A Simplified Viscosity-Pressure-Temperature Equation 630139

The viscosity-temperature relationship of most hydrocarbon oils and many pure compounds can be closely approximated by the Slotte-Fahrenheit equation η = kta where t is in deg F. The equation is as accurate as the Walther (ASTM) equation over the temperature range of 70–250 F. When expanded to include pressure-viscosity relationships (pressures up to 15,000 psi),
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This equation was examined with 122 oils taken from the literature and found to be accurate (<10% error) in 95% of the cases. Relationships were also developed between the constants with a high level of correlation, so that the constants may be reduced to only two, although with an attendant loss in accuracy. The above equation is shown to be related to the VTF (Viscosity-Temperature Function), which is the basis of a proposed modification to the ASTM VI system.


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