1.Premixed calibration gas
The method of premixing calibration gas is the sum method of gas sensor calibration. 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 cylinders can be divided into two categories: low-pressure and high-pressure gas equipment.
2.Utilize cross-calibration method
With the cross-calibration method, mainly each sensor suffers from interference from other gases. For example, to calibrate LEL ethane gas, 50% LEL 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. So it is difficult to say which mixture to use and keep it under 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.
3.Method for calibrating other gases with methane 1
There are two methods for accomplishing the use of methane as a surrogate for calibration of other gases. One 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 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.
4.Method 2 for calibrating other gases with methane
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 LEL pentane. Although methane gas was used during calibration, after the instrument is calibrated, its reading is the concentration of pentane gas.
Many low-range hazardous gas sensors can use cross-gas calibration. Similarly, infrared detectors absorb at the same wavelength for any gas, and a cross-calibration method can be used. The advantage of the cross-calibration method is that it allows the sensor to be calibrated using a gas whose range is easily available and easy to handle. However, there are also some problems with using cross-calibration methods. One is that the response factor of each sensor is different, because it is impossible to make every sensor the same when manufacturing the sensor. Additionally response factors cannot be used. The response characteristics will vary depending on the heater voltage setting. Therefore, it is a good method to use the actual target gas to calibrate the sensor for periodic detection.