Watercooling, different coolants

What to cool a laser machine with has been and probably will be a split opinion for many users.
When asked in forums and discussion groups the replies are many and spread across a lot of different mixes and products.

With this article i will explain the different mixes, what they do and how they fit our machines, and why some may not fit.
In the end, it´s up to you as the owner to decide what mixture you want to use, read this article to decide and see what properties the different mixtures have and what the goal is with your machine.


Preventing growth of bacteria, algea and other microorganisms. You do not want algae in your laser tube, it prevents heat transfer and may kill your tube.
I have seen usage of small amounts of bleach or chlorine in the coolant, it acts like a biocide too but in wrong amounts it will make the coolant very low pH and the mixture is corrosive for metal parts in the loop.
Biocides made for hot tubs or pools shouldn’t be used either.
There is readily products for this so i would suggest using a real biocide. Check out PC water cooling stores.


Or wetting agents is a premix, like Water Wetter, Purple Ice, HydrX and Hy-Per Lube with many more names depending where in the world you are.
Note 2018-01-20: Since i wrote this article it seems like Water Wetter has changed their contents or it differs between EU and US markets, so i would wait with adding water wetter at the moment until this is confirmed. It has showned higher conductivity than earlier.
Wetting agents lowers the natural surface tension of water. The effect causing water beads to form.
This helps water transfer, prevents bubbles from forming, lowers the viscosity and acts as a corrosion agent for metal parts in the loop.
Dish soap is one type of surfactant, but it also contains alot of other additives, like perfume and similar, read the bottle before using.
In auto stores you can find additives for your wiper fluid, DO NOT use this in your coolant, most of them are flammable and is dangerous to add to a 16000 volt system, just don´t do it to be safe!


Our laser tubes have ~16000 volts flowing trough the tube, the water inside the machine will get charged if it contains any particles carrying this charge.
Distilled water has almost no particles at all and do not get static charges. When you add anything to the water, you amp up the static charges also.
Why is this bad for a laser tube?
First off, having a couple of thousand volts in our coolant tank is dangerous, it hurts pretty bad if you happen to touch it.
Second, if alot of the energy in the tube goes away to charge the water, less energy is carried to the other end of the tube, thus lowering the power of your laser tube.
Third, static charges helps mineral build ups in your tube. With time these may create hot spots and a temperature difference in the tube, cracking it.
Remember, without any minerals/particles in the tube, you cannot have a static charge.
The static charge may also interfere with the beam, making it deviate from the path and hit the edges of the end mirror and you get a split beam inside your machine.

Cleaning and maintenance

Basic rules for your cooling tank
– Keep it out of sunlight
– Keep the return hose below water surface
– Have a lid on the tank, leave a small gap around the hose for pressure differences.

When changing the water in your tank, use 50-50 mix with Listerine+distilled water, or vinegear to flush your loop for a hour.
Then refill with your favorite mix of coolant.

Using tap-water for shorter runs

It works to use tap water for shorter runs. Use with caution, fill your tank with tap water, use the machine and empty the tank right after use. Add a mixture of vinegear+water or listerine+water and run the pump for 30-60mins.
I would not suggest keeping tap water in the tank longer than 3-4 days.
Depending on your tap water quality, it may go bad over night, or you´ll be fine for two weeks. Just as a precaution i never recommend tap water at all, even though i use it myself sometimes.

Different coolant mixtures, and their properties
Tap water
  • Good thermal properties
  • Microorganisms will flourish
  • Very high conductivity
  • Risk with mineral deposits creating hot spots, depending on water quality.
  • No corrosion inhibitants
Distilled water
  • Good thermal properties
  • With time, microorganisms may flourish
  • No corrosion inhibitants
Distilled water + RV antifreeze
  • Fair thermal properties
  • Corrosion resistance
  • Good protection against algea and other growth
  • False sense of security using a pre-mix.
  • Higher viscosity, less thermal pickup
  • Higher conductivity (about 400x more than distilled)
Distilled water + surfactants + biocides
  • Excellent thermal properties
  • Corrosion resistance
  • Not conductive, less than 2μS/cm (tap water: 1000-2000μS/cm)
  • Good protection against algea and other growth
Distilled water + surfactants
  • Excellent thermal properties
  • Corrosion resistance
  • With time, microorganisms may flourish
  • Good thermal properties, similar/identical to Distilled water
  • Will re-ionize very quick and go back to regular water.
  • With time, microorganisms may flourish
The “I have done it, you are wrong”-tale

