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Re: [PSUBS-MAILIST] oil vs. air compensation
The dielectric constant is a material property which indicates how
resistant the material is to current flow through it. (incidentally,
the "dielectric" is the material between conductors in capacitors and
coaxial TV cables). A high dielectric oil is what you would find in a
power transformer - i.e. mineral oil, which is good for immersing
electronics, but not ideal for fluid power transmission (hydraulics).
There are a variety of petroleum oils formulated with dielectric in
mind, but silicone oils are probably better suited to all submarine
applications, since they are non-flammable and do not produce the toxic
byproducts of combustion that petroleum oils do.
Look at Dow Corning 561 or something similar. Obviously, to minimize
drag you want a low viscosity oil, but that may not be the primary
consideration if you have, for example, other onboard oil systems and
want one oil for all purposes. Definitely stick with silicone oils, though.
With regard to simply exposing a flexible bladder to sea, that does
compensate the motor to ambient pressure, but there are a few
disadvantages to this method that should be considered. Compensating to
ambient only introduces the possibility of admitting seawater to the oil
system in the event of a leak, and does not provide any indicator of oil
loss in the event of a leak. Conversely, if you compensate with a
slight positive pressure (accomplished by spring loading the exposed
bladder, or by piping the oil back to a pressure controlled source), you
ensure that in the event of a leak oil will leak out and not seawater in
(disastrous in a motor), and you also have a visual indicator in the
form of the bladder volume to alert you to the presence of leaks. By
setting the system up initially at some volume less than the maximum
possible, you allow for thermal expansion of the oil without affecting
the compensation pressure. Additionally, having bleed points at high
points in the system allows you to easily remove any air, as the
positive pressure pushes it out. As part of the pre-dive, you would
just crack all of the bleed caps until oil seeps out and not air and
then tighten them again. This way, you verify that when you look at the
bladder volume, that actually corresponds to oil volume and will not
change unexpectedly as you dive.
-Sean
ShellyDalg@aol.com wrote:
Thanks Jim. I'll keep an open mind on what type of compensation for
now. The Minnkotas are good for 50 feet with no compensation, so I'll
be able to test launch and work out the bugs before I need to commit.
I've heard that just a short flexible tube filled with oil and sealed
to the housing is all it takes to compensate with oil. I've seem a
couple like that, and it certainly looks to be the simplest method.
That and the increased thermal conductivity are some of the
advantages. The oil creates a little drag within the motor housing,
and tends to get black from the arcing at the brushes so replacement
is a regular maintenance task. I've also heard that vegetable oil is
used. Have you heard what type of oil is best? There was a thread a
while back talking about "di-electric" what ever that is.
I'll e-mail Greg ( about my plexi disks) and see what he says.
Frank D.
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