Originally Posted by DriveHard
I appreciate the time you spent on that, thank you. I still wonder how that assumption around the cloud point of the fluid plays into this. Any chance you have that info. on those fluids as well. I have data that shows at least on one fluid it is a really good estimation of when the Walther equation falls apart. I have also heard rumor you can estimate around 8,000 cST is where the equation falls apart. Either way, I would estimate for the range I showed in the graph, the results should still be pretty accurate...
You are quite welcome. I believe we can see the 8,000cSt thing doesn't work based on the base oils I used in the examples. Generally when I graph, I'll go to 0C but not lower because of my lack of faith in the data below that point. One could probably do -10C though and it not be totally out to lunch. Would be nice if something could be put together that you could put MRV visc in + density and it would correct the curve below 0C based on that.
Cloud point is where visc goes off a cliff as the wax crystals start forming. That's not an issue with PAO, which is part of the reason (aside from availability) that I presented the numbers with that base, since it is unaffected by that phenomenon. You need PPD's to push down wax formation point on Group III and below bases.
I just stumbled across this page which shows measured KV for numerous oils down to 0C:
https://wiki.anton-paar.com/en/engine-oil/
If we look at his examples:
1. 0w-30
KV100: 11.7
KV40: 66.8
KV30: 102.66
KV20: 167.29
KV10: 291.93
KV0: 550.23
Putting those first two in a visc calc yields:
KV30: 102.1 (pretty good)
KV20: 165.4 (pretty good)
KV10: 286.8 (decent)
KV0: 539.2 (starting to deviate)
2. SAE 30
KV100: 10.58
KV40: 85.76
KV30: 146.70
KV20: 271.56
KV10: 553.20
KV0: 1257.25
Putting those first two in a visc calc yields:
KV30: 146.70 (bang on)
KV20: 262.60 (decent)
KV10: 560.50 (decent)
KV0: 1303.1 (significant deviation)
3. 5w-40:
KV100: 14.44
KV40: 90.90
KV30: 143.70
KV20: 242.10
KV10: 439.85
KV0: 868.78
Putting those first two in a visc calc yields:
KV30: 142.50 (pretty good)
KV20: 237.30 (decent)
KV10: 424.60 (starting to deviate)
KV0: 827.60 (significant deviation)
So this seems to support that once you get down to around 0C, things indeed start to fall apart.