Making a DIY water chiller

I was recently gifted an old water cooler that was taken out of service because it was leaking from the reservoir that the bottle sits in. Right now it is sitting in the back seat of my car. I know the cooling circuit works well. The unit is just the guts and frame with no outer case so I will need to figure something out for an enclosure.
I’m now sifting through my parts bins for controls and other pieces I will need to transform this into a functioning water chiller for my K40 laser. So far I have scrounged up a Watlow temperature controller.

and I think this is a temp sensor? I have to investigate it more.

I’m going to use an external pump in place of the pond pump that sits in the tank and adds heat to the water. I have a nice pump that I can use but I think it may be too much. It is a magnetic drive aquarium pump capable of, I think, 880 gal/hour.

I could make a bypass system that will regulate the flow to the tube and divert the excess back to the tank but I think it may be a waste. I know I have another pump somewhere. I do have an old junk RV that may still have the 12 volt water pump, hmmm.

More Later. . .

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It could also be a thermal cut-off. Obviously you’ll be able to tell with an ohmmeter.

It’s not that expensive to get new sensors that your watlow can read, and I’d think it’s worth a few $$ to keep the laser tube alive. A k-type thermocouple in a stainless steel probe that goes right into the water is a cheap part. There are approximately a thousand variants just on amazon. By way of example and not recommendation, here’s one available both in ubiquitous k-type (should be just fine) or more expensive PT100 (overkill for your need, may or may not be supported by your watlow) in 1/4" and 1/2" sizes:

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