Generally limited because of the concern about hydrolysis. Typically measured using ASTM D-1744 which makes use of a Karl-Fischer reagent. It should be noted that unlike non-detergent mineral oils, phosphate esters can contain considerable amounts of dissolved water. This is normally kept under control with the purification media and/or the use of desiccant breathers. In extreme cases, vacuum dehydration may be required to lower a very high water content such as that arising from a cooler leak. See also Hydrolysis and Hydrolytic Stability.