CO2 detector for cave air

[David Gibson]

various people have, over the years, discussed projects for monitoring CO2 in caves. Difficulties have included the cost and accuracy of the sensors, and the fact that they often consume quite a large amount of energy (e.g. they need to be heated before use) which is a problem for a long-term monitoring project.

The best sensor to use is probably an IR device but this doesnt seem to have attracted the interest of cavers, despite the fact that its straightforward to build a "home made" version, the only expensive part being the filter.

However, cheaper commercial IR sensors are now avaialble and a recent issue of Elektor magazine features a project using one. See http://www.elektor.com/magazines/2010/m ... 1993.lynkx

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[Rob Gill]

Thanks for spotting this, I'd obviously missed it in my lunchtime scan of the magazines in WH Smith...
Any idea what sort of concentrations might be regularly experienced in the cave environment? Clearly, there are cases of lethal levels but would we expect 'normal' cave levels to exceed the 3000ppm upper limit of this sensor? (A quick scan doesn't reveal a similar sensor module with wider range on ZyAura's site, though they do have a CO2 monitor unit with a range to 10,000ppm and one under development with a range to 50,000ppm.)
Would be interesting to play with one of these underground, but not much point if it always indicates maximum level...
- Rob

[David Cooke]

A reading of 5.8% (5800ppm) C02 was recorded in Cuckoo Cleeves last year at the height of the CO2 problems there.

I would say a typical reading for a cave would be around the 1% (1000ppm) level.

So a range of 10,000ppm would probably be more useful than 3000ppm otherwise you are in danger of going off-scale just as things get interesting.

There is a useful table of how CO2 affects humans here

[David Gibson]

Would be interesting to play with one of these underground, but not much point if it always indicates maximum level...
- Rob

Its not likely to indicate maximum if you just go into a cave to "play" since most caves contain low levels of CO2 and, if you found a place where you were getting a significant reading, you would be advised not to hang around anyway.

The real value of this sensor, in a cave environment, is long-term monitoring. It is known that the level of CO2 in some cave flcutuates significantly, and the 'research programme' is to try to predict when the levels are likely to be dangerous, e.g. is it associated with the time of year (bio activitiy), or with the type of weather (flushing of air).

But, certainly, you are correct in that there's no point in placing your data-logger underground and coming back a year later to find that the readings were off the scale for a significant part of the year.

I was hoping that this Elektor article might encourage someone to look again at building their own IR CO2 sensor. Its not a difficult thing to do (especially if someone takes apart one of these ZyAura sensors) - you just need a source of IR (e.g. a hot filament) and an IR filter at the right wavelength. Henry Lyall obtained for me, some time ago, a couple of diffraction gratings of the right wavelength, and getting such a component manufatured for us in bulk would not pose any difficulty.

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