Skip to content

Coolant

tl;dr
Use distilled water. Add disinfectant periodically.

"A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, and low cost, is non-toxic and chemically inert, and neither causes nor promotes corrosion of the cooling system. Some applications also require the coolant to be an electrical insulator."
from Coolant, Wikipedia

The coolant transfers heat from the body to the cold container.

Important features we want in a coolant:

  • Doesn't allow bacteria or algae to grow.
  • Is not electrically conductive.
  • Is non-toxic.
  • Doesn't stain if spilled.
  • Doesn't corrode other components.
  • Doesn't degrade plastic or rubber.
  • Doesn't form mineral deposits (scale).

We want these qualities in both loops. This is also why we use ice packs instead of plain ice. The ice will melt and turn to water. Unless you are going to make ice from distilled water, the melted ice will dilute and contaminate the coolant.

If you are just experimenting or developing, and you don't mind replacing water often and contaminating some of the components then you can use plain water. Otherwise, for long-term use, follow the advice above. I would even suggest not to manipulate any part of the system that is wet or underwater with your bare hands. Wear disposable gloves before making repairs.

Automotive cooling products

Designed primarily to prevent freezing which is not an issue here. Toxic. If spilled, it can be dangerous to children and pets.

PC water cooling liquid

Expensive. Can stain the bed or sheets if spilled.

Bleach

Can corrode metal parts. Can damage plastic and rubber components.

Hydrogen peroxide

Good for a "flash clean". Mix equal parts distilled water and 3% hydrogen peroxide. It decomposes into water and oxygen leaving no trace but will generate oxygen bubbles in the loop and it will be difficult to prime the loop afterwards.

Disinfection

Buy a Benzalkonium chloride (BZK)–based disinfectant. You can find it in a supermarket or a pharm in the cleaning products section. Can be a laundry/fabric disinfectant/sanitizer or a surface disinfectant/sanitizer. Can be a spray or a liquid concentrate. Check the ingredient list for Benzalkonium chloride. Specific brands depend on your geographical location. Note the concentration. What you purchase will typically be 0.1% or 1% concentration. Add enough disinfectant to reach a BZK concentration of 0.01% in the coolant. Add some more every few weeks. Another valid approach is to add a few sprays every few days. It might be better to add the disinfectant while the water is circulating for better distribution.

Note that BZK is safe with copper and nickel but might not be compatible with aluminum.

TODO: add a calculator or an example of changing concentrations.

Alternative methods

It might be possible to disinfect the coolant using UVC light. It is not clear how deep the light can penetrate into the water. UVC light is very dangerous in direct exposure. It will burn your skin and eyes. Such an approach is not currently recommended. Such setup must happen inside an enclosure. More research is needed.