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Vacuum at altitude

I have a problem drawing sufficient vacuum. The machine, a
combination caster and investment table, was used successfully by me
at near sea level. I now live and have my studio at 6500 ft. I have
been using the machine to do RTV molds but cannot draw sufficient
vacuum to draw out all the air from the mix. Anyone have a
suggestion? Thanks KPK.

KPK, I will assume you have checked all the connections and the
machine is set up as it was before but with fresh oil. Next I can
tell you that you will never pull as much mercury as at sea level
but while it takes longer to evacuate the gas where I am at 5500 ft
it can be done almost completely. Try letting it rise a bit and then
releasing the vacuum to allow it to break the bubbles then repeat
the process till your mixture doesn’t rise any more. I have used
several different mold materials and this has never made much
difference but just so you know the Dow Corning RTV E has given me
excellent results. I hope this helps.

John Sholl
J F Sholl Fine Jewelry
Littleton, Colorado

  I now live and have my studio at 6500 ft.  I have been using the
machine to do RTV molds but cannot draw sufficient vacuum to draw
out all the air from the mix 

I have read somewhere that you can also use a pressure chamber to
compress the bubbles to a very small size. It doesn’t seem that you
would have to use very high pressure. I would think that less than
50PSI should work. You mix an pour at at room pressure, then place
in the chamber to cure. The trick is to keep it pressurized until
the RTV or Investment has cured.

The article I read was talking about investment but I don’t see why
it wouldn’t work on RTV also.


First how long are you vacuuming? Are you using a surface tension

You make it sound like you are getting air bubbles in your RTV molds
when you cut them but you don’t state that as the reason you ‘cannot
draw sufficient vacuum’.

Your vacuum pump should not have any problems just because of
altitude. If you are saying it 'cannot draw sufficient vacuum’
because of your gauge reading. You are fooling yourself. Your gauge
will read different depending on the altitude but that doesn’t mean
you are not getting a full vacuum.

If you are using a surface tension reducer and you are getting less
than 25 inches of mercury on your gauge you may have a leak. You
should be able to hear the leak since the pump will be sucking air
through it. If you don’t hear it try using a soap bubble solution in
a spray bottle and spray around the bell jar and fittings. If there
is a leak you will see it being sucked inside and bubbles will form

If no leaks try checking your pumps oil. How long ago did you change
the oil and clean your pump? Is the oil level too low? If you haven’t
changed your oil you should do this about once a month. Also remember
to drain the water out each time before using it.

If it has been awhile since changing/cleaning the oil/pump you will
need to do this. Be sure to drain the oil then fill with cleaner
solution. This can be purchased or make it by mixing about half
vbacuum pump oil and half kerosene. Then run your pump. How long
depends on how bad the muck has built up inside. But about 5 minutes
should be enough. Drain this cleaner solution. and fill with new
cleaner solution and run again. Do this until the solution drains
clean. Then fill with vacuum pump oil to the fill line.

Depending on use the cleaning should happen about every month or

If you still have problems let me know. We can repair the pump or
send you the parts. We also have a couple of used pumps and a used
invest/caster like yours for sale as well as new ones.

You can reach me by email or by phone. By the way your vacuum gauge
should read between 25 and about 29 inches of mercury. It is also
possible your gauge is not working right.

Ken Kotoski
MPG Repair

Kelly: try pulling a vacuun on the mold and leaving it. The bubbles
will gradually release over nite. I am assuming that you are using
one of the RTV rubbers with a long curing time. Usually 8 to 12
hours. I did it this way for years until I goat a vacuum machine
that pulled a really strong vacuum. Frank Goss

Hello KPK, To compensate for your 6500 ft altitude you will need to
raise your water temperature. You will also need to adjust your
investment/water ration to compensate for the temperature increase.
You can find some of this subject at Look in “Afticles-Marc” for
"Curing Altitude Sickness"

TAH Handcrafted Jewelry
e-mail: @Timothy_A_Hansen1

Proper maintanance is crucial to keeping my vacum pump functioning
for use at vacums below 29" mercury. One other tip I was told by a
J/B rep, If you change the oil, cover the outlet with your hand and
turn on the pump while a little oil remains in the pump with the
drain open. This will force the oil out the drain and help purge out
crud from the pump. Also a pump can sometimes be rejuvenated by
using fresh oil and running the pump for 1-2 days continously,
rewearing and sealing the vanes. A source for Vacum pumps and
supplies is any Air conditioning parts wholesale house, they usually
do a bigger volume and have special sales on vacum pumps etc.

