I have constructed a device to be lowered into shafts to video the conditions and get gas readings, all it needs now is an amusing alliterative name !
My first thought was “Twiggs Terror” (as in what i’ll be feeling at the prospect of losing £500 of gear down a mine shaft if something goes wrong)
I may reconsider positioning the gas meter at the top as its not water-proof and everything else is, incase we have a splash down.
How about attaching one of those bath alarms on the bottom – they can be quite shrill – probably loud enough to be heard from quite some distace, particularly in an enclosed shaft, and will let you know if you reach water.
Only £4.50 and might just save your gas meter.
See http://www.byretech.com/acatalog/Blue-Fish-Bath-Alarm.html