Gas Detection

While measuring the concentration of gas in any use of gas detectors. what are the things they need to keep in minda What is the difference between ppm and lel. where to measure and where to use ppm lela

*... LEL is "lower explosive limit" which is a percentage of the concentration of atmospheric gases when the mixture ignites.
(There is also a HEL (or (UEL) or upper (high) Explosive Limit).
The% of gas in the air must be between these limits in order to burn. LEL eg propane is 2.1% in air and its HEL of 10.4% in air. Below or above these limits, propane will not ignite.

*... Ppm ... parts per million is usually used to detect harmful gases such as hydrogen sulfide (H2S).
*.. H2S is a flammable, toxic, and acid gas, which irritates the eyes and mucous membranes at concentrations between 20-150 ppm. Slightly higher concentrations cause irritation of upper respiratory tract.
*.. Inhalation of 500 ppm for 30 minutes causes headaches, dizziness, agitation and upset stomach.
*.. Concentrations above 600 ppm can be fatal within 30 minutes due to respiratory paralysis.
*.. Smell alone can not be invoked to detect the presence of H2S, because of the chilling effect it has on the olfactory nerves. The gas destroys the sense of smell at low concentrations.
As a matter of interest, flammable (explosive) limits for H2S is 4.3% to 46% in air ... A wide choice flammable.

I thought that two responders were very clear on the question: If this is not a question of gas combustion or ignition, use ppm. The toxicity is not an ignition problem, so use ppm. Flag

Ppm = parts per million.
LEL = lower explosive limit.

LEL measurement would be used only when the gas is suspected explosives (such as propane, for example). For each type of explosive gases in the air when a certain concentration is reached, it becomes explosive, but below that concentration, it can not explode. Each gas has a LEL different explosive.

PPM will normally be measured if the gas is toxic.

Hope this helps.
Bill