Explosimeter
Explosimeters , also called Ex measuring devices or EXmeters for short , can display explosive gas-air mixtures . This makes it possible to avoid endangered areas or to only enter them with equipment that is not a source of ignition. Explosimeters are often carried in areas where there is a risk of explosive mixtures being created as a result of malfunctions.
The concentration of an ignitable gas is displayed as a percentage of the lower explosion limit (LEL) of the calibration gas. Consequently, a value of 100% LEL corresponds to the lower explosion limit - ignition of the gas-air mixture is only possible from this concentration. To be on the safe side, warning thresholds are set for explosimeters that are well below the LEL. Usually these are in the range of 10 to 40 percent of the lower explosion limit. If the warning threshold is exceeded, the devices emit an optical and acoustic alarm that draws the user's attention to this and thus gives the opportunity to leave the danger area.
Often the explosimeters also have a built-in pump with which the gas to be measured is pumped into the device via a probe and a hose. This is especially important for manholes or channels.
calibration
In principle, all gases that the sensor can detect can be used to calibrate explosimeters. The measurement of gases other than the one used for calibration usually produce incorrect measured values. For standard calibration gases, however, there are tables with correction factors with which the correct value can be calculated.
The calibration gases most commonly used by fire brigades are nonane and toluene . In the meantime, methane is also being used more and more frequently as a calibration gas, since it is the main component of natural gas and the most explosive gas that occurs during fire service operations.
However, calibration with methane has the disadvantage that the concentration of other measured gases is usually displayed clearly too low. For example, the correction factor for ethine (acetylene) in an explosimeter calibrated with methane is approx. 2.8 - a value of 20% LEL displayed on the device would therefore correspond to a concentration of the gas that is almost 60% below the lower explosion limit. For this reason, explosimeters calibrated with methane are usually set to very low warning thresholds (10–20% LEL).
Measurement method
The most common sensors in explosimeters are catalytic catalytic and infrared sensors.
Catalytic exothermicity
Ambient air enters the sensor chamber through a porous sintered metal disk. Inside there are two heating elements that are heated to a temperature of around 500–600 ° C by applying a heating voltage. One of the heating elements, the pellistor, is coated on the surface with a catalyst so that any combustible gas that may be present is catalytically burned. The resulting increase in temperature leads to an increase in the electrical resistance in the heating wire of the pellistor. The second heating element, the compensator, is coated with a chemically inert layer and serves as a reference resistor to the pellistor. Environmental influences such as temperature and humidity, which lead to a change in the temperature of the heating elements, are compensated in this way. Since the difference between the resistance values of pellistor and compensator that occurs when gas is burned is very small, a Wheatstone measuring bridge is used to measure this.
The presence of substances such as lead and sulfur compounds as well as halogenated hydrocarbons in the measuring air can result in poisoning of the catalytic material and permanently damage the sensor.
Sufficient oxygen must also be available during the measurement, otherwise the measurement gas on the pellistor will not burn and the sensor will therefore provide incorrect measurement results.
Infrared measuring method
In the measuring chamber, which is connected to the ambient air, there is an infrared light source that emits a broadband infrared beam into the chamber volume. The light is reflected on the chamber walls and reaches a detector unit, consisting of a measuring detector and a reference detector. In front of the measuring detector there is a narrow-band infrared filter in the beam path, which is only permeable for the wavelength range in which the hydrocarbons absorb the IR light (around 3.8 µm). If there are hydrocarbons in the measuring chamber, depending on their concentration, they absorb infrared light, which leads to weaker radiation on the measuring detector. There is no blocking filter upstream of the reference detector, so that it can cover the entire spectral range of the emitted infrared light. If there are only hydrocarbons in the measuring chamber, the irradiation intensity on the reference detector does not change significantly in contrast to the measuring detector. However, the presence of other optical disturbances, such as smoke or steam, also leads to a lower radiation intensity at the reference detector. This allows the disturbance variables to be compensated when measuring the hydrocarbons.
Compared to the catalytic catalyzing process, the infrared measurement process offers the particular advantages that it is insensitive to catalyst poisons , measurement is also possible in inert environments and requires less maintenance. A disadvantage of this measurement method is that hydrogen is not detected.
Web links
- Gas measuring devices for flammable gases (accessed on February 14, 2020)
- Explosimeter Combustible (accessed February 14, 2020)
- Risk of explosion in the manufacture of test mixtures for explosimeters (accessed on February 14, 2020)
- List of function-tested gas warning devices (accessed on February 14, 2020)
- Dangers of liquid bulk goods (accessed on February 14, 2020)
Individual evidence
- ↑ Explosimeter. The Chemistry School, June 1, 2019, accessed on June 1, 2019 .