Newsgroups: sci.chem
From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Subject: Re: vacuum pump oil
Date: Tue, 31 Dec 1996 01:50:52 GMT

"R. Winthrop" <throp@bilge.com> wrote:

>I was wondering if anyone had any ideas as to a cheap substitute
>for the oil used in vacuum pumps. Last I checked, it goes for $15
>a quart.  

You will find there are several grades of vacuum pump oil,
and that there are "vacuum pump oils" sold in 20-200 litre
drums by the multinational oil companies that are much 
cheaper than the oils provided by the pump manufacturers.

The oils from the pump manufacturer will have lower
volatility, and originally came from a solvent refining
process to reduce carcingenic risk, but these days 
the oils come from a hydrotreating process that eliminates 
nitrogen, sulfur, oxygen and aromatic hydrocarbons.
The pump manufacturer's oil may also have anti-corrosion 
and/or antioxidant additives added. These can be important
as gradual breakdown of the oil will result in more
volatile hydrocarbons being produced.

Because they don't contain Viscosity Index improvers or 
Extreme Pressure additives, mineral oil vacuum pump oils 
are usually single-grade ( SAE 10, 20 or 30 ), and thus the 
equivalent single grade of vacuum pump oil from a multinational 
may be significantly cheaper, however..... 

A good system should not require frequent changes of
the oil - it usually means that there are insufficient traps
upstream of the pump, or that the ballast is not being
used correctly to purge the oil of condensible vapours.
The important aspect of using a cheaper oil is to monitor
both the ultimate vacuum and pumping speed ( time for
a specific volume to reach a certain vacuum ). You may
find the sklightly inferior vacuum is satisfactory for your
needs. Once the UV or PS starts to deteriorate, try runing 
the pump for a while ( 1-2 hours ) with the ballast open 
- if it improves fine, if not change the oil, and remember that 
the cheaper oil doesn't have the anti-corrosive additives 
of the expensive oil, so change more frequently if 
exposing it to corrosive gases..

              Bruce Hamilton