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Vacuum table PSI?


#1

What kind of PSI vacuum reading should I be expecting from a vacuum
casting table?


#2

Hi Dave,

Well, the smartass answer would be “One Bar”. (AKA “one atomsphere’s
worth of pressure”.)

Realistically, that’d be 14.7 PSI at sea level most places. If
you’re in death valley, you might get to 15 PSI. If you’re on a
mountain, not nearly so much.

Most low vacuum system gages are measured in inches of mercury. (if
it starts at zero, goes to 30, and maybe says something about “Hg”,
it’s a mercury gage.) That depends on the pump, but somewhere in the
28.5 to 29.9 range. (If you crack 29.3 inside a minute or two,
you’ve got a good pump)

No matter how good your pump, you can’t get more than that. The
pressure is limited by the weight of the atmosphere pressing down on
the void you created with the pump.

Regards,
Brian


#3
What kind of PSI vacuum reading should I be expecting from a
vacuum casting table? 

At sea level, roughly 28.5 to 29 inches of vacuum, or close to it.
But a better gauge, since vacuum gauges themselves may not be totally
accurate, is simply a glass of water at room temperature. When the
vacuum reaches it max, the glass of water should be boiling. This is
mostly the standard if you use this for investing. For casting, that
exact measure is less important, since air flowing through the flask
will generally not allow the vacuum gauge to reach anywhere near it’s
max. For casting, the volume of air the pump can move can be as
important as the maximum vacuum the pump can reach.

Also, vacuum gauges are not calibrated in psi. That measurement is
usually “inches of mercury” for the types of lower tech pumps we’re
usually using.

Peter


#4

The vacuum reading is not in PSI but commonly in inches or mm of
mercury or Torricelli. If the scale reads 0-33 then it is inches of
mercury and your reading will be something like 29-31 under normal
conditions. Any reading higher than this is better as an absolute
vacuum would be off the scale. If your gauge is in mm of mercury
then the smaller the reading the better (ie: 1). If your gauge reads
in Torr then you are unlikely to get a reading until you have a
decent vacuum but would run from 760 at atmosphere to 1 and below
that is in the high vacuum range so anything under 1 is good but
unlikely with a casting rig. If your gauge is really a positive
pressure gauge then approx 14 is atmosphere and 1 Torr would be -19
atmospheres. I hope that this hasnt caused confusion. By the way,
pressure is measuerd in Pascal (Pa derived from N/M squared) 1
atmosphere is approx 100kPa so a vacuum would be measued in Pa to
decimal fractions. Even more confusion.

Nick royall


#5

David,

What kind of PSI vacuum reading should I be expecting from a
vacuum casting table? 

In general around 29+". Sure test is boiling water at room temp in
under a minute.

jeffD
Demand Designs
Analog/Digital Modelling & Goldsmithing
http://www.gmavt.net/~jdemand


#6

It depends upon your altitude. Here at 5000 feet 23 inches of
mercury vacuum is normal. At sea level I believe it should be closer
to 29 inches of mercury vacuum.

Ken Moore
www.kenworx.com


#7

I have an AquaVac, which has a hose connected to the faucet in my
laundry tub. Water runs through the Vac and drains out another tube
into the sink. I usually get 30 on the gauge…the water pressure is
superb! It is a debubblizer as well as the casting. Absolutely the
neatest little casting machine that I bought from the instructor in
my Adult Ed class about 37 years ago…still performing!


#8

Your vacuum gauge should read almost 30 (inches of mercury, not PSI)
at sea level, less vacuum at higher elevations.

John
The Jewelry Equipment Dr.


#9

Hello David;

Vacuum is measured in inches of mercury. PSI is for pressure
measurement. 30 inches of vacuum would be a perfect vacuum, only
possible in theory. Realistically, a good pump will pull 28-29
inches, and that’s if everything is sealed well. 27 inches is about
what you need for most applications.

David L. Huffman


#10
If the scale reads 0-33 then it is inches of mercury and your
reading will be something like 29-31 under normal conditions. 

Since a “perfect” vacuum would be 30 in of Hg I would be quite
surprised to see a vacuum gage that read more than 30 inHg

By the way, pressure is measuerd in Pascal (Pa derived from N/M
squared) 

Everywhere but here in the US where for some reason we still use
measurements like PSI, inches of mercury, inches of water, Torr etc
for pressure measurements.

James Binnion
James Binnion Metal Arts


#11

Thanks, everyone!

The pump is getting about 15hg, which is about 1/2 what it should
be.

I thought I remembered getting a much higher number the last time.
One of the joys of buying inexpensive equipment on ebay is it doesn’t
always come with a manual.


#12

I’m at approximately 2200 feet here in Asheville, and my caster, with
a hivac dental pump pulls between 27-28. My old one, with a more
standard vacuum pump only pulled about 26.

There are other issues to consider as well, such as the condition of
your rubber/silicon pad on your vacuum table, and all the hose
connections, as well as how ‘clean’ the flask edge is of
plaster…ie, that it makes a clean seal.


#13
The pump is getting about 15hg, which is about 1/2 what it should
be. 

Most decent vac pumps require pump oil, and not just any oil, but
one made for this use. If you bought it on ebay, make sure it’s got
the right oil level in it. Too much or especially not enough, and you
could get that poor performance.

Peter


#14

Hi David,

Did you double check the oil level? Also, if somehow the vacuum
table power was turned off prior to releasing the vacuum suction,
sometimes the machine can suck in some of the oil and may need to be
cleaned out. It’s real important to always release vacuum before
shutting the machine off.

Just a thought on things to check for.
Teresa