Several calibration methods for gas sensor range

Calibration of the gas sensor range can be quite easy or very complex and expensive, depending on the type of gas and the concentration range. In principle, in order to achieve satisfactory accuracy, a balanced mixture of the target gas and the background ambient gas is the best calibration gas. However, although it can be done, it requires higher than normal operator skills. In practice, most calibration gases are purchased from chemical plants. The following sections introduce several methods of range calibration.

　　A. Premixed Calibration Gas

　　The premixed calibration gas method is the preferred and most popular method for gas sensor calibration. Premixed calibration gases can be compressed and stored in cylinders under pressure. These cylinders can be any size, but when calibrating in the field, people prefer small and lightweight cylinders. These small and portable cylinders can be divided into two categories: low pressure and high pressure gas equipment.

　　Low pressure cylinders are thin walled, lightweight and are usually non-recyclable and disposable. High pressure cylinders are designed for pure chemical hazardous materials. For calibration gases, these cylinders are usually thick walled and can withstand pressures of 2000 psi.

　　In order to calibrate the sensor, a pressure reducer is needed to make the high-pressure gas flow out of the high-pressure gas cylinder. It is composed of a pressure controller, a pressure gauge, and a flow limiting orifice. The flow limiting orifice is a very small wire hole that allows a certain amount of air flow under a given pressure.

　　During the calibration process, some sensors need to be moistened in order to obtain proper readings. This humidification process is similar to the sensor zero setting procedure.

　　B. Infiltration Equipment

　　The permeation device is a sealed container that contains a chemical in gas-liquid equilibrium. The gas molecules permeate through the lip or lid of the permeation container. The rate at which the gas molecules permeate depends on the permeability of the substance and the temperature. The permeability is stable over long periods of time. A constant calibration gas is mixed with the permeation chemical, and its permeability is known at a given temperature. This requires a constant temperature calibrator and flow controller. However, the permeation tube continuously delivers the chemical 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 while very low vapor pressure gas chemicals have permeabilities that are too low to be of any use.

　　Permeation devices are mostly found in laboratories and are often used in analytical instruments. For gas monitoring, the concentration required for sensor calibration is typical of high permeation devices. Therefore, its application is limited.

　　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, 50% ELE methane gas is usually used instead of the actual ethane gas. This is because ethane is a liquid at room temperature with a low vapor pressure. Therefore, it is difficult to use an accurate mixture and keep it at a high pressure.

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

　　There are two ways to use methane as a substitute for calibrating other gases. The first method is to calibrate the combustible gas alarm with methane and replace the readings of other gases by multiplying the readings obtained by the response factor in the manual. This is the most commonly used catalytic sensor.

　　Catalytic sensors have a line output, so the response factor used is consistent with the full scale range. For example, when the sensor is calibrated with methane, the output for pentane is only half of that for methane. Therefore, the response factor for pentane is 0.5. So when the sensor is actually detecting pentane and is calibrated with methane, multiply the reading by 0.5 to get the pentane reading.

　　The second method is to use methane as the calibration gas, but the calibration reading is doubled. For example, 50%LEL methane calibration gas is used to calibrate 100%LEL pentane. Although methane gas is used for calibration, after the instrument is calibrated, its reading is the concentration of pentane gas.

　　Many low-range hazardous gas sensors can be calibrated using cross-gases. Similarly, infrared detectors absorb at the same wavelength for any gas, so 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 single gas whose range is easily obtained and handled.

　　However, there are some problems with using 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 it. For example, in catalytic sensors, the heater voltage is specified in the manual. In addition, the response factor cannot be used. The response characteristics will change depending on the setting of the heater voltage. Therefore, it is a good method to calibrate the sensor with the actual target gas for periodic testing.

　　Stable, non-flammable and non-toxic gases are available from suppliers in various concentrations. Please contact the instrument manufacturer for details.

　　D. Gas Mixing

　　Not all calibration gases are available. Even if they are available, they may not be available at certain concentrations or fixed background gas mixtures. However, many gas mixtures can be diluted to calibrate low concentration range gas monitors.