is
the manner in which the viscosity of a given
fluid varies inversely with temperature. Because of the mathematical
relationship that exists between these two variables, it is possible to
predict graphically the viscosity of a petroleum fluid at any temperature
within a limited range if the viscosities at two other temperatures are
known. The charts used for this purpose are the ASTM
Standard Viscosity-Temperature Charts for Liquid Petroleum Products,
available in 6 ranges. If two known viscosity-temperature points of a fluid
are located on the chart and a straight line drawn through them, other
viscosity-temperature values of the fluid will fall on this line; however,
values near or below the cloud point of the
oil may deviate from the straight-line relationship.