Introduction to cross-calibration of gas detectors
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. So it's difficult to use a mixture 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.
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 system output, so the use of response factors is consistent 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.
