It is buried in sand with about 2 feet of dirt on top of the sand, tar covers the tank’s metal surfaces except the very top. I would still give it a try as long as I can connect it directly to metal. The tank’s natural resonance will change as the propane level changes regardless of what is touching the side. There must be some type of calibration process??
But I don’t see it being a good solution simply because its not intended for outdoor use or use in a hazardous environment. I’m sure it needs to be rated for an explosive environment after all propane is in the class IIA explosive gas group http://en.wikipedia.org/wiki/Electrical_equipment_in_hazardous_areas. I wold think they need to be rated for that to even attach it to a barbecue tank.