In refrigeration, any ANY moisture will knock the system to it’s
knees. Refrigeration supply houses have an oil testing kit if
you care to get one. Your oil will show high comtamination
undoubtedly because this kit tests for any negligable moisture.
I use a welch (I think, been awhile since I’ve looked and I’ve
had a few over the years, good pumps) and change my oil when I
get around to it. Much like a car. Since this pump is not used
to draw the deep vacuum as in refrigeration, just clean oil is
my criteria. And of course vacuum pump oil only.
Generally, you can look in your sight glass to see the oil. It
gets browner and milkier as time goes on. These pumps are built
to high standards and the clearances in them are ten time
tighter than in an automobile engine. Hence the need for clean
oil, since they run about $3grand.
I have added a refrigeration accummulator and filter so that I
won’t draw metal or investment into the pump in a mishap.
I don’t use the ballast, or you could leave it open and still
get enough vac for casting. Closed for boiling though, (it
adds a bleed) With the ballast off you should hear no sound
after it pulls down as there is nothing entering the suction
side. Usually with it open you hear a gentle burp burp as the
slight amount of air it admits gets compressed and expelled. The
real rules regarding this I have long since forgotten.
I also keep a large foam intake filter (from bulk foam filter
roll) folded a couple times and fastened over the air intake,
and tied there to keep the intake air clean.