Riddle me this........

[buff]

Here's the problem.
I recently returned from Bonaire..Upon disembarking from the plane in good ol' Minnesota I noticed my air tight photography case was bowed in i.e. concave. I turned the knob on the case to equalize the pressure and heard a great "swoosh" of air moving through the vent. The concave nature of the case would lead me to believe that the pressure was less inside the case than outside. Having last closed the case in Bonaire I would have to assume that the air pressure in Bonaire is less than the pressure in Minnesota. That would mean that Bonaire is at a higher elevation than Minnesota.....but how can that be???? I don't know the elevation of Minnesota but I am sure it is higher than Bonaire and that would mean that the case should be convex i.e. the relatively higher internal air pressure is expanding the case.
Is there something that I am missing in this "photo mystery"?

 

[joewr]

Buff,

The pressure in a pressurized airplane cabin at altitude is less than at sea level. So, the air in your case was slightly evacuated whilst thou wert traveling in God's great skies. Then, you landed in high pressure Minnesota and got the gas effect you described(Iguana Don, our local expert on gas would have answered this question, but he is off testing the frijoles of Cozumel.).

Joewr

 

[donooo]

Hey Buff, great place for me to sneak in a different thought.

Sure the pressure is different, that's why we have wind. Air blows from high pressure to low pressure. You need to look at the isobars ( lines of equal pressure ) on a weather chart, constantly changing. Over here in Washington, during the summer when the low pressure comes in, it clouds up, the wind comes from the south, might rain ,,,,, this can go on for ever, this is weather. Tropical storm are giant low pressure systems.

Your airplane was probably pressurized to 8,000 ft, but if your camera was in a baggage compartment the pressure might be different then 8,000 feet.

On a smaller scale, the pressure at my home (2,000 ft.)is different then Seattle every day and constantly changing. So if I drive the 90 miles to Seattle to dive, over a 3,000 foot pass, my nitrogen levels will be different to start with then somebody living in Seattle that I meet at the beach. ( if we wait long enough we will equalize )

Hhuuu ,,,, Same problem last winter when I was staying in Yuma Az. and wanted to dive in San Diego. Tremendous pressure differences plus 4,000 foot mountain passes and then go back to Yuma.

A combination time to fly and altitude diving problem.

So the bottom line, I'am looking at a Cochran Commander computer which is constantly on and keeping track of this stuff. I'am afraid to bring this up in Computers as they don't like Cochran. I haven't gotten over my Poseidon reg.

tnx donooo

Not riddle, but a mystery.

 

[blacknet]

Hello,

I have witnessed several things like this. The techniqe that's recomended to fix that problem is very simple and can safe and/or extend the life of your camera equipment!

What you should do is before transportation/storage of any/all camera/photo/housing/etc.. equipment is REMOVE THE O-RINGS! That's right. REMOVE THE O-RINGS that seals the unit up to make it water tight. Lets say you dive 4x a year and you don't remove them. They are stored in the closet sealed up. It is safe to assume that it is under pressure and the weakest link in the camera will eventually go out. May be the orings (could dry rot or something) or it could damage the camera (storage under pressure for extended periods of time)

If you are going to fly then remove the rings, pack it up and you'll never witness this again. With the rings out it allows air to circulate and not trap air, leading to a pressure difference.

Also most equipment makers will tell you that they certify the equipment to be pressured IN WATER, and that in air CAN DAMAGE the equipment. When they pressure test the unit durring service they dunk them in water and then pressure test it.

Ed

 

[buff]

Well I would have fallen for all of those answers but here's the rub. The case is supposedly water and AIR tight so that the pressure could not have equalized while in the plane....that would have explained the gush of air that ran into the case when I opened the valve on it-if Bonaire was at lower pressure than Minnesota-but there is one other possiblility.
The camera was not in the baggage compartment. It was in the passenger section with me(where $2300 bucks worth of equipment should be). The flight itself took about 12 hours - plenty of time for the case to equalize to the ambient pressure if it were not completely sealed. I do not remember opening the case on the plane. If I did open the case on the plane this would explain all of this phenomenon.
Am I going to have to start a self help group for those of us who have Poseidon regulators, Donooo??? Be proud that you bought something from the land that brought you the midnight sun, Bjorn Borg, and the Sweedish bikini team!!!!! Don't listen to all those DIR'rs-Poseidon is the regulator of the U.S. Navy and I consider those guys a little more knowledgeable than DIR when it comes to "tech" diving.

 

[blacknet]

Hello,

I have had the same problem with the pressure change, but going a mere 400 feet.

As for the regulators goes are you looking for parts to service it or just saying you like them? BTW I do tend to agree with your last statment to.

Ed

 

[The Chairman]

Hey Buff and all,

Nature hates a vaccuum (so does my son and daughter, 'cause they sure won't use one) and the mass of air (a gas mixture) is far, far less than that of water. Air will leak past many seals that will keep water out, and most of our dive equipment is designed to withstand positive pressure differential, rather than a negative one. Having worked in a cryogenic research facility whilst in college, I remember the problems of making things "airtight". It normally involved a bunch of goos of various sorts and lots of prayer. We approached vaccuums to the order of 10 -3 torr (space is like 10 -6 torr) daily and even had to deal with partial vaccuums that were probably equivalent or a little greater than cabin pressures at altitude.

On another note, heat and barometric pressure will also cause havoc in trying to maintain a standard pressure. This is why tire pressures vary as much as they do from month to month. If you had the camera sealed in the heat with an approaching storm (Hi pressure), then opened it up in a relatively calm and much cooler atmosphere, then you could experience the same phenomemon. Too many variables to be able to put your finger on the precise cause for this, and nothing to really worry about. It's just physics messing with your mind.

As for removing O-Rings, be very careful not to nick them, and please replace them before entering the deep blue (grin). And what ever you do, lighten up on that silicone grease... a tiny little dab will do ya!

 

[seaspyder]

Buff,

The answer is two fold:

1) pressurised cabins are not at 1 ATM, they are higher pressure than the outside at 30000 ft but lower than ATM at sea level, hence your ears pop when taking off and landing as they adjust the cabin pressure.

2a) since the cabin is at lower than 1 ATM, and your case was sealed at 1 ATM, it is at higher than ambient pressure during the flight.

2b) You are correct in saying the case is water and air tight but clam shell cases do better against something pushing down on them than somthing inside pushing out. In other words the pressure from the inside eventually leaks out during the flight, however, when the cabin is repressurised the case clamps down and seals the lower pressure inside. That is why pelican cases now have a valve which one can leave open during a flight.