Tri Tool 757RSS Hydraulic Power Supply - Manual (Page 19)

19
Model 757RSS, Hydraulic Power Supply
92-0565 : Rev. 070912
The viscosity index is a measure of how much the fluid’s viscosity changes as its temperature
changes. The higher the viscosity index number the less the fluid’s viscosity is effected by
temperature and the better it is for use in the system. Most ATF, Type F fluids have viscosity index
numbers around 165 whereas, most standard hydraulic fluids have viscosity index numbers around
100.
API gravity, is a measure of the density a hydraulic fluid compared to water. If the API gravity is
greater than 10, it is lighter and floats on water; if less than 10, it is heavier and sinks. API gravity is
used to compare the relative densities of hydraulic fluids. For example, with two hydraulic fluids, the
one with the higher API gravity will float on the one with the lower API gravity. Although
mathematically API gravity has no units, it is nevertheless referred to as being in “degrees”.