Posted By FAH
Hi Shahram

There are a range of possible options here.

First, you don't identify your general global position; this may seem irrelevant, but different parts of the world have different requirements for different substances.

Anyway, perhaps you might like to consider the following:-
The LEL sensors are normally set to sense purely flammable substance such as methane - the most common reference gas for flammable.
Although H2S is also flammable, with almost the same LEL as methane.

However, normally the sensors that look for H2S are looking for the toxic element, not the flammable. It is not unknown for one set of sensors to operate before the other; although it is normally the toxic that alarms first.

You must confirm the alarm levels of both the LEL & Toxic alarms in order to define what the real problem is. Perhaps you should also get an independant body to re-check the alarm systems altogether!

Now, as a general guide, & unless your sensors are set to different parameters, the LEL alarm would normally operate at somewhere between 20-50% of LEL - for hand-helds the T90 is normally just under 20% LEL. This should give an undefinable margin of safety to enable all exposed to attain a place of safety before they ignite - but there's no guarantee that the actual LEL won't be achieved before all achieve the place of safety.

For me, having a very well developed sense of self-preservation; at its most simple - when the alarm goes; so do I - every time.

Frank Hallett