general physics of equipment.....

[mpittman]

perhaps i am too young and uninitiated yet to understand why things are the way they are in our world (i relate this to a why is the sky blue type question).
with the birth of scuba , we have found a way to subject our bodies to the pressures of the deep without building armor around us keeping us at 1 bar of pressure , instead we have designed a system to elevate the level of pressure inside our air cavities to exert equal pressure outward towards the ambient pressure of the water... now , stay with me here.... why is it we have designed this system for ourselves... yet for most of our water sensitive equipment (especially us diy guys who might not be able to get 500' depth out of our housings) we build structures around them to hold them (hopefully) at 1 bar of pressure?
granted for many things it is no huge feat to do (our homemade lights and camera housings) but for things with moving parts such as sealed shafts on dpv's our job becomes increasingly difficult...
is there any specific reason say if i have 1 cubic inch of air space inside a specific housing ive built for "x" piece of equipment and build it so it will hold pressure without trying to deform and fill that cubic inch with water such as common routes go. why coulldnt i instead build a sort of flex chamber with a volume of 10 cubic inches of air at 1 bar that is designed to buckle and shrink (in this case up to 10 bar of pressure) and displace that 10 cubic inches of air into the 1 cubic inch of air space inside the original housing thus making it exert a force outward equal to that of the ambient force outside the case? wouldnt it be much easier to seal a spinning shaft on a dpv for depth if the only consideration was separation of water pressure equal to that of air pressure inside the housing?
ok now i know that is a raw concept and you run into issues such as the change in buoyancy at depth and surface , drag created by the water occupying space in the collapsed chamber etc etc.but now lets build on that original idea with the help of our old friend scuba...
10 bar of pressure is about 147 psi...or 1000 kPa is you prefer.. would it not be fairly simple to build (or modify an existing) sort of first stage regulator and a small low pressure bottle say the size of a can of pop install them on whatever housing your building , and have the regulator feed air into the housing to equalize the inside pressure to that of the ambient pressure outside , and a small overfill valve to bleed off the internal pressure on ascent? that way your not changing volume and buoyancy nearly so much as you would with the collapsable bag idea. ok almost done here......
now imnot looking into this idea to compensate for all differences in pressure , but lets say existing dpv technology for diy'er allow us to build one thatll run to 4 bar without leakage (although i know thenumber is higher for some of you more crafty folks) with a sort of first stage pressure equalization method in place thatd compensate for say 4 bars of pressure , wouldnt you have just bought yourself a total of 8 bars of working pressure between the build design and pressure compensation? just a thought here guys , seems like something a bit more feasable than drowning all our enclosed gear in a non conductive newtonian fluid (mineral oil and transmission fluid).
i wouldnt see this idea working on anything with a large volume of air space inside the chamber , but if you can manage to keep air pockets to a small size your not having to add much at all to re-equalize.
any ideas, suggestions , further elaborations , curses , trolls or flames , are as always , quite welcome.

 

[miketsp]

You also have to think about whether the parts inside the pressure housing will accept the higher working pressure or not. First thing that comes to mind are batteries (lead-acid - with an internal air space and others with dangerous contents like lithium) and LCDs which have large surface areas and are relatively delicate. After all you wouldn't go to all this trouble unless you were planning to design for deeper use.
One big advantage comes to mind and that is that the increased air density inside the housing would significantly improve the cooling of any internal components that depend on air cooling (in comparison with 1ATM air). High pressure air has better thermal conduction capability. However you should not overlook that some designers oil-fill precisely for this reason - to improve component cooling. Especially motors.

 

[jonnythan]

Human bodies are very convenient in that, for the most part, they're completely sealed from the outside world and the only compressible part is the lungs.

Machinery, on the other hand, doesn't just come sealed. It may be possible to immerse most things in a liquid to make them incompressible (just like our bodies are essentially all liquid and incompressible). However, the problem is again sealing. If you were to fill your battery canister up with mineral oil, you wouldn't need to create a housing to withstand great pressure... however, you *would* still need to seal it off completely from the outside world, and it would be a gigantic mess whenever you had to open it up. This applies to anything... if you get rid of all the air spaces by replacing them with liquid, you eliminate pressure buildup as you go deeper.. but you need to deal with the extra complexities of having completely liquid-filled items.

Additionally, many things will not work well immersed in any kind of liquid. SPG's and compasses are two items that come to mind which are routinely liquid filled, so you're not the first person to have this idea. I have no doubt that many things designed for deep water use are designed with that in mind.

 

[mpittman]

yeah we had a similar discussion about liquid filling/immersion cooling vs. the big mess in the dive light msn group , im not much of a fan of the idea .
mike tsp you bring up some real valid points on the battery and lcds. but those both are items that would be relativly easy to build a chamber to seal off at 1 atm of pressure . my biggest focus on this entire thread (which i should have clarified to begin with) would be an engine compartment for a dpv . beig able to in the very least balance the inside pressure of air with the outside pressure of water using a shaft seal between the two seems like itd be a much easier feat than trying to use a shaft seal to not only separate the inside of the chamber from the outside , but to also hold a great amount of pressure as well.
does anyone out there have the low down on a good 24 volt motor to use? going off of gavin's site it looks like they are using 24 volt motors putting out around 430 watts of power , no ideas on rpms , blade pitch , or the slip clutch they use though. id gladly just out and buy a gavin , but i mean really , the cost of a new gavin is twice as much as i paid for my last car heh. that and id really just like to try building my own.

