Vacuum Pumps

Having used and abused 2 of the ‘for jewelers’ vacuum pumps I
opted for a Vacuum pump made for the oil business-a WELCH, built
to run all day long in the hot sun for days, weeks, and YEARS. I
found it at a used equipment(oil related) business called
’Practical Engineering’ in Tulsa. I paid $500 for a rebuilt(new
condition) REAL machine and it didn’t pull quick enough. So, I
traded pullies around and found the pair of pullies that enabled
the pump to pull 29.9 inches of mercury in about 40 seconds. What
a pleasure. The need for speedy evacuation is simple: you want
to pull air out quickly, for the investment will set up within 10
to 12 minutes. So, efficient bubble removal, along with total
de-bubbling makes for supreme investment. Steven

Steve and others, I also have a large Welch vacuum and love it

  • especially because it found me at the flea market and I only
    paid $300 for it.

–Does anyone have a reliable way of seeing when the oil needs
changing? When the manufacturer tells me " every six months", it
seems ridiculous for the amount of use it gets. I go through 50
lbs of investment in six months and live in the desert (L.A.). I
changed the oil (4 quarts) when I bought it a year ago and it
looks the same when viewed through the sight glass.

–Also, am I supposed to hear any sound, distinct from the
normal running sound , when I close the intake port and open the
gas ballast knob? And how often and why am I supposed to do this?

I tried getting this info from welch but they dont seem to
understand the needs of the jeweler or I just don’t communicate
them well enough.

                Thanks, Peter slone

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.

John g

Hi Peter Slone, I don’t think you need to change the oil on that
pump for quite a long time…50 lbs of investment in 6 months is
not even a warm up excersise for that pump. due to the climate
your in, i doubt that the pump will accumulate any water that
won’t evaporate quickly on it’s own. great pump at a great
price!!! good luck with it. Daniel grandi

OK, Vac Guys:

I have folowed the suggestions about water in the vac oil. I am
using a car AC pump attached to a washing machine motor, run off
the “Suct” side for a vac pump to replace the defunct vac pump in
an old perf caster. I haven’t used it yet to cast, but have
tested it with the bell jar. I can’t boil water unless I am at
about 68 to 70 degrees. The vac comes up in about a minute to
boil at that temp, but the gage shows just 28". Could water in
the oil be a problem here, and if so, would I just drain and
refill with auto AC compressor oil??

Roy (Jess)

Hi Jess, The water should be at east 70oF ( I use 72oF) to test
the pump…i have used those as pumps before and it is a really
inexpensive way to make a vac pump…you might be able to pulley
it up so that you get a little more speed and increase your vac.I
used that pump in Thailand as a casting pump. i cannot tell you
how investing and water will affect it… but i can say that
water is not good in Any Oil type pump.

Daniel Grandi

Hi Roy, By all means keep water out of the oil, Since guages are
notoriously inaccurate, don’t worry too much about the numbers.
Just use your system and if success strikes, keep using it. I,
too, use an old air conditioning compressor. It works fine. Have
fun. Tom Arnold


An auto A/C pump is considered a high back pressure pump as
opposed to a low back pressure. (lo backpressure pumps pull
deeper vacuums) The a/c pump is not designed for an extremely
low vacuum, and to boil water with it may not be possible (at
normal ambient temps) However it will certainly do the job
casting. But I’m afraid that changing the oil won’t help. You
can contact me off list if you want.

John g


Thanks for the input. Glad to know this works for someone, do
expect it to work for me as soon as I get the workshop built —
weather’s getting warm enough to do it!!