Vacuum presses at higher elevations

[andrew whicker]

Hi,

I live at around 4300 ft. That puts me around 12.5 PSIA atmospheric pressure.

If I were to use a vacuum press for veneer, will I need to go down more than the recommended 25 mmHg to 27 mmHg?

[roger wiegand]

I think your units are confused (or mine are)-- 1 atmosphere = 29.9 inches of mercury = 760 mm mercury = 14.7 lb/in^2 at STP. Most gauges I've seen for woodworkers are calibrated in inches Hg and kPa.

12.5 lb/in^2 is still a lot of pressure, I'd be surprised if it made much difference. That ought to be plenty to flatten out veneer. I'm sure someone who lives at altitude can convey actual experience. The cost of achieving higher vacuums goes up exponentially while the clamping pressure only goes up linearly-- with an upper limit of the atmospheric pressure. If you need more clamping pressure than that I'd go to a mechanical clamping system rather than higher vacuum levels.

[andrew whicker]

you're right.. sorry 27 inHg.

[andrew whicker]

I'm assuming it's fine. I doubt all mountain location woodworkers are running special contraptions

[ChrisA Edwards]

I don't think you can achieve higher levels of vacuum clamping, once you achieve max vacuum, you can't make it go higher. If you need more pressure, you need to go mechanical.

12.5psi x 144in (1 sq/ft) = 1800lbs per sq/ft.

Someone, not much smarter than me, please correct me if I'm wrong.

[Ben Ellenberger]

Chris is right.

You gauge pressure us the difference between inside the bag and outside the bag, so if you get 25 inches Hg at sea level and 25 inches Hg at altitude, you have the same pressure difference and the same clamping force. You can only get the inside of the bag to zero, so the maximum vacuum you can draw is equal to the outside pressure. 25 inches is about normal atmospheric pressure for 5000 feet, and 20 inches is about normal for 10,000 feet. So if you are drawing 25 inches at 4,300 feet, your set-up is working pretty well!

[Bill Dufour]

Brian jumped in before I did. A standard pressure gauge or mercury column measures relative pressure not absolute. A high quality pressure or vacumn gauge will state somewhere on the face "gauge pressure". So most gauges when left open to the air will read zero pressure or vacuum. Not 14.7 PSI.
Bill D.

WOG on the face just means the bourdun tube is suitable for water, oil or gas.

[Tom Bender]

If your vacuum will generate 25" Hg at sea level it will probably only draw 21.5" at 4300 ft. It's a function of the difference between 0 and your local 1 atmosphere, say 4.9"

That said, you will be ok.

[David Dalzell]

I live at 5000 ft elevation. I use, not often, a vacuum press for my veneers and have no problems. I use Unibond 800. I have had no problems ever. I have never had a problem with veneers coming loose, nor does the press cycle more than normal during the vacuum process. Caution: use a chemical filter mask if using Unibond.

[Mick Simon]

I'm at 7000 feet and use a Vacuupress Compact 150 with no problems, including bent laminations.

[Brice Rogers]

At 4300 feet elevation, no matter how hard you try or how expensive of a multi-stage vacuum pump you have, you cannot get 27 inches of Hg. The best you can do (in THEORY) would be about 25.5". In practice? Less. Tom Bender's estimate might be close.

The "perfect vacuum" measures about 1" less Hg per 1,000 feet above see level.

Here's a table from the internet:

Altitude in feet	Pressure In Hg	Temp F
0	29.92	59.00
1,000	28.86	55.40
2,000	27.82	51.90
3,000	26.82	48.30
4,000	25.84	44.70
5,000	24.89	41.20
6,000	23.98	37.60
7,000	23.09	34.00
8,000	22.22	30.50

[Dwayne Watt]

The pressure working on the vacuum bag is the barometric pressure at your location minus whatever low pressure your pump can generate. For example, if sea level normal pressure is 14.7 psi (29.92 mm Hg), then at 4300 ft the barometric pressure would be 12.55 psi at same ambient temperature. Assuming the vacuum pump can pull down to 1.55 psi (-11 psi on a vac gage at your location) the net pressure on the vacuum bag is 11 psi or 1584 lb per square foot. Plenty lots of force!

[Jery Madigan]

I'm at 5400 ft and no problems with a standard setup. I open the release a bit the whole time.
I have switched to Unibond 800 also and no problems there. I recommend an electric blanket to cure it. They can be had for about 50 bucks. I have a twin size.