My idea is to stick that probe somewhere on the geyser where at least a warmer than ambient reading will result. Lets say my geyser heats to 60 degrees and this reading is 40 when it switches off having reached temp.
If this 40 reading is fairly consistent, then I know my geyser is at 60 degrees when the probe tells me 40.
All I really want is some way of knowing how long the geyser still has to go to reach its set temp.