Thanks, Jon,
I found the chart below that helps. An increase in
cabin air pressure of just half a pound (14.7 to 15.2 psi) would register
1,000 feet on the altimeter. I wouldn't have guessed that. If
that happened in 10 seconds I would definitely notice with my ears; if it
happened over 2 minutes I probably wouldn't. An increase of 1.9 psi
would register 3500 feet on the altimeter.
In order to avoid false alarms, I surmise the main thing to consider
is how much variation in pressure might occur in normal operation of the
scrubber and O2 metering system. I think some auto-metering O2 systems
respond to the drop in air pressure caused by the removal of CO2 by the scrubber
and replace it with the indicated amount of O2. I have no idea what drop
in pressure it would take to trigger the O2 release, but it must be rather
small. A while back someone set the O2 feed manually based on calculated
consumption and found it worked well without requiring adjustment. Was
that you, Alec?
Jim
| Altitude Above Sea Level |
Absolute Barometer |
Absolute Atmospheric Pressure |
| feet |
meters |
inches Hg |
mm Hg |
psia |
kg/cm2 |
kPa |
| -5000 |
-1524 |
35.7 |
914 |
17.5 |
1.23 |
121 |
| -4500 |
-1372 |
35.1 |
898 |
17.2 |
1.21 |
119 |
| -4000 |
-1219 |
34.5 |
882 |
16.9 |
1.19 |
117 |
| -3500 |
-1067 |
33.9 |
867 |
16.6 |
1.17 |
115 |
| -3000 |
-914 |
33.3 |
852 |
16.4 |
1.15 |
113 |
| -2500 |
-762 |
32.7 |
837 |
16.1 |
1.13 |
111 |
| -2000 |
-610 |
32.1 |
822 |
15.8 |
1.11 |
109 |
| -1500 |
-457 |
31.6 |
807 |
15.5 |
1.09 |
107 |
| -1000 |
-305 |
31.0 |
793 |
15.2 |
1.07 |
105 |
| -500 |
-152 |
30.5 |
779 |
15.0 |
1.05 |
103 |
| 01) |
0 |
29.9 |
765 |
14.7 |
1.03 |
101 |
| 500 |
152 |
29.4 |
751 |
14.4 |
1.01 |
99.5 |
| 1000 |
305 |
28.9 |
738 |
14.2 |
0.997 |
97.7 |
| 1500 |
457 |
28.3 |
724 |
13.9 |
0.979 |
96.0 |
| 2000 |
610 |
27.8 |
711 |
13.7 |
0.961 |
94.2 |
| 2500 |
762 |
27.3 |
698 |
13.4 |
0.943 |
92.5 |
| 3000 |
914 |
26.8 |
686 |
13.2 |
0.926 |
90.8 |
| 3500 |
1067 |
26.3 |
673 |
12.9 |
0.909 |
89.1 |
| 4000 |
1219 |
25.8 |
661 |
12.7 |
0.893 |
87.5 |
| 4500 |
1372 |
25.4 |
649 |
12.5 |
0.876 |
85.9 |
| 5000 |
1524 |
24.9 |
637 |
12.2 |
0.860 |
84.3 |
| 6000 |
1829 |
24.0 |
613 |
11.8 |
0.828 |
81.2 |
In a message dated 3/27/2012 3:08:21 P.M. Central Daylight Time,
jonw@psubs.org writes:
Probably somewhere around 2-3psi I would think. Not sure
about the rest of you but I have to start clearing my ears at around 6-8 feet
of water pressure. If you need to clear your ears in a 1ATM during
operations, that indicates a problem of some kind which should send you to the
surface. The two main issues I can think of that you want to avoid is an
internal pressure/dive duration that ends up requiring decompression; a high
enough pressure level that introduces O2 toxicity. Both of those
situations would be difficult to get to without the pilot knowing something
was wrong since even without any gas monitoring you'd be clearing your ears
plenty of times. Unless you are drawing a vacuum as part of the pre-dive
systems check (like DW-2000 does) there should be no clearing of ears unless
there's a problem maintaining 1ATM within the cabin.
Jon
On
3/27/2012 1:35 PM, JimToddPsub@aol.com wrote:
I think I'll add an onboard checklist for possible causes if the alarm
goes off. This brings up the question: What level of
cabin pressure increase should activate the alarm?
I'd like to hear some answers on that one.