The calibration of the gas sensor range can be quite easy or very complicated and expensive, depending on the type and concentration of the gas. In principle, in order to achieve satisfactory accuracy, an equilibrium mixture of target gas and background ambient gas is the best calibration gas. However, although it can be done, the skill requirements for the operator are higher than normal. In fact, most calibration gases are purchased from chemical factories. The following sections describe several methods for span calibration.
A. Premixed calibration gas
The method of premixing the calibration gas is the preferred and most popular method for gas sensor calibration. The premixed calibration gas can be compressed and stored in a cylinder at a certain pressure. The size of these bottles can be arbitrary, but on site, people prefer small and light cylinders. These small and portable cylinders can be divided into two categories: low pressure and high pressure gas equipment.
Low-pressure gas cylinders are thin and light in weight and are usually not recycled and disposable. High pressure cylinders are designed for the purification of hazardous materials. For calibration gases, these cylinders are typically thick and can withstand 2000 psi.
For the calibration of the sensor, a high pressure gas is required to flow out of the high pressure gas cylinder, and a pressure reducer is required. It consists of a pressure controller, a pressure gauge, and a flow restrictor. The flow restrictor is a very small wire hole that allows a certain amount of air flow to be applied at a given pressure.
Some sensors require moisture in order to get proper readings during the calibration process. This humidification process step is set with the sensor zero point.
B. Osmotic equipment
The osmotic device is a sealed container containing a gas-liquid equilibrium chemical. Gas molecules penetrate through the edges or caps of the infiltration container. The rate of penetration of gas molecules depends on the permeability and temperature of the material. Permeability is stable over long periods. A constant calibration gas formed by mixing with osmotic chemicals knows its permeability after temperature is given. This requires a constant temperature gauge and flow controller. However, the permeate tube continuously delivers chemicals at a constant rate, with storage and safety issues. The permeability of a given gas may be too high or too low for the application. For example, a gas with a high vapor pressure permeates too quickly and a very low vapor pressure gas chemistry has a permeability that is too low for any use.
Most osmotic devices can be found in the laboratory and are often used on analytical instruments. For gas monitoring, the concentration required for sensor calibration is a typical high permeability device. Therefore its application is limited.
C. Cross calibration
Using cross-calibration methods, mainly each sensor is subject to interference from other gases. For example, to calibrate 100% LEL of ethane gas, usually 50% ELE methane gas is used instead of the actual ethane gas. This is because ethane has a low vapor pressure in a liquid state at room temperature. Therefore, it is very difficult to use a precise mixture and keep it under high pressure.
In other words, methane has a very high vapor pressure and is very stable. In addition, it can be mixed with air and kept under high pressure. Compared to ethane mixtures, methane can be used in more calibration applications, while it has a long life. A 50% ethane mixture is readily available. Therefore, manufacturers of flammable gas alarms recommend the use of methane as a substitute for other gases.
There are two ways to accomplish methane as a substitute for calibrating other gases. The first method is to calibrate the flammable gas alarm with methane and, at the same time, multiply the response obtained by the manual by the response factor in the manual. This is the case with the most commonly used catalytic sensors.
The catalytic sensor is the line output, so the response factor is used in accordance 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 of pentane is 0.5. So when the sensor actually detects pentane and meters with methane, the reading is multiplied by 0.5 to get a pentane reading.
The second method still uses methane as the calibration gas, but the calibration reading is doubled. For example, 100% LEL pentane is calibrated using a 50% LEL methane calibration gas. Although the calibration uses methane gas, after the instrument is calibrated, it reads the concentration of pentane gas.
Many low range hazardous gas sensors can be calibrated using cross gas. Similarly, infrared detectors absorb at the same wavelength for any gas and can be cross-calibrated. The advantage of the cross-calibration method is that it allows the calibration of the sensor to be easily obtained and processed using a gas.
However, there are some problems with the method of cross-calibration. First, the response factor of each sensor is different because it is impossible to make each sensor the same when manufacturing the sensor. For example, in a catalytic sensor, the heater voltage is described in the manual. In addition, the response factor cannot be used. The response characteristics will vary with the setting of the heater voltage. Therefore, it is a good method to calibrate the sensor for periodic detection using the actual target gas.
Stable non-flammable and non-toxic gases of various concentrations are available from suppliers. Please contact the instrument manufacturer for details.
D. Gas mixing
Not all calibration gases are available. Even if it is available, it is possible that the calibration gas is not available at a certain concentration or a fixed background mixture. However, many mixtures can be calibrated for low concentration range gas monitors after dilution.
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