Cooling System                                                                                                                             Last update: 2019-08-13
Some calculations:
The specific heat of water is around 4200 Joules per Kg per Kelvin. (you need 4200 Watt-seconds to heat 1 liter of water 1 degree Celcius)
Let’s say our laser tube dissipates 420 Watt and we have a buffer vessel with 10 liter water. Now we can operate the laser for 100 seconds before the water becomes 1 °C warmer.

Basic Operation
We implemented a buffer vessel from which the water is pumped into the laser tube, then passes the chiller, then the flow meter, and back into the buffer vessel. The flow meter is at the end of the circuit, so a loose hose anywhere in between will be detected.

Temperature measurement devices (DS1820) are placed at the tube’s input, output and at the chiller’s output.
The chiller can be of various principle. One option is a heat exchanger with tap water on the other side. After streaming some time tapwater will arrive at some 10 - 12 °C (at least in our premises). I did an experiment with a heat exchanger from an old central heating (gas) unit where it was used for hot tapwater for the shower etc. In the tapwater circuit was an electric valve from a dismantled laundry machine. When the temperature on the output of the heat exchanger was to high the valve was opened shortly.

Another option is a Peltier based chiller. I made one, but due to bad Peltiers it did not work.

Or a compressor based one. A (second-hand) household refrigerator or freezer may also do a great job in providing a large amount of cold water. But do not pump it directly into the Lasertube, the temperature shock may destroy the tube.

F.t.t.b we will use a compressor based chiller, the type used to provide small amounts of cool drinking water.

In our situation the chance of temperatures below zero °C is very low, so we did not use anti-freeze or take other measures to prevent the laser tube to get damaged by frozen water.

Dewpoint calculations
We should not cool the system to such a low temperature that moist from the surrounding air condenses on any part of the system, especilay not on the output window of the laser tube. How far can we cool down given the room temperature and the relative humidity?

Well there is a formula [found on wikipedia] giving the relation: It is a two step sum:

γ (gamma) = a * T / (b +T) + ln (Rh / 100)       a = 17.27, b = 237.7 °C, T = room temperature, Rh = Relative Humidity in %, ln = natural logarithm.

Tdew = b * γ / (a - γ)     [°C]

We implemented a combined temperature and humidity sensor in the electronics compartment, send these data to the LaserControl program on the Pi and calculate how far the system may be cooled down without the risk of condensation.