If one was to “guesstimate some numbers”, it’s not hard to calculate the approximate heat input to the coolant.

I’ll toss some numbers out for fun.

My 80 watt red&black draws about 160 watts (measured) at idle. That includes the coolant pump.

At 65% “indicated output” on the Ruida controller my 80 watt tube runs at about about 24mA which is probably around 70 watts.

At that power level the machine draws about 610 watts. (measured)

So subtracting the idle power from the loaded power gives about 450 watts that presumably the tube and power supply are consuming.

Modern switch-mode power supplies are pretty efficient so if we assume an efficiency of 89% that gives us 400 watts of power delivered to the tube and 50 watts wasted in the power supply.

So, with 400 watts of input power and about 70 watts of optical power coming out of the “bright end”, that leaves us with about 330 watts of heat wasted in the tube that needs to be be dealt with by the cooling system.

One watt = aprox 3.412 BTU/hour so 330 watts = 1126 BTU/hour

One BTU is the amount of heat required to raise 1 lb of water 1 degree F.

IF the cooling system holds 5 gallons that’s about 42 lbs.

1126 BTU / 42 lbs = 26.8

So for every hour of operation at that power level, the water temp rises by about 26.8 degrees F.

But wait, there’s a little bit more. My “aquarium style” coolant pump (which I no longer use) draws about 26 watts. And since it’s a submersible pump ALL of that energy ends up in the water.

So that’s an additional 81.7 BTU/hr which adds an additional temperature rise of almost 2 degrees F/hour.

Unfortunately the coolant pump runs all of the time the machine is powered up so that energy is always heating water whether the tube is firing or not.

So to summarize:

In the above example, with the laser idle the coolant is being heated by the pump at a rate of about 2 degrees F / hour and with the laser firing it’s about 28.8 degrees per hour.

The math for all of that should be good. Plug in whatever power numbers you like for your particular machine and cooling system capacity. If you’ve got a “Kill A Watt” meter or similar it’s easy enough to measure idle and running power on your machine. If you’ve got one of those fancy laser power measuring tools then you can directly measure the output power of the tube, otherwise you’ll have to guess.

Got all that? There will be a short quiz after recess.