13th August 2005 14:31
ppo2 housing oil filled
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?
13th August 2005 17:23
Originally Posted by denzel
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.
14th August 2005 03:55
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 Mark’s 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 you’d 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 ½ 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 end’s 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.
I’ve built several of these housings w/ above methods and have never had one leak. The deepest I’ve had one is to about 55 m. Good luck!
14th August 2005 23:54