Cold Cranking Cold Pumping Viscosity 1

[Akro101]

I am confused. In reviewing the SAE J300 Table, I note that for 10W the maximum viscosity for cold cranking is 7,000 cP measured at -25C and the maximum viscosity for cold pumping is 60,000 cP measured at -30C.

I know that these measurements are done using different test apparatus and procedures, but if the viscosity units are the same (cP), how can an oil at the maximums allowed by the spec have a viscosity of 7,000 cP at -25C and a viscocity of 60,000 cP at -30C?

I understand that most oils would not actually be right at the max for both tests, I just don't understand the large difference in the maximum numbers when they are measured only 5 degrees apart.

Thanks for any help you could give me.

 

[drwebb]

You raise a good question. A rule of thumb is that mineral oils approximately double their viscosity for each 5 C reduction in T, so the comparison is ca. 14K vs 60K, still a big difference but less than 9X. I was hoping a rheologist would weigh in with a good theoretical answer, but for now here are some differences to consider. The MRV uses a very long cooling cycle and a low shear rate with a paddle rotor in a tube with about 1 mm clearance. The measurement is started from a dead stop. The CCS uses a shorter cooling cycle (affording smaller wax crystals) and (I think) a flat rotor against the bottom of a cup and is spun for 60 secs before reading the vis.

It sounds to me like the MRV is more of a bulk viscosity, while the CCS is more thin-film- sort of like kinematic- versus high temperature high shear testing. Also, I gather that the real key parameter in the MRV is the absence of yield stress, and the measured viscosity isn't usually the killer. So a passing oil could have a vis considerably below the 60K max while just squeaking under the CCS limit.