An explosimeter is a device which is used to measure the amount of combustible gases present in a sample. When a percentage of the lower explosive limit (LEL) of an atmosphere is exceeded, an alarm signal on the instrument is activated. "Explosimeter" is a registered trademark of MSA.http://www.msanorthamerica.com/catalog/product1084.html

The device, also called a combustible gas detector, operates on the principle of resistance proportional to heat—a wire is heated, and a sample of the gas is introduced to the hot wire. Combustible gases burn in the presence of the hot wire, thus increasing the resistance and disturbing a Wheatstone bridge

Wheatstone bridge

A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. Its operation is similar to the original potentiometer. It was invented by Samuel Hunter Christie in 1833 and...

, which gives the reading.

A flashback arrestor

Flashback arrestor

A flashback arrestor or flame arrestor is a device most commonly used in oxy-fuel welding and cutting to stop the flame from burning back up into the equipment and causing damage or explosions. The two main types are dry and wet. Each has its own advantages and disadvantages...

 is installed in the device to avoid the explosimeter igniting the sample external to the device.

Note, that the detection readings of an explosimeter are only accurate if the gas being sampled has the same characteristics and response as the calibration gas. Most explosimeters are calibrated to methane

Methane

Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 or hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

.

External links

• http://www.marineengineering.org.uk/testequipment/explosimeter.htm (select explosimeter from the left frame)
• Explosimetry