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

[james accordino]

WOW!  Thanks for the knowledge. I previously replied,
but you REALLY know your stuff.  Thanks again for the
viscosity lesson.

Jim Accordino

--- "KUNZ,BOB (HP-Boise,ex1)" <bob_kunz at hp.com> wrote:
> 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
>
>


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