You know, if you're doing work, or going into enclosed spaces, you really should just get the best gas detector you can get your hands on. It's a lot better than dying.

I worked for a short time at an installation that refurbished small oil rigs. It's about the worst kind of modern work environment: lots of poisonous substances (eg. heavy oil/crude leftovers), welding in closed spaces, tight crawlspaces and the whole thing is a basically a set of Faraday cages, so both VHF and cell reception is pretty bad.

Basically when you opened up a tank, you'd first have someone lower down a high precession meter to sample it over 12 hours. If that looked good, you have at least two gas meters per team, and the team is in constant contact with someone outside (either by said crappy VHF, and/or supplemented with a rope - pull the rope every minute - sound the alarm if the signalling stops, and send in a rescue team. If you figure an excellent 5 minutes from alarm to the team is inside, that's 6 minutes without air).

Worst accident we had was due to faulty documentation: a rig had stopped over and done an undocumented paint job. A team was repainting that section, on the inside of an enclosed space (a shaft) while another team was doing hot work (welding and/or cutting) on the outside. The old paint on the inside had cadmium in it - the result was 5-7 people airlifted to the hospital with cyanide poisoning.

Brilliant anecdotes; thank you. I didn't realise quite how seriously it was taken (multiple samples over extended periods, etc) in industry. Cyanide poisoning sounds horrific.

For the record I sit at a desk all day poking at computers -it's just occasionally I and some friends have the urge to go exploring WW2 structures or old railway tunnels. If I were doing anything commercially or even if I were doing it more often then proper air monitoring would be essential. At the moment we're more than happy to eyeball something (or stand at the entrance and check for a breeze) and just decide "nope -not worth the risk".

I'm also hyper aware that actually, we don't really know what the heck we're doing (even though I spend a lot of time reading up on how similar has gone wrong for other people); so discretion has to be the better part of valour. If anyone reading this happens to be based middle of the UK, has a better idea what they're doing than I do and wouldn't mind me tagging along to learn some new things, then please please please, PM me :)

It so much more important to take this stuff seriously if you only do it occasionally and for fun/recreation! Suffocation is the least of your worries, both H2S and CO are poisonous. And you don't have a team standing by with with air tanks. So response time is likely to be closer to an hour, on the off chance that there's even anyone to call for help. Think about the kind of brain damage you're liable to suffer if you do survive such an ordeal.

A couple of gas meters (the reason to have two, in case it's no obvious, is to try and make sure at least one sounds an alarm in case of faulty equipment etc) - and an "active" monitor to someone outside (aka: a rope) is generally as good as you'll get. And a lot better than what most people do.

Don't become the next anecdote at a search and rescue seminar.

I... umm... yeah. Those are all extremely good points. The idea of surviving with major brain damage is almost more scary than thinking I wouldn't come out at all. It also goes towards explaining why when I ask my friends for advice who do have industry experience, they're not willing to do more than tell me I'm an idiot and that if I want to do it safely then I just shouldn't do it.

I feel I should point out that I do try to be mindful not to become a statistic or anecdote. Apart from the obvious reasons, in 2015 it's hard enough to get away with having unsupervised, unstructured and unlitigated fun. If I do myself in through stupidity whilst exploring, climbing or gliding etc, then apart from anything else it's going to make it that much more difficult for anyone who comes after me. Imposing more restrictions or difficulty on a hobby I love is the last thing I want to be remembered for.

H2S, CO and O2 seem like the obvious gases to be mindful of; is there anything else you'd suggest looking for in an air monitor pr being especially wary of? Massive thanks for your time and knowledge :)

I suppose there are other hydrocarbons to worry about (both naturally occurring from compost, and from leaking gas pipes/old tanks). Not so much because of suffocation (that's why you have the O2 gauge) but from the not blow up/be burned alive part. I suppose it depends a bit where you go exploring - AFAIK old coal mines can be dangerous wrt fires/explosions. It's a good reason to avoid open flame/non ex certified electronics, if you have reason to suspect an explosive atmosphere. Fires can be dangerous just by binding oxygen - in addition to the more obvious danger.

I think the main thing is to realize that enclosed spaces can be really dangerous. Farmers die in grain/grass silos every year. And people die from CO poisoning from using coal as heat-sources indoors, or just from theoretically sound, but flawed fire based heating solutions (eg: paraffin/diesel/oil heaters).