Online viscosity calculators

[paulri]

This is not a Molakule, Everybody else is going to learn from this question, but a Paulri, I am going to learn from this question. Here goes:

Yesterday I wanted to compare the viscosities of certain oils—not at operating temps, but at startup temps (for me), which would be freezing to 60 F. I found this online viscosity calculator http://jiskoot.com/services/calculations/viscosity-temp, and by plugging in the viscosities of a given oil at 40C and 100C, they could tell me what the viscosity would be at 0C.

My question is—are these numbers they were cranking out, reasonably certain to be the actual viscosities of the oils at 0C? Or are they simply an extrapolation, as if to say, we have no idea what the oil viscosity would be at 0C, but we just drew a line going through the 100C and 40C viscosities, and this is where the trajectory led to?

 

[Linctex]

Originally Posted By: paulri

My question is—are these numbers they were cranking out, reasonably certain to be the actual viscosities of the oils at 0C? Or are they simply an extrapolation,


I can't see how it ISN'T an extrapolation... -40*C difference is a huge jump to make, given X and Y and probably guessing at Z.

 

[Linctex]

Originally Posted By: paulri
but we just drew a line going through the 100C and 40C viscosities, and this is where the trajectory led to?


Can you use their numbers to graph several examples?

If so, if you also end up with a straight line - then there your answer shall be.

 

[Quattro Pete]

Originally Posted By: paulri
My question is—are these numbers they were cranking out, reasonably certain to be the actual viscosities of the oils at 0C? Or are they simply an extrapolation, as if to say, we have no idea what the oil viscosity would be at 0C, but we just drew a line going through the 100C and 40C viscosities, and this is where the trajectory led to?

They are extrapolations. The further away from 40C you go, the less accurate they may be.

Here is another tool you can try:
http://www.widman.biz/English/Calculators/Graph.html

 

[A_Harman]

Originally Posted By: paulri

My question is—are these numbers they were cranking out, reasonably certain to be the actual viscosities of the oils at 0C? Or are they simply an extrapolation, as if to say, we have no idea what the oil viscosity would be at 0C, but we just drew a line going through the 100C and 40C viscosities, and this is where the trajectory led to?


It wouldn't be a line that you draw from 100 to 40 to 0C, unless you were plotting on logarithmic graphing paper.
The viscosity index calculation is very good at predicting viscosities above 40C, but below that, the accuracy depends on the oil. If it is conventional oil, the higher wax points of the heavier hydrocarbon chains can cause the viscosity to be higher than predicted, so I wouldn't trust the numbers below ~15-20C. Synthetic oils are more predictable down to 0C due to their more uniform hydrocarbon chain lengths.

 

[paulri]

I was looking at synthetics and synthetic blends. So I'll just use those calculators for full syns only.


Originally Posted By: A_Harman

It wouldn't be a line that you draw from 100 to 40 to 0C, unless you were plotting on logarithmic graphing paper.
The viscosity index calculation is very good at predicting viscosities above 40C, but below that, the accuracy depends on the oil. If it is conventional oil, the higher wax points of the heavier hydrocarbon chains can cause the viscosity to be higher than predicted, so I wouldn't trust the numbers below ~15-20C. Synthetic oils are more predictable down to 0C due to their more uniform hydrocarbon chain lengths.

 

[SuperSalad]

Using the viscosity index to calculate viscosity at higher temperatures is much more accurate than calculating it at lower temperatures. Low temperature viscosity depends very much on the specific base oils, viscosity modifiers, and pour point depressants used in the formula, which can all effect high and low temperature viscosities differently.

 

[Shannow]

There's an ASTM that uses a log equation to calculate the viscosities at different temperatures, and these calculators use that.

The charts from people like shell which show the straight lines on the log paper don't go past -10 to -20C due to all the effects described above.

I use this app...
https://play.google.com/store/apps/details?id=se.assalub.viscosityapp&hl=en

MotoTribologist's point is a good one too...for example if you find a paper with KV100 and KV150 (rare but they are out there) the error in 9.1 and 4.5 cst (respectively as an example) makes the KV40 number stupidly inaccurate...

swinging a -10C number off the KV40 Kv100 is going to similarly push that mark.

Some BITOGers believe them accurate down to -30 or -40 to support their mixing beliefs, but they are pretty well delusional.