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Old 11-21-2020, 10:45 AM
DanH's Avatar
DanH DanH is offline
Join Date: Oct 2005
Location: 08A
Posts: 9,719

Here's a fine example of dumb.

Bill previously mentioned his pump would suck foam flat. I said I was pulling about 20 inches. I'm not.

I've been using this pump about 15 years, when it was removed from a medical machine at a friend's large recycling yard. The gauge came with it. There is no notation of units on the gauge face, and all these years I have assumed the marked range of 0 to 30 was inches Hg. Turns out it is actually centimeters Hg, an odd unit, or torr, or mmHg when read as indication x 10.

The obvious clue is the pump label (duh!). If I ever looked at it, it didn't dawn on me to convert torr to familiar units. Certainly explains why I'm not sucking foam forms down to pancakes.


This may all be a coincidence of terms. Bill says 210 torr may be an absolute value, 210 more than zero. If so the gauge really is indicating 21" below atmospheric, i.e. 29.92 adjusted for elevation. I'll check it with an altimeter or something tomorrow.


There's an old (bad) joke which goes something like "I thought I was wrong, but I was wrong".

Dug out old altimeter and hooked it up in parallel with the USG dial gauge. Photos attached. Bill nailed it. The 210 Torr pump rating is indeed absolute, not depression, meaning it is rated to pull down to 210 mmHg above absolute zero. The USG dial, on the other hand, indicates depression below atmospheric.

I originally set 300 feet as my local altitude above MSL. After releasing the vacuum it settled on 350. No big surprise I guess. That poor old altimeter has never seen 30K+ with anything I’ve owned, and I don't know if it has ever been checked. It's also hard to say if the USG gauge is real accurate given its unknown pedigree.

The big picture is close enough; I have indeed been squeezing the layups at something in excess of 20" Hg deltaP, or roughly 10 lbs per sq inch.
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Dan Horton
Barrett IO-390
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Old 11-21-2020, 05:33 PM
RVDan RVDan is offline
Join Date: Jun 2012
Location: Frederick, MD
Posts: 864

I have a lab vacuum pump which will pull to 1 X 10E-004 Torr or very near a complete vacuum. I have been making molds from pink insulation foam and pulling full vacuum without a noticeable change in dimensions. However none of the parts I am making are that dimensionally critical. BTW, I am at 300 ft msl, so get pretty high pressures from the atmosphere.

I did have concerns about this and before making the molds, I vacuum bagged a 2” thick block of foam that I cut into a dimensioned block. I applied the vacuum and then measured the block with the vacuum applied. It did change some and the corners tended to round, but nothing major.
Dan Morris
Frederick, MD
Hph 304CZ
RV6 built and sold
N199EC RV6A flying
Retired Aerospace Engineer and A&P/IA

Last edited by RVDan : 11-21-2020 at 05:42 PM.
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