[PSUBS-MAILIST] Yet another compensator method

Mon Jul 6 18:33:50 EDT 2020

  I disagree.  The small amount of air in the bellows is nothing to worry about.  I would fill with oil the best you can and go for it. A half full bellow is hard on the bellow because it will collapse a lot,  but a small amount like 10%, who cares.  I looked at oil filled thrusters at Nuytco on a older DW and the oil compensation hose is under 2 inches long.  I looked at ROV hose compensators and the same thing.  I personally do nothing but fill with oil.  No compensation at all.  So a little water gets in-big whoop,  so a little oil escapes, again big whoop.  You should change the oil often anyways.  I make the thruster mounts super easy to remove a motor and my fill plug is under the prop.  I can change oil in a 10 minutes.Hank
    On Monday, July 6, 2020, 11:35:55 AM MDT, Brian Cox via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

 I think a way to simply leave it open should be figured out.  
Brian

--- personal_submersibles at psubs.org wrote:

From: Alec Smyth via Personal_Submersibles <personal_submersibles at psubs.org>
To: Personal Submersibles General Discussion <personal_submersibles at psubs.org>
Subject: Re: [PSUBS-MAILIST] Yet another compensator method
Date: Mon, 6 Jul 2020 13:15:52 -0400

Sorry, my understanding is the exact opposite. Thermal expansion/contraction volumes of the oil are significant, but of much smaller magnitude than gas volume changes due to pressure if the amount of air in the system is not absolutely minimal. Lets say your air bubble was half the volume of the bellows, which is about what it looks like in the photo. If you go to 350 feet, you will be adding a little more than 10 atmospheres, so the bubble will only be about a tenth its original volume and the bellows will have to contract about 45%. I'm not saying it cannot, you might be just fine. But as you dive to your 350 feet, the water will also get cold, so the oil will contract too. Quantifying how much would depend on the temperature differential and the volume of oil. But my point is BOTH the air and the oil are contracting. If their deltas were in opposite directions, say with the air contracting and the oil expanding, then one could argue an air bubble absorbs the expansion. I did once do the math for thermal expansion/contraction on Snoopy's thrusters, and it made me switch from just using the coiled length of hose method to adopting what looks like the exact same bellows you have in the photo. I selected that bellows because it had a volume appropriate to the thermal expansion calculation. However, if you add the gas delta to that, I'm not sure the math would have still added up.
And yes, filling these things with oil so there's no or minimal bubble is a REAL pain!

Best,
Alec
On Mon, Jul 6, 2020 at 10:29 AM James Frankland via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

The air bubble is for a few reasons.
1. To allow easier expansion of the oil if required, even though its enclosed.2. Seems impossible to completely eliminate it, so thought id go with it.3.  Some people said you should have a bubble last time i brought this up!  

On Mon, 6 Jul 2020 at 15:16, Alec Smyth via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

Looks good to me, but why have an air bubble? I always tried like crazy to eliminate it, since that is what expands or contracts most.
Thanks,
Alec
On Mon, Jul 6, 2020 at 10:00 AM hank pronk via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

 James, that looks great-the air bubble is good also.  Hank
    On Monday, July 6, 2020, 7:31:46 AM MDT, James Frankland via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

 Hi All
Getting Jodie B ready for diving in the next week or so.I have changed the motor compensator yet again as the last enclosed one didnt really have a proper flexible part and I think the oil expanded a bit over time and leaked out through the seals.  Just a tiny bit, but enough to put some air back in the pipe.
Anyway, the new method is simply a tube with belows.  These are from a toilet flush and have been sitting full of oil for a couple of weeks to see if anything happens.  They seem good, so i have installed them. The motor is completely filled with oil and the bellows have a small amount of air.  They compress easily, so should provide pressure compensation while also allowing the oil to expand if required.
The bellows will be covered with an open ended tube for protection.  Just got to make a strap for those.
Any comments?
ThanksJames

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