I'm not familiar with that gadget but it got me thinking ...
Being a liquid gas why couldn't you measure the gas bottles contents by gauging the bottles difference in temperature? I may be wrong but it would seem that where the gas sits the temperature would be lower than where the gas isn't sitting.
Seems to me if I remember right when I was teacing physics there was a plastic strip of material we used in the labs that could sense such as change in temperature and simply change color from red to green or maybe it was blue.
I'll have to find out what it was - Damned CRS!
Guess they've already figured this out here's a product that does just that:http://www.wdrake.com/walterdrake/Sh...070300&Affid=4