I'd like to fit an operating temperature indicator to the oil boiler.
At the moment a mercury lab thermometer is puttied onto the outgoing
(flow) pipe. It was one of those quick temporary fixes which became
too permanent & it's now time to recover the said thermo.

Any suggestions for a permanent replacement please? A simple and
cheap-ish solution would be ideal. Accuracy takes second place to
readinngs being consistent as I could calibrate against the mercury
thermo. An ideal solution would be a digital display showing both flow
& return temperatures.

TIA