1. Combustible Gas Indicators or Explosimeters
An explosimeter is a device used to detect the amount of combustible gases present in a sample of the given atmosphere. This gives the reading in terms of percentage of the LFL (lower flammable limit).
“Resistance proportional to heat" is the working principle.
The equipment consists of a Wheatstone bridge in which one of the resistances is variable. The circuit is shown below:
It consists of four resistances in which one varies according to the amount of the gas present. A hand pump is used to draw the gas or the atmosphere containing the gas inside the device. A filter and flash back arrestor is used to filter the gas and also acts as a flame arrestor.
The device is switched on. As the hand pump is operated to suck a sample of gas from the cargo tank, simultaneously the filament gets heated. Any combustibles in the sample will land on the filament in the sample chamber. The combustibles will burn as the filament is already hot causing an increase in resistance which disturbs the Wheatstone bridge. The reading can be read from the indicator. The instrument gives the reading in percentage of the Lower Flammable Limit or Lower Explosive Limit which is 1%.
This type of gas meter can only be used if the gas content is very low (i.e.) this instrument should not be used if the atmosphere contains:
- H/C + inert gas – then the gas will not burn as there is no oxygen
- H/C + oxy-acetylene – then the burning will be too violent
- H/C + oxy-hydrogen – Same as above
- Lead petroleum vapors – Lead oxide deposits on the filament cause a reduction in sensitivity
All meters require calibration. This meter requires the following before using:
- Zero check
- Span check
- Battery check
Proper working of the equipment can be achieved by regular maintenance. Clean the filters regularly and it is advisable to have it serviced by the manufacturer once every six months.