Of course there is users who have cooled their laser with tap water, RV-antifreeze, camel urine or whatever, just because it works for them isn´t a rule to say it works for everyone :)
How fast a laser cooled with camel urine will fail depends on so much more than the coolant, as power usage, how often and how long it´s used, room temperatures, lasing temps of the water and so on.
The facts are there, you be the judge of how you want to cool your machine, i can only suggest what to do, or not to do.

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HP Persson

Digging up the strange ideas and convert them into useful tools and addons for the lasers.
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8 thoughts on “Watercooling, different coolants

  1. Tony

    Well, here’s the “I have done it, you are wrong” comment.

    Over 6 years and 5 lasers (3 x k40’s) I’ve only ever used tap water. I toss a cup of bleach in there to stop algae and then forget about it.

    My brother who lives in a colder area adds anti-freeze, I don’t need that.

    The oldest functioning tube (not killed by me being dumb) I have in a K40 is 3 years old, it works fine (down on power but that’s expected considering its age). Water flow rate hasn’t slowed.

    Now I wouldn’t advise using hard well water, but if your local city water is low mineral, then use it.

    When first started I read all the scare stories, including the bollocks “it’ll corrode your mirrors!” one. As usual no-one can produce a tube that’s been ruined by running plain tap water through it. By the time any minerals have built up to cause a problem the tube will have died of old age.

    If distilled water is cheap where you are then use it, note that it can rust metals more than plain water will (fittings and the stainless steel tank in your chiller). De-ioned water can be more damaging to metals.

    My main problem was the plastic tubing going brittle. Get black stuff, UV doesn’t attack it.

    1. HP Persson Post author

      Thanks for sharing! I use tap water too now and then, but i keep from suggesting it as water is very different even down the road. And we have seen very high conductive water acting on the power output aswell as the regeneration process. In my summer home i can use the tap water directly from the spring, but in the city it´s so bad the PSU starts to coil whine :)

  2. Paul

    Why do you say “keep the return hose below water surface”? I like to hear the water trickling and so don’t do this at the moment.

    Thanks for the website and all the useful info.

    1. HP Persson Post author

      The trickling water creates air bubbles in the tank, get a visual indicator or a flow sensor that breaks the connection for the machine to use the laser if the flow stops :)

  3. Alf

    Hi again,

    Why not use Destilled water and some antifreeze over tap water? Seems like better combo over tap water even if it costs some fe NOK.

    1. HP Persson Post author

      There is so many different antifreeze brand and blends out there, and many of them contain additives that will create problem in the laser tube. Either by making the water too conductive so the water is field charged (output power and gas regeneration process lowered), and some will act on the hoses, so it´s easier to just stay away from them. If you find one with low conductivity and not eroding the hoses, you can use it :)

  4. Jeff

    I have built several custom PC rigs and I am researching cooling for a K40 I plan to buy. I have seen very little info on using mineral oil (zero conductivity) and heat sinks/fans for tube cooling. If you could provide any information on why not to use mineral oil that would be great. I know it is a bit more expensive than water but as far as use of length it can not be beat. I am not very familiar with laser properties and how it would react to mineral oil. Thanks!

    1. HP Persson Post author

      Mineral oil are denser and will cause problems inside the laser tube. The flow needed for it to work properly will endanger the glass tube due to the force applied. I have tested regular mineral oil without problems, but the gain is very slim. And cooling the oil through a heat exchanger or radiator will not work for most users, as the cooling will be limited to ambient temp, and that is often too high for the health of the laser tube. A laser tube wants 15-18c temperatures. Maybe if there is a oil with lower viscosity it can work, but with that there is probably a loss on the thermal pickup/exchange instead.

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