Interesting. I never really thought of low oxygen before, but then most of the spelunking we've done isn't far below the ground - usually in culverts and under city streets with numerous openings to (hopefully) provide fresh air.
But I still may start saving my pennies for an oxygen sensor at some point - preferably one with a visual (LED?) alert rather than audible.
The professional ones usually have three warning alerts, visual (flashing lights), audible (beeping), and tactile (vibrating). That way if it's in your pocket and you're running a jackhammer you might still notice it. You may be able to find a model that allows you to disable the noisemaker.
The other problem with gas sensors is that the actual sensor module is only good for a limited time. They start to go out of calibration and need to be reset using a special mixture of calibration gas. Kind of like breathalizers, you can buy those on ebay too, but without calibration they'll either say you're legally dead or perfectly sober after 8 beers.
I've bought a couple gas sensors on ebay, and I found that the best deals (if you can find them, it's rare), is a kit including the calibration gas so you can periodically do your own resets on the sensors. You can also get ones like the MSA models that have drop-in replacement sensors, but those use the printer ink price model.
For oxygen levels (not flamable gas!) an open flame will tell you if there's enough air to breath. When a candle or match won't stay lit, GTFO. Explosive gas isn't too likely except in coal mines, lots of TC explorers smoke in sewers and no one's blown up yet.
Cool - I'll have to look into those when I have some cash to burn. Of course, it's a little easier to justify spending money on something that could potentially save your life.
Hey Guys, I live in Hot Springs. I've been from one end of the tunnel to the other 4 times. Yes the entrances(2 of them) are out in the open and no, they are not gated. As for the tunnel, It's built in sections, some of it concrete, some of it stone work,,,and some of brick work. If someone is really interested in going PM me. You will need a couple flashlights and plenty of water, it does get really hot and steamy towards the lower end from all the hot spring water being piped into the underground creek. There is an old bowling alley down there somewhere(the reason i have been 4 times) but i haven't found it yet. it is not rumor, it does exist and was photographed less than two years ago as seen in the local gangster museum. oh, and the water level is usually no more than waist deep in one spot and knee deep the rest of the way. I highly recomend only those in pretty good shape attempt the trip. I've had to cary two people out that went along and didn't make it quite all the way due to the heat.