Detailed introduction to gas detector calibration method
A. Premixed calibration gas
The method of premixing calibration gas is the gas sensor calibration and method. Premixed calibration gases can be compressed and stored in cylinders under pressure. These bottles can be any size, but when calibrating in the field, people prefer smaller, lighter bottles. These small, portable gas bottles can be divided into two categories: low-pressure and high-pressure gas equipment.
Low-pressure gas cylinders are thin-walled, lightweight, and generally not recyclable and disposable. High-pressure gas cylinders are designed for pure chemical hazards. For calibration gases, these bottles usually have thick walls and can withstand pressures up to 2000 psi.
In order to calibrate the sensor and allow high-pressure gas to flow out of the high-pressure gas bottle, a pressure reducer is required. It is composed of a pressure controller, a pressure gauge, and a flow restriction hole. A flow restriction orifice is a very small linear hole that is suitable for allowing a certain amount of air flow at a given pressure.
Some sensors require moisture during the calibration process in order to get proper readings. The humidification process steps are the same as the sensor zero point setting.
B. Penetration equipment
A permeation device is a sealed container containing chemicals that equalize the gas and liquid phases. Gas molecules permeate through the rim or top of the permeation container. The rate of penetration of gas molecules depends on the permeability of the substance and the temperature. The penetration rate is stable over the long term. A constant calibration gas mixed with the penetrating chemical, the permeability of which is known given the temperature. This requires a thermostatic caliber gauge and flow controller. However, permeate tubes continuously deliver chemicals at a constant rate, which creates 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 permeate too quickly and very low vapor pressure gas chemicals have permeability rates that are too low to be of any use.
Osmosis equipment can mostly be found in laboratories and is often used on analytical instruments. For gas monitoring, the concentrations required for sensor calibration are typical of high permeability equipment. Therefore its application is limited.
C. Cross calibration
With the cross-calibration method, mainly each sensor suffers from interference from other gases. For example, to calibrate 100% LEL ethane gas, 50% ELE methane gas is usually used instead of the actual ethane gas. This is because ethane is a liquid at room temperature and has a low vapor pressure. Therefore it is difficult to use a safe mixture and maintain it at high pressure.
In other words, methane has a high vapor pressure and is very stable. Additionally, it can be mixed with air and maintained at very high pressures. Methane can be used in more calibration situations than ethane mixtures, and it has a long life. A 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 methods for accomplishing the use of methane as a surrogate for calibration of other gases. The first method is to calibrate the combustible gas alarm with methane and, at the same time, replace the readings for other gases by multiplying the readings obtained by the response factors in the manual. 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 line with 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 for pentane is 0.5. So when the sensor actually detects pentane but is calibrated with methane, the reading is multiplied by 0.5 to get the pentane reading.
The second method still uses methane as the calibration gas, but the calibration reading is doubled. For example, use 50% LEL methane calibration gas to calibrate 100% LEL pentane. Although methane gas was used during calibration, after the instrument is calibrated, its reading is the concentration of pentane gas.
D. Gas mixing
Not all calibration gases are available. Even if it is available, it is possible that at a certain concentration or fixed background mixture, the calibration gas is not available. However, many gas mixtures can be diluted to calibrate low-concentration range gas monitors.
