[talcum]

Originally Posted by unregistered

well, I'm making progress - s l o w l y
re #1, now have the board schematic, and have been told the 22°C fixed setpoint is by means of a fixed resistor - but the specific resistor was not identified (I don't think the tech knew, and was not sympathetic given my use of the incubator)
the board schematic is here if someone cares to look (while I was told it was a fixed resistor, I suspect that R1 is the factory adjustment - make any sense ?)

re #2, a bit snowed under by a bunch of stuff that I know nothing of; i.e. digital pots
but I (think I) understand that I can find a 5k digital pot (though I have not yet seen such), but I have a problem with only 256 steps
- I've seen reference to 1024 steps, but no 5k pot having such
I suspect such a pot would work, closest I've found is a AD5231, 10k, 1024 position
- can these be 'stepped' in parallel ?

for reference, my voltage controllers on a 0-10V or 0-1V scale have a resolution of (binary) 2^12 = 4096 steps

I concur with Murray13 on the resistor uses.
It would be useful to change the setpoint using your 5k pot, measure the value on the pot and then the temp setpoint in the chamber. Since we know that 22 C is R6/(R4+R6), this would give you a second point to establish the linear reltionship on the resistances. Then you could change R2 and R5 to give you the precision and range needed with the digital pot.