<div dir="ltr">Exactly what River says! There was a time with Snoopy when the thrusters were driving me crazy. I'd surface and the little hose that was supposed to act as a compensation bladder was inflated to the thickness of a thumb. Release a hose clamp and it was a miniature version of Beebe's photo. What was happening? I had very little air in the motors, but there was a bit. It was also like 95 degrees on the surface when I put in the oil, and I was diving to 250 where it was really cold. Because the little hose couldn't shrink enough, the whole thruster was getting pressurized. These issues were what led me to try air compensation instead. So far, so good.</div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Jul 6, 2020 at 7:19 PM River Dolfi via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org">personal_submersibles@psubs.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div><br></div>Just to clarify:<div>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.</div><div>-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. </div><div><br></div><div>-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.</div><div>-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.</div><div><br></div><div>-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.</div><div> </div><div><div><div dir="ltr"><div dir="ltr"><div>Thanks,</div><div><br></div><div dir="ltr">-River J. Dolfi</div></div></div></div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Jul 6, 2020 at 3:34 PM via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Send Personal_Submersibles mailing list submissions to<br>
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Today's Topics:<br>
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1. Re: Yet another compensator method<br>
(hank pronk via Personal_Submersibles)<br>
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Message: 1<br>
Date: Mon, 6 Jul 2020 22:33:50 +0000 (UTC)<br>
From: hank pronk via Personal_Submersibles<br>
<<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>><br>
To: Brian Cox via Personal_Submersibles<br>
<<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>><br>
Subject: Re: [PSUBS-MAILIST] Yet another compensator method<br>
Message-ID: <<a href="mailto:830388013.3406258.1594074830394@mail.yahoo.com" target="_blank">830388013.3406258.1594074830394@mail.yahoo.com</a>><br>
Content-Type: text/plain; charset="utf-8"<br>
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?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<br>
On Monday, July 6, 2020, 11:35:55 AM MDT, Brian Cox via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>> wrote: <br>
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I think a way to simply leave it open should be figured out.??<br>
Brian<br>
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--- <a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a> wrote:<br>
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From: Alec Smyth via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>><br>
To: Personal Submersibles General Discussion <<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>><br>
Subject: Re: [PSUBS-MAILIST] Yet another compensator method<br>
Date: Mon, 6 Jul 2020 13:15:52 -0400<br>
<br>
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 !<br>
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.<br>
And yes, filling these things with oil so there's no or minimal bubble is a REAL pain!<br>
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Best,<br>
Alec<br>
On Mon, Jul 6, 2020 at 10:29 AM James Frankland via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>> wrote:<br>
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The air bubble is for a few reasons.<br>
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!??<br>
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On Mon, 6 Jul 2020 at 15:16, Alec Smyth via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>> wrote:<br>
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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.<br>
Thanks,<br>
Alec<br>
On Mon, Jul 6, 2020 at 10:00 AM hank pronk via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>> wrote:<br>
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James, that looks great-the air bubble is good also. ?Hank<br>
On Monday, July 6, 2020, 7:31:46 AM MDT, James Frankland via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org" target="_blank">personal_submersibles@psubs.org</a>> wrote: <br>
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Hi All<br>
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.<br>
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.<br>
The bellows will be covered with an open ended tube for protection.? Just got to make a strap for those.<br>
Any comments?<br>
ThanksJames<br>
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