Posted by Evilbunny If you can warrant the price for sure go for a standard 4 gas (o2,LEL,h2s,co)...If you really want one my recommendation Is a RKI instruments GX-2009 Only because I like how easy it is to use and its what I personally was trained on. But the cheapest ive seen would be one of the many from BW Honeywell.
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$700 is pretty pricey but probably doable for a
group of explorers. The problem would be the recurring cost of keeping it calibrated.
Posted by Evilbunny Otherwise I'd just go for an O2 sensor an if its below 19.5% something is displacing it and I probably would think twice about going in. Just drop the sensor down with a stick or something before you go in and wait a minute or two. Note that most have to be calibrated every few months at least. Ideally before every use but not really economical. Kinda pricey but worth it in my opinion
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It looks like we would be at almost $300 for a devise to monitor just O2. Again, there is the cost of keeping it calibrated.
Posted by Evilbunny Im thinking a storm drain CO (Carbon Monoxide) or something similar would be a bigger problem than H2S...
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Now, it can get very affordable to monitor just CO. Perhaps "monitor" is too strong a word here, but one can buy consumer grade CO alarms for under $20 from home center stores. Let me stress that these are not certified or designed for people who go into confined spaces underground; these are for monitoring "indoor air quality". Seems to me that these beat having nothing and they are cheap.