Re: [PSUBS-MAILIST] Pressure compensation

["Gary R. Boucher" <protek@shreve.net>]

Nuge,
     A trolling motor has seals that keep out water.  If the seal is not 
present the motor case will fill with water in a while by simply being 
operated below the surface.  This seal like any other water-tight seal is 
good to only a certain amount of pressure.  You exceed the pressure and it 
leaks filling your motor.  Not good!  Now, if the air pressure inside the 
motor housing is the same as the water pressure at depth the seal has the 
same pressure on both sides and it is much the same as it would be operated 
on or slightly under the surface.  It should not leak.
     The trick is having the pressure inside the motor the same as the 
outside pressure, or at least close.  If you set the pressure inside the 
motor at a certain value and ascended the pressure inside is greater than 
outside.  If you dove beyond the set pressure you have more pressure 
outside.  One method of pressure compensating a motor is to place a rubber 
bladder with air in the water close to the motor and run a tube from the 
bladder to the motor housing.  As the sub dives the pressure on the bladder 
increases and so does the inside motor pressure.
     There are several things you should consider, but this is not 
necessarily all you have to consider.  If the tube or bladder leaks you 
will get water inside the motor.  If the volume of the bladder is too small 
it will not be able to furnish enough compressed air to the motor and at 
some certain depth the outside pressure on the seal will start to become 
greater than the inside motor pressure.  Any bag of air placed outside a 
sub (assumed to be in some kind of holder) will get smaller in volume when 
the sub dives to greater depth and pressure.  If this bag is too large the 
sub will loose a substantial amount of buoyancy when diving.  In other 
words; the deeper you go the more the sub becomes negative and the more it 
wants to sink.
     You can use regulators to do pressure compensation where compressed 
air is injected into the motor as needed in a descent and released as you 
ascend.  But this wastes air and is far more complicated.  Also watch the 
amount of moisture in the air used in the bladder.  You do not want 
condensation inside the motor if you can help it.  I would use a dry gas if 
feasible.  Nitrogen is probably better than air in such an application.  I 
would think a dessicant could be used somehow in the line to take out 
unwanted moisture but be careful not to allow it to get into the motor itself.

GB


>      Hey guys,
>
>    I've pretty much scoured the archives looking for
>posts which deal with pressure compensating motors
>such as the trolling motors we've been talking about.
>Although I've read a lot on there, there didn't seem
>to be much in the way of explanation.  Anyway, there
>was no crack of lightning that hit me, so I'm still in
>the dark.  What exactly IS pressure compensation, why
>is it important, and how do you do it?  From what I
>could glean, it apparently is a way of making your
>motors or what-have-you more resistant to the high
>pressures encountered at depth by encasing them in
>some kind of airtight case and then pressurizing it
>with a gas or liquid until the pressure inside will
>roughly equal, or compensate for, the pressure
>outside.  Am I on the right track here?  Do I need to
>even worry about it if my boat's only going to go to
>about 150 feet max depth or so?
>     If somebody wants to jump in and clarify a little
>for me, I'd appreciate the enlightenment.  Thanks!
>
>                                     Laters,
>
>                                         Nuge
>
>=====
>"Noisy outside, empty inside." - Confucius
>
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