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Old 13th August 2005, 14:31   #1 (permalink)
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Question ppo2 housing oil filled

Hi,

The design of mark munro's ppo2 monitoring housing, the "tube" has gotten me thinking. Because off the not 100% smoothness of the pipe, the o-ring might fail to seal 100%, specially at depth.
When I was to make such a housing, and fill it with oil, say like an uwatec aladin compu, would that stop the flooding, since the pressure differance would not be there anymore?

denz.
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Old 13th August 2005, 17:23   #2 (permalink)
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Quote: (Originally Posted by denzel)
Hi,

The design of mark munro's ppo2 monitoring housing, the "tube" has gotten me thinking. Because off the not 100% smoothness of the pipe, the o-ring might fail to seal 100%, specially at depth.
When I was to make such a housing, and fill it with oil, say like an uwatec aladin compu, would that stop the flooding, since the pressure differance would not be there anymore?

denz.

Filled with oil a pressure differential will still exist (oil will be at ambient pressure to when it was filled) Oil will make it less crushable and that may help stop pressure deforming the housing and loosing the o-ring seal.

If it does leak of course you would get a very small amount of water in (beacuse its aleady fully filled with oil) before the internal is at ambient.

Some observations Ive had designing and working with o-ring sealed pressure vessels with water pressures up to 2000 bar:

O-rings seal better when they have a pressure differential across them. Often a bad o-ring seal (where the mating surfaces arent smooth or non geometric but the oring is intact) will leak a little at low pressure but seal up dry when the ring is extruded enough to form a full seal by larger presure differentials.

If it doesnt leak at low pressure its usually fine at high pressures provided there is no deformation of the housings.
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Old 14th August 2005, 03:55   #3 (permalink)
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Filling a housing w/ mineral oil to make it less susceptible to water intrusion works well in the case of the MK15 primary, but it is not a practical solution for one of Markís ppO2 units. The reason is that there is an O ring sealed port plug in one of the end caps that must be removed to gain access to the calibration potentiometers. Every time you calibrated the unit youíd be dipping your screwdriver into the oil and then topping up the tube w/ more oil when finished. What a mess!

There are several things you can do to reduce the likelihood of leakage. First and easiest is to eliminate one of the end caps all together. Replace it w/ a Ĺ inch thick disk of plexiglass. The disk is drilled and tapped to accept the port plug the same way as the stock end cap, and is slotted to hold the board like the stock cap as well. It is then slipped into one end of the tube and epoxyed permanently into place.

Another thing you can do is to make the other endís cap a bit longer and add a second O ring groove, your basic belt and suspenders methodology.

Finally, make your tube from polycarbonate/ Lexan instead if acrylic. It is much more impact resistant and considerably stronger.

Iíve built several of these housings w/ above methods and have never had one leak. The deepest Iíve had one is to about 55 m. Good luck!

Ken

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Old 14th August 2005, 23:54   #4 (permalink)
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Talking ambient then...

Thanx for the tips.
Guess I didn't re-invent the wheel.
I'll go with the lexan/ polycarbonate, and will keep it open, at ambient pressure.
Already decided before to glue one side, but the double o-ring on the other side makes perfect sense.

denz.
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