Dan Wellman HVAC Service

Hello KPK, vacuum is a measure (expressed in in. of mercury,in.of
water,ft.of water or any other liquid of ones choice) of how high
the local air pressure will push a liquid in a vertical tube from
which all air has been removed.

The air pressure at sea level can push mercury to a hight of 29 in.

But since the air pressure at 6500 ft. altitude is considerably less
than it is at sea level,it cannot push the mercury as high
therefore, a vacuum of 29 in. of mercury cannot be reached. As
suggested by MPG and perhaps others.

This is also a partial explaination of why your water gets hotter in
a pressure cooker than in an open pot.


Hi Ralph,

I did not say a vacuum of 29 in Hg could be reached at higher
altitudes. I said between 25 and 29. There are variables that come
into play. The one you stated is only one. Another is the fact that
very few people use high quality vacuum gauges so the reading on
their gauge may say as much as 29. This is not to say it really is

I may not have made this clear enough before. Or you may have

Ken Kotoski
MPG Repair

Hello to kpkelly!

Since you operated your machine when you lived at sea level I can
tell you are probably quite knowledgeable on how to use your own
machine. I have a moment to address your question. I operate a
shop where my specialty is my ability to repair every piece of
equipment a jeweler uses and I have designed casting machines during
my career.

Vacuum pumps work the same regardless of altitude. If your machine
is located at 6500 feet or 6900 feet as we have here at our shop the
pump actually has a “head start” on pulling as complete a vacuum as
it is capable of. The need of the vacuum pump is to attain nearly
an “AbsoluteVaccum”.

It sounds as though the most likely difference between your sea
level experience and your high altitude experience is that during
the transition, your system may have developed leaks or water may
have already damaged parts of the pump causing the inability to draw
sufficient vacuum for your work. Please check your system for

There are 2 ways to measure your vacuum : (1) compare the vacuum
chamber to atmospheric pressure; this is what the gauge on your
machine does (2) measure absolute vacuum…(the gauge on your
machine is using the comparing vacuum pressure to atmospheric
pressure method because it’s easy and inexpensive).

We test vacuum pumps here in our shop using the more expensive
"AbsoluteVaccum" gauges and we require casting machine vacuum pumps
to test between 50 and 100 microns before we consider them truly
repaired. Traditional differential pressure gauges just don’t have
the necessary resolution to measure vacuum down that far.

For most of your casting needs you’ll be relying on a combination of
normal gauge readings and your skill and experience to get the
results you need.

Our philosophy on maintaining vacuum pumps is to drain the water and
the old oil; fill with new oil; as often as you can afford to do
so–we recommend draining and filling about once a month with
regular usage and more frequent is better of course. Two important
things (1) maintain the proper level of oil in the pump (2) don’t
let the pump sit for very long with water in the crankcase.

Our philosophy, based on many vacuum pump repairs and “rebuilds” is
that the only thing that should be put into a vacuum pump crankcase
is good quality, fresh, vacuum pump oil – (sometimes we see other
things which people put in the crankcase such as cleaning solvents,
trapped in pockets near the bearings and the seals, which increases
the frequency of problems, thus the need for repairs. Good quality,
fresh, vacuum pump oil will result in the best performance and less
frequent need for repairs. Vacuum pump oil is considered as just
one of the many materials consumed in the process of casting.

I wish you the best of luck and if you have more questions you may
email me any time. Best Regards, JOHN CRANOR, The Jewelry Equipment
Dr (web address)=

I’m looking for a good vacuum system for small to medium size RTV
mold making and down the road possibly some casting down the road.
Cost is of a concern. Any suggestions or advise is welcome. Please
email at: Thank you, JD