At risk of reiterating what folk have already said in this thread, the first answer to your question is that an oxygen monitor alone does
NOT give you sufficient protection in ironstone mines.
The chief dangers in most ironstone mines where chalk is also present are twofold: (a) low oxygen
via any iron (II) compounds oxidising and scavenging elemental oxygen to do so to form acidic iron (III) compounds and (b) those soluble, acidic, iron (iii) compounds reacting with limestone to give off carbon dioxide. The UK's legislated short time exposure limit (15 minutes) to CO2 is 1.5% by volume. Assuming a nominal oxygen concentration of 21 % v/v, then you can reach the toxic level for CO2 and still maintain an oxygen level of 19.5%
which is above the default alarm threshold on most monitors (19 % v/v
).
CO2 sensing technology has improved greatly in the last 24 months. Old sensors used to top out below 1% v/v - I use the predecessor to this, which has an upper limit of 5% v/v:
https://www.co2meter.com/collections/handheld/products/personal-co2-monitor Yes, it's not cheap, but what value do you put on not being poisoned??
For personal O2 monitors, Ribble Enviro do single gas meters from about £75 +VAT or so:
https://www.ribble-enviro.co.uk/product/protoge-zm-single-gas-monitor/?gclid=EAIaIQobChMI_fCc16Ps4gIVCLDtCh0aNADQEAYYAyABEgKYAPD_BwE
You're unlikely to get any flammables such as methane unless you're very close to a coal seam too. H2S is a remote, but not impossible, hazard since rotting matter can emit it and there's been plenty of damp rubbish in the ironstone mines I know. If you want to go for a 4-gas sensor (N.B. to my knowledge there aren't any that combine CO2 measurement with a 4 gas - certainly the case 2 years ago), perhaps this:
https://www.tomtop.com/p-e5806.html?currency=GBP&Warehouse=CN&aid=gplaukyly&mid=10000018477&utm_source=SEM&utm_medium=Google+Adwords&utm_campaign=TT_PLA_YLY&utm_content=2945&gclid=EAIaIQobChMIsZ6v5aTs4gIVzb3tCh2hHwYFEAYYBiABEgK7IPD_BwE
Don't forget the importance of regular calibration. These monitors can drift. And if you're trusting it with your life (which is essentially what you're doing), you want to be sure that it's accurate.
Also, in ironstone mines prone to CO2 don't forget the importance of measuring at different heights. It's possible for the CO2 to stratify - since it's twice as dense as air, it can exist in a layer on the ground. Diffusive and advective mixing is likely to be minimal due to the low and constant temperatures in these mines. That means, if you only measure CO2 concentration at head height and you kick up a low-lying CO2 blanket behind you, your exit is now highly compromised. Measure at ground level, measure at mid height and measure at breathing height. Bear in mind that the mine topology and geology will change as you go through it - so will the gas profiles. Get used to, and get in the habit of, constant dynamic risk assessment. Make sure that your contingency plan isn't invented on the hoof once shit has hit the fan...
This will be on top of the usual hazards in abandoned mines such as roof collapse, getting lost, vertical structures above you or below you and the possibility of rotting, false, floors. Do some research beforehand and know your enemy!
Hope this helps and keep safe.