Hi all,
I am looking to set up a vacuum system around the workshop.
This will be used (hopefully) for clamping down work stock, with the 'cups' for handeling sheet steel and de-gassing resin / bagging laminates.

I have moved on from a fridge compressor and now have a larger pump with a rating of 9.4CFM pulling down to 7.9" WG.
First question - What is 'WG' in terms of a vacuum rating? How does 7.9"WG compare in terms of vacuum?

Anyway, I also have a steel air reciver and a vacuum switch so the pump can 'charge up' (draw down) a tank and turn off automatically. Rather like a normal air compressor backwards.

The steel tank is 50litre and rated at 10 bar working pressure.
Obviously this is desgined to take pressure to 10 Bar but is it safe to use it with a vacuum?

Next, The vacuum switch is adjustable from -0.2 to -1 bar. I presume that to adjust it i turn on the system, let the pump reach it's maximum vacuum then back off the screw on the switch until the pump stops?

In terms of connecting the switch to the pump - The pump is 240v Ac and has a 13Amp fuse fitted to the plug.
The switch is rated at 250V AC but only 2 Amp contact rating. It has Normally open contacts.
There is no starter as such on the pump motor, just 2 large caps fitted to the case.
I presume I need a contactor to power up the motor but what sort would work with the N/O contacts of the switch.
I presume once the desired vacuum has been reached the switch will close and I have to wire the contactor to stop the motor, I will need it to turn back on once the vacuum drops (rises) again.

Finally, in terms of plumbing, on the compressed air system we have we just use quick couplings and 1/2" ball valves (for water) these all work fine at around 12 Bar.

Does anybody have any input on using these sort of fittings on a vacuum system?
Will the quick fit (pcl / rectus) type couplings hold vacuum or are the valves only designed to work under pressure?

Finally, anybody know of any good sources of info for diy resin traps and diy de-gassing chambers?

Sorry for the long post but couldn't find much info so thought it may be usefull for others also!

Within a couple of days, I am going to start building my dust collection system...it will be DIY and very cheap... Keep watching my thread...http://www.cnczone.com/forums/showthread.php?t=39588

First of all the term "vacuum" is just a convient way of regarding pressures that
are less than what is around you. So at any point the "absolute pressure"
(that is in relation to an absolute zero) can be measured in many units.
For example: weight per unit area, as in pounds per square inch etc.
or in the height of a column of some material. For example Hg (mercury) which
would be on the order of 29.x inches OR as in your case the height of a water
column. WC. A more or less standard atmosphere will support a column of
water about 15 feet ( I havent done this for about 40 years so the numbers
may be a little off) So your WC of 7.9 inches is about 0.04 of an atmosphere.
Now for the other way that vacuum is often measured in inches of Hg as
a depression from ambient, it will depend on at what altitude you are as to
what a perfect vacuum would even be. If you are locatred near sea level then
an absolute perfect vacuum would be on the order of 29.7 in of Hg but if you
were on a high mountain (or even here in Albuquerque or Denver at 5000 ft)
the perfect vacuum would read less (don't have the table handy) .
So the best way to measure a vacuum is the ultimate pressure . The units
used will depend on what range of pressures you ar discussing. In chemistry
labs vacuum pumps are ususally in the range of 10 to minus 6 on down to
10 to minus 9 or so microns ( that is using Hg coulmn ) .
So for your vacuum table your pump should be adequate PROVIDED the
area over which the force is applied is great enough to supply the required
holding power.
:-)
...lew...

......column. WC. A more or less standard atmosphere will support a column of water about 15 feet ( I havent done this for about 40 years so the numbers may be a little off) So your WC of 7.9 inches is about 0.04 of an atmosphere. ..... .

One standard atmosphere at sea level will support a column of 32 feet in water; this is equivalent to 14.7 lbs per square inch.

This means that if you have a vacuum cup with an area of 10 square inches on a surface and you can pump out all the air inside the cup the atmospheric pressure holds it down with a force of 147 lbs.

It is impossible to get all the air out, most good vacuum will get over 90% of the air out so you have a residual pressure in side of about 1.5 psi and the force, or holding power of the vacumm cup is down around 130 lbs.

As mentioned 7.9 inches of vacuum is practically nothing.

Normally vacuum is measured in inches of mercury and vacuum pumps or vacuum venturi operated by compressed air are often rated in "in Hg". A vacuum of 29" Hg is pretty good because a full vacuum is about 29.7".

To estimate how much holding power a vacuum cup, or the surface of a vacuum table has just calculate using the rating for the pump or venturi and the 29.7 to get psi and use that for the area in question to get the force.

With a venturi vacuum generator that can draw 15" Hg you have about half an atmosphere left; 15 divided by 29.7 is about 0.5. So inside the cup you have about 7.4 psi, outside you have 14.7 psi, the difference is 7.3 psi so the holding power, the force over 10 sq in. is 73 pounds.

This does not sound like much but on a vacuum table with material that covers several square feet the holding power can be impressive; do the calculation, 1 sq foot is 144 sq in. so this holds well over 700 lbs at half an atmosphere vacuum.

With vacuum hold down you also have to consider leakage and the pumping capacity of your system because while a pump may pull a very good vacuum in a well sealed system even a pinhole leak may overload the pump capacity to get better than a small fraction of its leak-free rating.

Do a serch on cnczone and you should find many posts on vacuum tables: CNCRob and Greolt both had large threads on this subject.