Ah, so you've got an IG5 then! We used them a lot when I was at BT, (
http://www.jmwlimited.co.uk/IG5_Indi..._Detector.html)
They are pretty robust,they had to be, the quickest way to kill one was to suck water into it, they have a filter that goes in the sampling line to prevent debris and water being sucked into the sensor, they are a handy tool for checking for flammable gas in chambers and cabinets, although iirc they are designed for natural gas and don't like petrol fumes.
Somewhere I have my original BT training manual and I'm sure it has the operating manual in it, I'll dig it out and copy it for you.