Vacuum gauge question- altitude

General Composites Discussions Engineering Talk
#1

I’m just a fabricator and accordingly lacking in theoretical understanding. Maybe ya’ll can help me with this question about measuring vacuum.

I’m running a vacuum pump/bagging system at 1000m (3300 ft) elevation. Not super high, but high enough to to lose a little atmospheric pressure.

If I had a gauge that was calibrated at sea level, and brought it here, it would read about 85mm Hg (3.5" Hg) in normal atmosphere, without the pump on, right? And then if I had a pump that could achieve a 750 mm Hg vacuum, then the gauge would hit 750 mm Hg with the pump on? And the clamping force would be about 10 kPa less.

So what happens to a calibrated gauge? Seems to me that since the gauge is measuring a relative difference, if it’s zeroed at 1000m then I would only see a possible 675 mm Hg with the 750mm Hg pump running.

I’ve got two gauges, and at my (worn out) pump’s max vacuum one reads 680 mm Hg and the other reads 630mm Hg. Both are calibrated with the rubber plug. Obviously they are cheap gauges and one or both are wrong, but I’m just wondering what I could see theoretically.

Thanks!

#2

looking forward to an answer!

Lots of guys here attest to an absolute pressure gauge instead…

#3

Yea, if you use an absolute pressure gauge is much easier to understand the actual pressure both in and outside your bag. All you need to know is what the actual atmospheric pressure is, and what is the pressure level inside your bag, the differential is the pressure consolidating your laminate.

For example: Average atmospheric pressure is 1013mbar at sea level. If you pull a vacuum on your part and inner bag pressure is 10mabrs, you then have 1003mbars (1bar) of pressure consolidating your laminate if atmospheric pressure that day was 1013mbars.

In your case at 1000m elevation your atmospheric pressure is at ~900mbars (depending on the day). So if you measure the pressure level of 10mbars in your bag you then have ~890mbars of pressure consolidating your laminate.

I would recommend you buy an absolute pressure gauge, the GDH200-14 works great, and price is reasonable.

#4

That’s a much easier way to think about it. Thank you, it’s very helpful.