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You're right it's measuring the pressure applied to the outside of the system. This is pretty standard for course vacuums. For someone who works with fine vacuum this would be confusing no doubt.

NB: Fitting and connections are evil in vacuum systems.

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Jim Watts
http://jameswattsguitars.com

I was able to pin down a couple of leaks in my deck by sprinkling baby powder around the frame while it was sucked down. Clean everything really well first, then carefully open the press & see where the powder has accumulated.

I'm a big fan of extreme overuse of teflon tape. I tape up my fittings until they look like I dipped them in frosting, and then screw them in. I get squeezeout on my teflon tape. It's not elegant, but I always get great seals. Using tapered pipe thread fittings and taps (NPT) is important as well, as well as having enough wall thickness to ensure a good seal. Using a vacuum tank in membrane or bag clamping situations is disadvantageous, IMO.



In the lab we tend to be more concerned with stuff (ie: how many particles per CM^3) where in industry they're more concerned with suck. Reading backwards from atmospheric measures suck pretty well, and measuring upward from vacuum measures stuff pretty well.


Disclaimer: This is for Randy, and it's going to get REALLY geeky, so be warned!
<GEEK>
There is definitely a pressure gradient in the system, and I've even done empirical testing to determine which factors effect it. I've been a bit too stingy to go out and affix a bunch of vacuum gauges at various places on a setup to determine the exact amount of difference, but I'll get around to it sometime. I had the same reaction the first time I was told about the pressure drop in long air hoses. Most of what we learn about pressure in science degrees is only applicable to non-flowing systems (which generally works, for lab and HVAC setups). As soon as there's flow, everything changes and there's a pressure gradient (relatively speaking, vacuum clamping systems are like sieves compared to lab and HVAC setups). It was actually John Watkins who turned me on to the 'what' in this one (that adding a bit of airspace under parts makes the vacuum work better) but it took quite awhile to figure out the 'why' given how that conflicted with my academic knowledge (the flow is the 'why')!
</GEEK>

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

OK, the systems I worked with were definitely static, or as close to static as you can get. With a significant amount of flow I can see there being a gradient. I would be curious as to how great that gradient is for typical clamping setups. Two gauges ought to do it. One right off the pump and one rigged to the bag.

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Randy Muth
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