As a hydraulic medium, water can be with or without additives.
Water-based hydraulic media are covered by part of a well defined classification system intended for "fire resistant fluids", which in short can be described as follows:
- HFA fluids are oil-water emulsions with a maximum combustible component of twenty percent. In practice, the additive concentration is generally five percent or less.
- HFB fluids are invert emulsions containing more mineral oil than water. They are little used outside of mining nowadays.
- HFCs are aqueous polymer or co-polymer solutions with at least thirty five percent water content. The water glycols fall into this group and usually consist of up to fifty percent water, with the balance being ethylene or propylene glycol.
- Tap water meaning water without any additives at all. The terms used are various: pure water, mains water, raw water. Essentially they all mean the same thing. If you are not interested in checking and maintaining concentration levels, etc., or if environmental legislation are particulary tough, then tap water will interest you as an alternative.