FW: pentosin substitutes revisited - verrrrrry interesting st uff (viscosity vs. VI)

[KUNZ,BOB (HP-Boise,ex1)]

Stephen Kormilo stephen at kormilo.ca wrote:


> A lower viscosity index means the oil is 'thinner'.  This is
> probably 'bad' at higher temperatures.

Actually the viscosity index is a unit-less value that describes the ability
of an oil to resist viscosity change. Viscosity is a value at a specific
temperature. Viscosity is measured in a unit called poise most often
expressed as centipoise or cp (1/100 of a poise). A poise is defined as the
force required to move a surface over another surface with a certain amount
of oil between them. I think they're all one centimeter distances and the
force then is in dynes.

Anyway, that's viscosity. The viscosity index can be considered the inverse
slope of the curve if you plotted temperature against viscosity. So higher
numbers mean less change in viscosity over temperature or a flatter curve.

>> Also, the viscosity for DTE (as an example) at -40 C is 5110 whereas for
>> pentosin it is listed at <1100.  Again, I don't know if this is good or
>> bad.

This means that Pentosin is thinner at -40 C than DTE. That's good. It pumps
better. Pentosin's viscosity is going to change less as temps go up while
DTE will change more because Pentosin has a higher VI. This is also good
because the oil will not thin out as temperatures rise.

Hydraulic designers like high VI oils in designs that need to operate over a
wide temperature range. If the range is narrow, then a lower VI oil will do.

> At cold temperatures, a higher VI is 'bad' because the oil will not
> flow as readily, just like regular motor oils, e.g. 5W-30, the 5W
> is the VI for cold (Winter) temperatures and the 30 is the VI at,
> or near, the normal operating temperature, IIRC.

Here you just need to substitute viscosity for viscosity index.

Bob "ex-Mobil Customer Service Lab dependent" Kunz

'86 5ks Avant
'02 TTQR