In all areas where flammable gas
are handled and processed there is the potential for gas leaks leading to fires and explosions. Explosimetry simply means the measurement of flammable or explosive conditions, normally in the atmosphere around us. The history of explosimetry dates back to the salt mines of the late middle ages. Specialist teams would work through the mines making intentional "controlled" detonations to burn the methane gas that had accumulated in pockets inside the mines... on some occasions, these detonations were larger than expected and the consequences were often fatal, in other situations the accumulation of methane was faster than anticipated and an uncontrolled detonation resulted in explosions, wreaking tremendous havoc through the mine.
In modern environments, both above ground in petrochemical and natural gas processing and below ground in a wide range of mining activities, the risks of flammable and explosive gas leaks building up to dangerous levels are mitigated by detection of these flammable or explosive gases at levels below the LEL (Lower Explosive Limit). Most measurement takes place at typically 25% or 50% of the LEL to ensure an "early warning" system is in place. The most common explosimetry sensors detect methane, hydrogen and carbon monoxide.
Explosimetry sensors are often integrated into fixed detection systems installed in locations around chemicals complexes. Portable versions of similar instruments are also frequently carried by personnel when entering areas where flammable gases are handled or where these gases build up as an inevitable consequence of nature, such as underground mines.
To test and calibrate these senors a range of test gases and calibration gases are used. A basic tests to demonstrate that a sensor is functioning involves exposing the sensor to a relatively high concentration of the flammable gas, this is often referred to as "bump testing". More accurate set-up of the sensors is determined using a more precise and lower level of the test gas in the form of a calibration gas mixture. This mixture will be accurately produced with the concentration normally close to the detector level, perhaps 25% or 50% of the LEL. For these tests greater care must also be taken over the delivery of this calibration gas mixture to the sensor to ensure that the gas mixture does not mix with excessive ambient air and distort the calibration event.