Re: [PSUBS-MAILIST] Thruster pressure compensation

Mark,

The problem with water. If it is deionized it works great. The problem......when you immerse any metallic item in water...the water absorbs molecules from the metal and is now contaminated. If you want to test this theory.....put about 5 oz of distilled(or deionized) water in a glass container (clean..... that has been thoroughly rinsed with distilled water) in a microwave for about 5 minutes. Then carefully and safely from a distance (trip mechanism or something) drop a fork in the water. The water will explode. The reason......pure water will not boil, only water with contaminates in it (metal, dirt, dust, salt,etc.) will.

After contamination, the waters dielectric strength is reduced. The amount of the reduction is the important part. With corrosion, molecule absorption and such......... the problems associated with water outweigh oil. Oil is also a lubricant. So any bearings in the device are lubricated. Also, the oil should be changed at some point. Bearing wear will eventually contaminate the oil to a point of have less dielectric strength.

Dielectric strength is very important for any kind of electrical device. Every material in the world allows electricity to conduct(spark) at some point(even air). Knowing that point is critical when designing with it.

I wouldn't use a bladder, I would just let the thruster structure take most of the pressure.

James Long
Owner/Designer
Lil Brother LLC (Instrument Division)

----- Original Message -----

From: Mark Roberts - Harbortronics

To: personal_submersibles@psubs.org

Sent: Sunday, July 29, 2007 3:19 PM

Subject: [PSUBS-MAILIST] Thruster pressure compensation

I'm curious why these devices don't use water filled pressure compensation?

Positives:

1) Water is a MUCH better heat conductor

2) The viscosity of water is much lower than oil, leading to less internal drag

3) No environmental concerns. Cheap and easy to handle.

Negatives:

1) Corrosion must be considered. Ferrite magnets, steel laminations, etc.

    The metal-working community uses various amounts and types of oil in coolant water to

    reduce or effectively eliminate corrosion. I wonder if this can mitigate this big concern.

2) If you have a leak, you won't know until enough salt & minerals diffuse into the motor to create problems

3) Must use distilled water (tap water has additional minerals which may deposit onto surfaces)

Neutral:

1) Both water and oil are compressible to about the same amount. Water is about 0.34% per KSI, and Oil is about 0.5% per KSI.

    Either fluid will require a flexible bladder/accumulator to compensate for volume reduction at significant depths.

2) Both oil and distilled water are non-conductive.

3) Dielectric strength would seem to be irrelavant for a low voltage motor.

Thoughts?

Mark

----- Original Message -----

From: Lil Brother LLC

To: personal_submersibles@psubs.org

Sent: Sunday, July 29, 2007 9:08 AM

Subject: Re: [PSUBS-MAILIST] Diver Lockout.... serious question here!!

Vance,

Air is the least effective means to cool something that is enclosed. The problem with air, if it is exchanged in a fast enough rate, it works great. On the other hand....if you can't exchange the air, you end up with an oven. Oil on the other hand can carry more heat away faster. The density is the factor here. Air is not very dense, so it will only absorb so much thermal energy per unit.Oil will absorb many more btu per unit than air. Also because oil will thermocycle.....the oil will transfer the heat to the container that contains it faster.

I think an air filled thruster would work good too, but there are problems with compression. If there are any bearings housed within the casing......they will bind under extreme pressure if filled with air. An oil filled enclosure will not compress as far. There will still be compression problems, but not to the extent of an air filled unit.

There was a project a few years ago....a guy made a gaming computer that used oil to keep all the expensive parts cool. You should look it up. It worked really well. (Oil cooled computer)

I do not plan at this point to run a reduction gear. BLDC motors are real good at direct drive.

I would like to build smaller thrusters that can go very deep.