 

[jonnythan]

my biggest focus on this entire thread (which i should have clarified to begin with) would be an engine compartment for a dpv . beig able to in the very least balance the inside pressure of air with the outside pressure of water using a shaft seal between the two seems like itd be a much easier feat than trying to use a shaft seal to not only separate the inside of the chamber from the outside , but to also hold a great amount of pressure as well.

So you're suggesting hooking up your DPV to some sort of compressed gas cylinder, so you can add air during the dive to alleviate the squeeze (aka a drysuit)? That seems to introduce many more problems than just sealing the thing off and making it strong enough to withstand the pressure. If you don't do that, and you don't fill the thing up with a liquid, there's no way to "balance the pressure."

 

[cool_hardware52]

Mpittman,


If you choose to pressurize your scooter, make sure to have some provision to vent the pressure as you ascend. I don't want to be there when somebody opens the cover on a DPV that has an internal pressure a couple atm higher than 1.

I have often thought about putting a schrader fitting in a battery can and precharging the can to a few (3-4) so on a typical dive to say 100 fsw the can would be above ambient even at depth.

I never have however given the concerns about pressure on the batteries themselves, and the "excitement" associated with opening the can.

Regards,



Tobin

 

[miketsp]

I never have however given the concerns about pressure on the batteries themselves, and the "excitement" associated with opening the can.

The only "excitement" that comes opening a battery can is
a) It leaked at depth and is now pressurised ie the lid will fly off as soon as you open the latches

or

b) You haven't opened it in a year and there is accumulated H2 inside so you don't want to risk generating a spark as you prise the lid off. There won't be enough H2 to do any real damage once there is a free opening but there may be enough to pop the lid a bit faster than intended.

On my DIY canister I never open it as I have external charging terminals but I have 2 pressure release screws which are taken out for storage.

 

[mpittman]

yeah my first post i coulda swore i typed a small provision for a one way dump valve (much like the dump valve on a good lift bag) , but in my ramblings i may have left that part out. as far as batteries go , id have no problems building a sturdy section of the case to hold those at 1 atm. perhaps before i go on any more about counter pressurising the actual engine compartment i should ask how exactly do others out there seal a spinning shaft against ambient water pressure? iall i can think of remotly similar to this would be something like a wheel bearing seal (sized down to the size of your shaft of course) and inverted to bulge in toward the compartment when ambient outside pressure is greater than inside pressure... i would imagine the tolerances on the steel ring that holds the seal itself would have to come close to the diameter of the shaft itself ( im talking like .020 or less) or the rubber part that makes up the seal would just stretch and have a large amount of water blowby...anyone got specs in regards to those numbers? or better yet specs on allowable numbers for shaft runout and crown?

 

[miketsp]

yeah my first post i coulda swore i typed a small provision for a one way dump valve (much like the dump valve on a good lift bag) , but in my ramblings i may have left that part out.

Yes you did mention it and No you didn't leave it out.

...and have the regulator feed air into the housing to equalize the inside pressure to that of the ambient pressure outside , and a small overfill valve to bleed off the internal pressure on ascent?...

Too many people answer without reading the original post. :frustrate

i should ask how exactly do others out there seal a spinning shaft against ambient water pressure?

You need to look at standard seals for water pumps, boat propeller shafts, oil drilling, etc. This is off-the-shelf technology that's been around for many years.
In most basic pump applications you just have 2 Teflon (or Ceramic or other) disks, one fixed to the spinning shaft and the other fixed to the housing. They are pressed together by a spring located on one side. They slide against each other almost without friction and the liquid can't pass.
Just run a Google search for "high pressure seals" and you will find all sorts of techniques.

 

[cool_hardware52]

yeah my first post i coulda swore i typed a small provision for a one way dump valve (much like the dump valve on a good lift bag) , but in my ramblings i may have left that part out. as far as batteries go , id have no problems building a sturdy section of the case to hold those at 1 atm. perhaps before i go on any more about counter pressurising the actual engine compartment i should ask how exactly do others out there seal a spinning shaft against ambient water pressure? iall i can think of remotly similar to this would be something like a wheel bearing seal (sized down to the size of your shaft of course) and inverted to bulge in toward the compartment when ambient outside pressure is greater than inside pressure... i would imagine the tolerances on the steel ring that holds the seal itself would have to come close to the diameter of the shaft itself ( im talking like .020 or less) or the rubber part that makes up the seal would just stretch and have a large amount of water blowby...anyone got specs in regards to those numbers? or better yet specs on allowable numbers for shaft runout and crown?

You did and I missed it. More paragraphs would help my old eyes, but I should have seen it anyway.



Regards,



Tobin