How much vacuum?

[johnm]

Folks -

I finally got around to hooking up the vacuum power switch to my Frankenvacuum unit.

I have the fridge pump hooked into a 5 gallon vacuum chamber. We live at about 1300 AMSL and the pump will generally pull just over 25" HG, which I equate to an uncorrected clamping force of approximately 10 to 12 PSI.

With the vacuum switch installed at the "factory" settings, the pump cuts out at 22" HG and cuts back in at 15" HG.

Obviously my clamping force is directly related to the vacuum surface area, and a large object could be held down safely with a lower amount of vacuum than could a smaller object.

Where should I set my initial cut in/cut out points? I don't want to burn the motor out, but I don't want the machine to have my sudden, undivided attention either.

I'm interested in all y'all's feedback, and would like to know what the actual correlation between vacuum HG and PSI for my elevation might be, for those of you in the know...

TIA,

John & Rose

[richards]

The vacuum switch that I use is factory adjusted to have about a 2-1/5 inches of mercury range. I set it so that it stops at 22". It automatically kicks in at about 19-1/2". Before installing the automatic switch, I felt safe when vacuum reached 15". But, as you stated, "clamping force is directly related to the vacuum surface area". When cutting plaques, my smaller pod/puck has 25 sqare inches of surface area and my larger pod/puck has about 90 sq. inches of surface area. Both work well. When cutting odd-sized material, I normally use one or more 4x4 inch pods. Two 4x4 pods easily stabilise long narrow strips of material.

[mikejohn]

John
I assume your reading of 25"hg is the holding tank pressure.
Up to 5000', pressure reduces approximately 1" per 1000' (close enough for government work).
Normal weather systems can alter pressure +/1".
Standard pressure is 1013.2 millibars at 15ºC at sea level, which is 29.92" at 59ºF at sea level.
When you see isobars on the weather chart (lines of equal pressure) they are calculated as if at sea level at that point.
So, check the weather chart for pressure for that day, deduct 1" for every 1000 feet of altitude of your workshop, that is the pressure on the work surface. Now deduct the vacuum at the work surface and you have the effective applied pressure.
1"hg is approximately 0.5psi.

To calculate millibars to inches, divide by 29.92, then multiply by 1013.2 .
As a point of interest, all aircraft throughout the world, once they are above about 5000feet, all set 1013.2 on the altimeter irregardless of local pressure, except Americans, who set 29.92"
However, as these are equal, it still stops the JFK-heathrow flight bumping into the Heathrow-JFK flight


..........Mike

[billp]

John,
If you have a vacuum guage inline with your system you should be able to see where the pump starts to "stall"(the guage will start to oscillate and you won't get much progress beyond that point). I'd see where that point is, and "back off" your settings to a little below that number. As you suggest above, the higher the vacuum your source is pulling, the better "grab" you will get, (and therefore less anxiety). If your switch has a 7" range then your actual time to "recharge" the pod/grid shouldn't be more than a minute or so (IF your gasketing is tight, and you have no other leaks in your system) so there shouldn't be much concern about burning out your pump. With a tight system you can sometimes go for hours before you bleed down to the point where your switch will kick in again...On my rig I was set at 25" at the top end, and the pump would then
turn off until I bled down to 18": . Even at the lower end of that range nothing would move on my pods...