[PSUBS-MAILIST] oil compensation

[Alan James via Personal_Submersibles]

Hank got me thinking about compensators again.
I had made an initial design for a compensator, using a spring
to produce over-pressure. This design is pretty standard in operation,
with lots of variations.
One problem with the spring is that it might be producing 8 pounds of
force at the start, but as the oil level goes down & the spring extends,
the over-pressure is reduced. Some of the springs I have seen are
pretty large, possibly in an effort to keep the pressure range in a narrower
band. 4-5 psi seems a normal over-pressure.
If I replaced the spring with a cylindrical 5lb weight, I would get the same
4lb (weight in water) over-pressure throughout the range of the diaphragms
travel.
Can anyone see a problem with this? Any comments. The lead may bounce 
around a bit
during transport but I don't think this would be a problem. 
My initial design is attached!
Alan



From: hank pronk via Personal_Submersibles <personal_submersibles at psubs.org>
To: Personal Submersibles General Discussion <personal_submersibles at psubs.org> 
Sent: Thursday, September 14, 2017 12:37 PM
Subject: [PSUBS-MAILIST] oil compensation

Hi All,
Thank you Greg, just sent them a request.

I have tested my air cylinder compensator for my thrusters both in the pressure test and a couple days ago to 100 feet.   The set up seems to work with no water intrusion into the motors.  I have the cylinder mounted below the motors witch causes a small air pocket in the hydraulic fitting on top of the motor.  I am not worried about the small amount of air except the potential of oil being forced up past the air pocket.  This makes me think the compensating cylinder should be above the motors.  Then I wonder why does the compensating cylinder need oil in it at all.  As long as the bore in the cylinder has enough volume so the piston is not bottomed out at max depth.  I also have a small spring pushing the cylinder rod to create a small internal pressure above ambient.  Is my logic flawed?  it would be very convenient to eliminate the oil in the cylinder and would make the compensator  faster to react to pressure change say in a fast ascent.  
Hank
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