John,

I assume you are adding a cabin altimeter to your instrument panel?  It reads out both the cabin differential in psi and the cabin altitude.  It is a must have IMHO to monitor and prevent cabin over-pressurization.  It is the only test instrument needed for calibrating the Dukes and sealing the cabin.

I know that there is an overpressure valve that mounts next to the Dukes.  However, my Dukes head was not working properly during testing and was not opening.  The overpressure valve was no match for the amount of pressurized air coming in the cabin, and the pressure quickly went over 6.5 psi before I was able to do a manual dump.  If you look at the teeny size of the overpressure valve and compare that to the volume of pressurized air coming into the cabin on climb-out, you will quickly come to the conclusion that it provides inadequate over pressurization protection.

I think you are using a G900 like me.  It has an annunciator input called CABIN ALTITUDE.  I connected that annunciator to a pressure switch set to 6 psi that compares the pressure in the cabin to the pressure under the floor in the baggage compartment (where the switch is mounted).  In case the Dukes valve fails and the cabin pressure rises too high, at least I will get some notice (see note above about not depending on the manual vent valve).

I do have a digital differential manometer that measures PSI you are welcome to borrow.  I used it to calibrate the overpressure valve that mounts next to the Dukes (I set it to 6psi).  I built a plenum from a large plastic food container that you can pressurize with a compressor (carefully) that you can borrow as well.

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