Explosimeter

Explosimeters, also called ex- measuring instruments or short EXmeter may indicate an explosive gas-air mixtures. This makes it possible to avoid hazardous areas or where the latter entered only with equipment that is not a source of ignition. Often explosimeters be carried in areas where the risk of explosive mixtures caused by malfunctions.

The display of the concentration of a flammable gas is in percent of the lower explosive limit (LEL ) of the span. Consequently, a value of 100 % LEL lower explosive limit - only starting from this concentration is an ignition of the gas -air mixture is possible. Warning thresholds are determined For your safety in Explosimetern that are significantly below the LEL. Usually these are in the range of 10 to 40 percent of the lower explosive limit. When exceeding the warning threshold, the devices give a visual and audible alarm by itself, makes the user aware of it and thus gives the opportunity to leave the danger zone.

Often the explosimeters do not have a built-in pump, with a probe and a hose, the gas to be measured is pumped into the unit. This is especially important for shafts or channels.

Calibration

For calibration of Explosimetern all gases can be used, which can detect the sensor principle. The measurement of other gases, as employed for calibration, normally provide erroneous readings. For standard calibration gases, however, with which the correct value can be calculated, there are tables with correction factors.

The most common calibration gases in the fire place nonane and toluene dar. More and more is now also used as a methane calibration gas, as it as the main component of natural gas is the most commonly occurring with explosive gas for firefighting operations.

However, calibration with methane has the disadvantage that the concentration of other gases is usually represented measured clearly too low. The correction factor for ethyne ( acetylene) with a calibrated with methane explosion meter for example, is about 2.8 - a displayed on the unit value of 20 % LEL thus correspond to a concentration of the gas, which is already at almost 60 % below the lower explosion limit. For this reason, very low alarm thresholds are set when calibrated with methane Explosimetern usually ( 10 - 20 % LEL ).

Measurement methods

The most common sensors in Explosimetern represent catalytic catalytic bead and infrared sensors

Catalytic heat tint

Through a porous sintered metal disk ambient air into the sensor chamber. Inside are two heating elements, which are heated by applying a heating voltage to a temperature of about 500-600 ° C. One of the heating elements, the pellistor is surface coated with a catalyst, so that any flammable gas present is catalytically combusted. The resulting increase in temperature leads to an increase of the electric resistance in the heating wire the pellistor. The second heating element, the compensator is coated with a chemically inert layer, and serves as a reference resistance for the pellistor. Environmental conditions such as temperature and humidity, which lead to a change temperature of the heating elements will be compensated in this way. As occurs in the combustion of gas from pellistor difference of the resistance values ​​and the compensator is very small, a Wheatstone bridge for the measurement, this measurement is used.

The presence of substances such as lead and sulfur compounds and halocarbons in the air to be measured can lead to poisoning of the catalytic material and permanently damage the sensor.

Infrared measuring method

In the associated with the ambient air measurement chamber is an infrared light source which emits a broadband infrared beam in the chamber volume. By reflection on the chamber walls, the light passes to a detector unit consisting of the measuring detector and a reference detector. Prior to the measurement detector is a narrow-band infrared filter in the beam path, which is permeable only for the wavelength range in which the hydrocarbons absorb (about 3.8 microns ) the IR light. There are hydrocarbons in the measuring chamber, they will absorb as a function of the concentration of infrared light, which leads to a weaker radiation at the measurement detector. Said reference detector is not a blocking filter in front, so that it can cover the whole spectral range of the emitted infrared light. If only hydrocarbons should be located in the measuring chamber, the radiation intensity changes from the reference detector not significant in contrast to the measurement detector. The presence of other optical disturbances, such as smoke or vapor, but also lead to the reference detector to a lower irradiance. This allows a compensation of the disturbance in the measurement of hydrocarbons.

Compared with the catalytic heat tinting process provides the infrared measuring method in particular the benefits of being insensitive to catalyst poisons and less maintenance required. A disadvantage of this measuring method is that hydrogen is not detected.

• Firefighting Equipment
• Fuel gas technology
• Chemical meter
• Explosion protection