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John,
The formula you are looking for is PSIA = 14.696PSI *(1-(6.8755856*Altitude Ft./1,000,000))^5.2558797
If you want to know the differential pressure calculate the outside pressure then the inside pressure then subtract the two pressures.
In your example, the outside pressure at FL290 is 4.583 PSIA and the cabin is at 10,000' or 10.108 PSIA, the differential is 5.525 PSID.
If you subtract the altitudes first, the pressure at FL190 is 7.046 PSIA, this is meaningless. Who cares what the pressure is at FL190 unless you are at FL190.
I have attached a chart of the ICASO Standard Atmosphere.
BTW, all standard Bourdon Tube gauges are differential pressure gauges by nature. Aneroid (vacuum reference) gauges (altimeters) are absolute.
When I tested my IV-P I used a large mechanical guage on the outside connected to the inside. I routed the air line for the door seal outside the cabin so I could control the door seal. I then pressurized the cabin using a 10 Hp shop air compressor while monitoring the gauge. I kept finding and plugging leaks until I got to 7.5 PSID. Your airframe only cares about differential pressure, regardless of altitude.
Regards,
Brent Regan
On 3/8/2013 8:11 AM, John Barrett wrote:
>
> I didn’t think I’d need help with measurement issues, but I’m hitting some stone walls.
>
> I have a pocket altimeter I thought I’d use but wonder if it will have a stop on negative altitude. Also I went online to see how to convert altitude differential to PSID. Not much luck. The one site I found had conversion formulae that don’t seem correct. It lists a correlation 1” of mercury to 1,000 feet of altitude and that should be ok. It then goes on to list a .49 conversion factor to get from inches of mercury to psi.
>
> My calculations show that at a service ceiling of FL290 the altitude difference to 10,000 foot cabin alt is 19,000 feet. .49 times this difference is about 9.5 PSID. That’s WAY higher than the 5.0 PSI we’re supposed to be seeing in the LIVP at FL 290. I had hoped to test to about 6 PSI.
>
> Short of an altimeter that allows negative readings all the way to something over 19K and a formula that works, the only other instrument I can think of is a gauge that reads psi. I found some online that sell for from a couple hundred bucks to $450 or so. They all appeared to be set up for plumbing into a pipe system and I don’t know if they would read correctly if placed inside the cockpit.
>
> How have you IVP builders out there gone about getting measurements when sealing up the pressure vessel?
>
> Thanks,
> John Barrett, CEO
>Leading Edge Composites
>PO Box 428
>Port Hadlock, WA 98339
>www.carbinge.com
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