To add to the debate, I'm using an MSA Altair 4. I'm not keen on electronic gadgets, but unlike the flame safety lamp, it chirps to tell you it's still OK, and makes things very clear if you are somewhere risky. It does not require different tests to check for different gases - you switch it on above ground, check levels are OK and start exploring... it sees to itself. It has failed, it had a defective Oxygen sensor, but it failed safe and started squawking whilst still above ground. It's pretty simple to use, in a ruggedised case and is compact... much easier to take through crawls than, say, a flame safety lamp. I'm still not 100% with it, then again I'm not 100% on going below ground. I've had it yelling in anger a few times, always in coal mines. Once was after having walked through waist deep orange goo, which bubbled as it was disturbed. Who'd have thought it.