Re: [PSUBS-MAILIST] Pressure compensation

["D. Blake" <dblake@bright.net>]

The last thing you need is an oil that impedes shorts.  You need something
neutral such as used in refrigeration compressors and submersible water
pumps.  Again, it is vital that it be isolated from contaminating O2.
Still, I would be excited by any information about an oil that serves both
purposes ie. pressure compensates and is not ill-affected by the presence of
O2.
A little aside, here's an article I came across to provide food for thought.

                        Submersible Pump Construction
                           and Sacrificial Anodes
                              by Andrew Spear

In general, water pumps designed for garden pond applications are
constructed
using methods and materials to make them long lasting in a
submerged-in-water
environment. Some pumps are designed so that they may be used either
submerged
in water or out of the water. While still others are designed so that they
must
be used out of the water.
How the pump is used often dictates the materials and construction methods.
Pumps that are designed to run totally submerged all the time are relatively
easy to keep cool and keep lubricated since water is a good conductor of
heat,
and under limited conditions is a good lubricant.
Very pure water is also an excellent electrical insulator. But when water
dissolves many substances, these substances release ions. Ions are
electrically
charged atoms or molecules which will conduct electricity.
Pond water, and most water we come in contact with, has many substances
dissolved in it, (e.g. chlorine and chloramine, salts, minerals, heavy
metals,
organic matter, other liquids, water conditioners, etc.) and is a good
conductor
of electricity. Therefore, you do not want water to come in contact with the
electrical connections of the motor.
Most of the heat generated in a pump comes from the motor windings. These
windings are thin wires that conduct the electricity that generates the
magnetic
forces that turn the motor shaft and the attached pump to move the water. So
the
problem is how to remove the heat from hot electrical wires without shorting
the
wires to ground or to each other.
Air is a good electrical insulator, but it also is a good heat insulator.
Some
manufacturers totally fill the void between the windings and the case with a
rigid epoxy material that is a good electrical insulator but also conducts
heat
better than air. The epoxy also provides good mechanical support over a
large
area between the motor and the case. The case surrounding the pump is
usually
made out of thin plastic because it is inexpensive to manufacture and is
relatively inert (non-corrosive). Plastic is not as good a heat conductor as
metal. But since the case is thin and will always be used submerged in
water,
the heat can easily pass through the thin case and be carried away by the
water.
Other manufacturers fill the void between the windings and the case with a
non-toxic oil that is a good electrical insulator but conducts heat better
than
epoxy. Since the oil is a liquid, other electrically insulating means of
supporting the internal motor with respect to the motor case must be used
and is
usually a small plastic or nylon bushing or washer near the motor ends. The
case
could be a thicker plastic for the mechanical support, but would be less
efficient at conducting the heat from the oil to the surrounding water.
Typically the case is made from metal that is stronger than plastic and is a
much better conductor of heat.
Frequently manufacturers use aluminum as the case material because it has
good
heat transfer properties and because it is easy to manufacture and
inexpensive.
The next most common metal for submersible pumps and motors is stainless
steel.
But this is more costly to manufacture.
Stainless steel has a relatively low galvanic potential while aluminum has a
high galvanic potential. Galvanic potential is the willingness or ability of
a
metal (or other substance) to lose or gain electrons in a chemical reaction.
These electrons create unwanted electrical currents using the electrically
charged ions in the pond water which, if left to themselves, will corrode
the
metals. Since a material with a higher galvanic potential will acquire a
higher
proportion of the available ions, placing a material with a higher galvanic
potential in contact with a material of a lower potential will effectively
protect the lower-potential material from the unwanted corrosive electrical
currents.
This is the concept behind a sacrificial anode. Most sacrificial anodes are
constructed using zinc which has a higher galvanic potential than aluminum.
And
most manufacturers of pumps that have aluminum housings recommend that a
sacrificial anode be attached to the pump body. Without this sacrificial
anode,
corrosion could destroy the motor or pump in as short as two months, but
typically in 6 months to a year. Many manufacturers will not warrant their
motors against failure to use a sacrificial anode.
In summary, when you buy your next pump, determine from the seller whether
the
pump has an aluminum body. And if it does, obtain a sacrificial anode to go
along with it. And check out your existing pumps in your pond. If they are
aluminum case pumps, adding a sacrificial anode could extend their useful
life
for many years.

© 1998, LCS and Andrew Spear. All rights reserved.
Last update: 27 Aug 1999

TTYL,
Big Dave

-----Original Message-----
From: TeslaTony@aol.com <TeslaTony@aol.com>
To: personal_submersibles@psubs.org <personal_submersibles@psubs.org>
Date: Saturday, August 12, 2000 11:14 AM
Subject: Re: [PSUBS-MAILIST] Pressure compensation


>In a message dated 8/11/00 9:23:43 PM Pacific Daylight Time,
>protek@shreve.net writes:
>
>> Jon,
>>       You're right in that the mentioned method is a much better way of
>>  compensating.  I wonder if anyone has ever been successful at using a
low
>>  viscosity oil in a trolling motor.  It would be an interesting
experiment
>>  in the direction of a low cost compensated thruster.  It almost makes me
>>  want to go get a trolling motor and experiment.  I wonder if the
armature
>>  would have too much drag against the oil and how much power you would
>loose
>>  in the process.  I would think that cooling would not be a big problem.
I
>>  would suspect that real thrusters for submersibles that are compensated
in
>>  this way have special armatures.  Probably the same oil used in
>submersible
>>  pump applications would work.
>>
>>  GB
>A good oil would be that oil that is used to lube guns and sewing machines,
>it has a pretty low viscosity, lubes all the moving parts, fairly easy to
get
>(I dare you to find a hardware store that doesn't carry the stuff) and
should
>seal anything that arcs to prevent shorts. The biggest drawback would be
the
>quantities that you could buy it in since the biggest I have seen has been
4
>oz. (although you probably could get bigger).
>
>Anthony
>