Temperature Dependence of Oil Viscosity 2

[DavidMBrown]

I am in need of equations describing the temperature dependence of viscosity of various motor oils, hydraulic oils, gear oils, and ATFs. Typically, product data sheets only give the viscosity at 40 C. There are graphs available that show an approximately log-linear dependence of viscosity with temperature, but the log scale of viscosity on these graphs is not uniform, ie, the distance between decades varies.

Does anyone have the mathematical basis for these graphs, ie, an equation for viscosity versus temperature given one viscosity, temperature data point for a particular product?

Thank you,
David Brown
The Fourth River Company
Pittsburgh, PA

 

[butelja]

ASTM D341 Gives the equation:

Log10(Log10(n + 0.7)) = A - B*Log10(T + 273)

Where:
n = kinematic viscosity in cSt
T = temperature in °C
Log10 = logarithm base 10
A, B = constants determined from 2 temperature-viscosity points, which are commonly 40 °C and 100 °C.

I got the previous information from:

http://www.solartronusa.com/downloads/datasheets/b782704usa.pdf

 

[electricpete]

good answer.

 

[hero123]

as it can be seen from the equation above, it's very difficult to make either kinematic viscosity or temperature as the subject of formula and hence we don't have graphs directly plotting the kinamatic viscosity to the temperatures. the equation is very complex one and hence it's advised to make use of the graph as far as possible.

 

[butelja]

Hero123,

I agree that the equation is too complex if you are doing hand calculations. But, if you are including it in a computer program or have a good scientific calculator (HP 48G, but I'm biased) with an equation solver, it is quite easy to handle.

 

[DavidMBrown]

Yes, it should not be too hard using Excel. Our first
step will be to verify that it reproduces the graphical
curves we have been using.

Thanks, butelja. Your answer was everything I was
hoping for.

Dave