Circuit for Hydrogen Sensor

I maybe overly paranoid, but this is a simple comparator circuit. But you're measuring the concentration of a flammable gas and asking for a trip condition at twice the minimum explosive threshold. Lastly, you post that you are from a volatile region of this world right now.

You might have an honest reason for this question but I don't feel right answering this.

Hydrogen, in air, has roughly the following limits, by volume:

4% = Lower Flammability Limit (LFL)

15% = Lower Explosive Limit (LEL)

58% = Upper Explosive Limit (UEL)

72% = Upper Flammability Limit (UFL)

Sometimes the LFL and LEL are assumed to be 4% and the UEL and UFL are assumed to be 72%, to be safe in avoiding both deflagration and detonation.

The 1% setpoint is reasonably protective. I've seen 2% used as well.

I'm not I&C so I can't answer the instrumentation circuit design question.

Other options are:

• concentrate on why the process hydrogen is so poorly contained and improve it
• concentrate on improving the ventilation in the area

http://en.wikipedia.org/wiki/Electrical_Equipment_in_Hazardous_Areas