[PSUBS-MAILIST] Personal_Submersibles Digest, Vol 85, Issue 35

Mon Jul 6 19:18:34 EDT 2020

Just to clarify:
It's totally acceptable to have no compensator whatsoever, just an
oil filled enclosure, however you must be aware of the repercussions of
that choice.
-Firstly, you must take great care to eliminate ALL of the bubbles in the
oil. With a compensator, you have plenty of lee-way so it's not nearly as
big of an issue. Oil compensated lead-acid battery systems on deep diving
subs have a relief valve at a high point to burp off hydrogen as it is
generated. There will always be a bubble (of a perfect explosive mixture of
hydrogen and oxygen mind you) and it's not a big deal because it's allowed
to escape as it expands, and has a flexible compensator to make up for lost
volume as it compresses.

-Second, your enclosure must be prepared for the change in oil volume due
to temperature and pressure to manifest as hydraulic pressure. Most likely,
the oil will find somewhere to escape,  like the u-cup seals on a Minn Kota
thruster. After the item cools down, you will find the opposite problem.
The housing now doesn't have enough oil, is at a slight vacuum, and will
tend to suck in seawater.
-typically oil-filled enclosures have some sort of quick-connect oil
fitting and an air vent at the top, as well as a relief check valve at the
lowest point. Oil can be pumped in, slightly overpressurizing the
enclosure, and expelling any accumulated water through the check valve at
the bottom.

-If you're building a subsea housing of any kind, compensated or 1-atm, I
would advise you to carefully consider what happens in an internal
over-pressure situation. If a housing were to flood at depth and then be
returned to the surface, it could potentially be internally pressurized to
dive pressure, and there would be no way of determining that until it
either explodes on it's own, or explodes while trying to open it. ALL
subsea enclosures commercially available today come with an internal
overpressure relief valve. Famously, there was an incident where the
Bathysphere came up flooded after an unmanned test dive. The hatch was
removed standing off to one side with a sledgehammer, and went rocketing
across the deck at the tip of a column of water. The impact dented an
inch-thick steel plate. I personally had a near-miss incident where an
enclosure naively designed without a proper relief valve, came out of a
test chamber with a 3000psi bubble of air in it.

Thanks,

-River J. Dolfi

On Mon, Jul 6, 2020 at 3:34 PM via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

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> Today's Topics:
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>    1. Re: Yet another compensator method
>       (hank pronk via Personal_Submersibles)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Mon, 6 Jul 2020 22:33:50 +0000 (UTC)
> From: hank pronk via Personal_Submersibles
>         <personal_submersibles at psubs.org>
> To: Brian Cox via Personal_Submersibles
>         <personal_submersibles at psubs.org>
> Subject: Re: [PSUBS-MAILIST] Yet another compensator method
> Message-ID: <830388013.3406258.1594074830394 at mail.yahoo.com>
> Content-Type: text/plain; charset="utf-8"
>
>  ?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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