Problem: Have medical device which runs 3kg of perfusate in a fluid circuit with a heat-exchanger that cannot cool below 20C. For a new experiment we need to go to 5C.
Objectives: Minimise cost. Have ability to track a temperature profile.
Constraints: Small. Needs to fit in our wet lab

Proposed solution:
Use the refrigeration loop from a craigslist fridge, with the plastic shell discarded.
RP2040 will control the fridge compressor, and a heater cartridge using feedback from a thermistor in the inbetween fluid line, which runs between the fridge 'internal' hex and the hex to the medical device.
A pump will run this circuit at a fixed speed.

Any alternative solutions are welcome. We expect this is pretty cheap to pull off as the fridge should be 50 or less, rp2040, thermistor and drivers will come to less than 15 and an additional pump will be probably 20 or less.