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[PSUBS-MAILIST] Silicone oil compensation
I recently found that Dow 561 is really hard to get... and really
expensive if you can get them to sell in 5 gal buckets. I found a much
better vendor for silicone oils, Clearco Products,
www.clearcoproducts.com. I just today called and placed an order for 5
gal of Pure Silicone Oil 50cSt, at $50 per gal. It looks like there are
a variety of viscosities available. I don't know if the price is
different for different viscosities.
IMHO, oil compensation sure seems like it would be vastly simpler &
cheaper, and much more reliable than using an air tank.
- Mark
Sean T. Stevenson wrote:
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
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