External tank level sensing via density ?

hotshotharry:
I stumbled across an interesting sensor that I cant seem to find any information on. The way I understand it is that this strip sensor is applied tithe outside of a tank and the resistance changes as the level changes inside the tank... It says the resistance increases as level increases from 50 - 500 ohms. Somehow it can tell the difference in density inside the tank, supposedly works on propane as well...

Holding Tanks Solution – Garnet Instruments

Does anyone know anything more about this ?

it can not see thru metal dude