber measuring device and a flow controller. However, the permeation tube continuously transports chemicals at a constant rate, resulting in storage and safety issues. The permeability of a given gas may be too high or too low for the application. For example, high vapor pressure gases penetrate too quickly, while very low vapor pressure gas chemicals have a permeability that is too low for any purpose.
Most penetration devices can be found in laboratories and are often used in analytical instruments. For gas monitoring, the concentration required for sensor calibration is typical of high permeability equipment. Therefore, its application is limited.
C. Cross calibration
By using cross calibration methods, each sensor is mainly affected by interference from other gases. For example, to calibrate 100% LEL of ethane gas, 50% ELE of methane gas is usually used instead of the actual ethane gas. This is because ethane is liquid at room temperature with low vapor pressure. Therefore, it is difficult to use a mixture of * * and maintain it at high pressure.
In other words, methane has a high vapor pressure and is very stable. In addition, it can be mixed with air and maintained at high pressure. Compared with ethane mixture, methane can be used for more calibration purposes and has a long lifespan. 50% ethane mixture is easily obtained. Therefore, manufacturers of combustible gas alarm devices recommend using methane as a substitute for calibrating other gases.
There are two methods for using 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 readings of other gases. The most commonly used catalytic sensors are like this.
The catalytic type sensor is a linear output, so the use of response factor conforms to the full scale range. For example, when calibrating a sensor with methane, the output of pentane is only half that of methane. Therefore, the response factor of pentane is 0.5. So when the sensor actually detects pentane and uses methane calibration, the reading is multiplied by 0.5 to obtain the reading of pentane.
The second method still uses methane as the calibration gas, but the calibration reading is double. For example, using 50% LEL methane calibration gas to calibrate 100% LEL pentane. Although methane gas was used for calibration, the reading of the instrument after calibration is the concentration of pentane gas.
D. Gas mixing
Not all calibration gases are available. Even if it is available, it is possible that the calibration gas may not be available at a certain concentration or a fixed background mixture. However, many mixtures can be calibrated for low concentration range gas monitors after dilution.
