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The other factors to consider deal primarily with the impact of exposing various materials to liquid and keeping “bugs” out of your fluid. Some polymers tolerate exposure to water better than other polymers. Some additives may cause a given polymer to fail when pure water would not. Some metals may suffer corrosion when there is a conductive path connecting them. Some water sources are less pure than alternative sources.
The most common tubing materials of silicone and vinyl each handle water and typical additives without trouble. I personally prefer clear tubing because you can visually see the condition of the fluid.
Water cooling components frequently include both copper and aluminum. Because of their different electron configurations, copper will corrode aluminum when there is a conductive link between the two. Numerous additives are available that greatly reduce the rate of corrosion and may often be found at your local auto parts store. Anodizing of aluminum components also greatly reduces the rate of corrosion.
If you have anything other than a single metal present, you should use distilled water with an anti-corrosion additive. Various anti-corrosion additives are available at most auto supply stores. Use of tap water may be fine, but depending on where you live the quality of the tap water may be pretty poor. Contaminants like minerals and bacteria may cause deposits to form that hinder performance.
A blend of water and anti-freeze is an acceptable substitute for “anti-corrosive water wetters”, but should generally be avoided. The main reason is the effect anti-freeze has on viscosity and heat transfer properties. Anti-freeze really does not possess a very good combination of properties at the temperature range that a water cooling system would operate. It really excels in its ability to deal with temperatures ranging from –20°C to 120°C. This range occurs frequently in cars around the world, but not too often in computers. If you absolutely can not find an anti-corrosion additive and must use anti-freeze, use no more than 25% anti-freeze to 75% water.
The temperature of fluid in a computer cooling system lends itself well to harboring various bacteria and algae. I generically refer to anything that grows in the system as “bugs”. Green algae rely on light for photosynthesis. Simply eliminating light prevents algae from growing. If your system is entirely within the case, algae should pose not problem. If some tubing resides outside the case, you may wrap it in an opaque material to block light. This also has the effect of making a visual check of your fluid difficult.
Perhaps the easiest and most widely available means of controlling both algae and bacteria is bleach. One ounce per gallon of standard laundry bleach is enough to ward off bugs for a long period. For you metric folks, that’s about 8 milliliters per liter. I recommend against using more as chlorine also tends to attack some metals and plastics. PVC tubing resists chlorine attack quite well while silicone breaks down under high chlorine concentrations. If your system is completely sealed, a single chlorine treatment may last indefinitely. If your system is open to atmosphere, you may need to dose it every couple of weeks. For open systems, if you can no longer smell chlorine and begin to see indications of contamination then it’s time for another dosing.
Bleach is not the only means of controlling algae and bacteria. There are a plethora of other chemicals used to keep pools and ponds free of algae and bacteria. Feel free to research these options before deciding on a means to keep your system operating cleanly.