From: Dennis <IEDXW@ASUVM.INRE.ASU.EDU>
>One ignorant question about oil: I was told that 20-50wt oil for example
>has a viscocity of 20 when cool and 50 when hot. Surely this is not correct.
Yes, it is not correct. A designation such as 20W-50 (W stands for
"winter") simply means that the cold viscosity is approx. that of a
straight 20-wt at that temperature, and the hot viscosity is approx. that
of a 50-wt oil at the higher temperature; i.e. a straight 20-wt oil would
be much less viscous at the higher temp. As always, a picture tells it
best:
(Highly exaggerated, qualitative plot:)
| o,
V | \ 'o,
I | \ 'o,
S | \ 'o <---50-wt oil
C | \
O | \
S | \ <-- 20W-50 oil
I | o, \
T | 'o, \
Y |20-wt oil-> 'o,\
| 'o
|
+-----------------------
TEMPERATURE
Of course, the real curves are not straight lines; the multi-weight
curve is probably asymptotic w.r.t. both the upper and lower curves.
The temperatures of the "intersections" are defined in some SAE report
somewhere - anyone know what the temps are? (Although they possibly
*never* intersect.)
Lee M. Daniels Laboratory for Molecular Structure and Bonding Texas A&M
daniels@lmsbvx.tamu.edu (409) 845-3726 Fax (409) 845-9351
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