High temperature part suggestion

[Ice-Tea]

 

[avogadro]

Small-ish thermos bottle filled with water, boiling keeps the temp at 100C

Thats basically a pressure cooker. It wont work, the pressure will just keep rising and water boils at higher temps at higher pressures. I guess you could put a valve so you keep pressure constant but you'll run out of water eventually.

Anyway, maybe you could use thermoelectric cooling down there?

[ali_asadzadeh]

Batteries are available toat least 270C (but probably not in Iran).  Regulators will typically have thermal shutdown at 155C so probably roll your own.

I've got pucker thermal chambers for -65C to +125C (-85F to 257F), and a small Memmert oven that goes to over 200C (or so it says on the knob) - just fired it up to see what it'll get to - will see what happens to the following parts at temperature:

PIC micro controller (probably an 18F something or other) running an on-board ADC, spitting out serial.
AA or AAA cells (bog standard)
LiIon 18650 - standard COTS

Will film this for YT and see what happens (got a fire extinguisher...), watch this space

Sound a very good Idea and experiment

[mzzj]

Small-ish thermos bottle filled with water, boiling keeps the temp at 100C

Thats basically a pressure cooker. It wont work, the pressure will just keep rising and water boils at higher temps at higher pressures. I guess you could put a valve so you keep pressure constant but you'll run out of water eventually.

Anyway, maybe you could use thermoelectric cooling down there?

Nothing last forever. But IF you read OPs post he needed 72 hours of operation.
Anywayz, as later found out he has no place to went the steam or ambient pressure is too high in the borehole.