<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8" /></head><body dir="auto"><p dir="ltr">Pressure can help, of course, but your sealing arrangement should provide a positive seal even in its absence. The spiral action can be avoided by careful attention to surface finish and eliminating machining marks on your shafts. This doesn't eliminate the problem though. Some seals are designed to be hydrodynamically lubricated by their encapsulating fluids, lifting the seal lip away from direct contact with the shaft (thin film lubrication). The bias pressure ensures that any leakage across the seal is of oil out (which has only environmental consequences) instead of seawater in (which can ruin your day in a hurry). It also allows you to monitor either the compensation pressure or the displacement of the compensator (via either a displacement transducer, which would give you rate information, or a limit switch, which would just tell you if it had bottomed out) so
that you can alarm on loss of compensation.</p>
<p dir="ltr">As for the air / oil debate, I would look carefully at the dielectric constants of each versus your intended operating depth. Oil behaves pretty much identically at all pressures. Gases increase in density at depth, reducing their effective isolation voltage. Probably not an issue at PSub depths, but for a much deeper diving sub, were I to consider gas compensation, I might look at something like sulfur hexafluoride instead of air, but then you're still dealing with venting the excess gas on ascent. Oil is messy and consumes more power, but provides additional lubrication and an excellent heat transfer mechanism.</p>
<p dir="ltr">Sean<br>
</p>
<br><br><div class="gmail_quote">On April 5, 2017 10:33:08 PM MDT, Alan via Personal_Submersibles <personal_submersibles@psubs.org> wrote:<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
<div>Greg,</div><div id="AppleMailSignature">I use 2nd stage octopus regulators on my ambient sub, but feel this is better</div><div id="AppleMailSignature">because you can adjust the regulator to your desired pressure over ambient. </div><div id="AppleMailSignature">With the second stage regulator you might be getting 1psi above ambient if </div><div id="AppleMailSignature">you have it 2ft below the thrusters. Industry standard seems to be around</div><div id="AppleMailSignature">4-8 psi above ambient. Most of the cheaper commercial compensators rely</div><div id="AppleMailSignature">on a spring whose force varies depending on it's extension which is relative</div><div id="AppleMailSignature">to the oil volume, whereas with Hugh's method you should be able to keep</div><div id="AppleMailSignature">a narrower range.</div><div id="AppleMailSignature">I don't know why the commercial units work in that range. Obviously the more</div><div id="AppleMailSignature">pressure you
have the less chance there is of water getting in. I believe mechanical</div><div id="AppleMailSignature">seals like a bit of pressure to help oil their contact faces. Also I have read that</div><div id="AppleMailSignature">you can get an unpredictable pumping action from microscopic spiralling on the </div><div id="AppleMailSignature">prop shaft, that can pump water in against an over-pressure.</div><div id="AppleMailSignature">Alan</div><div id="AppleMailSignature"><br />Sent from my iPad</div><div><br />On 6/04/2017, at 3:54 PM, james cottrell via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org">personal_submersibles@psubs.org</a>> wrote:<br /><br /></div><blockquote type="cite"><div><div style="color:#000; background-color:#fff; font-family:verdana, helvetica, sans-serif;font-size:13px"><div><span></span></div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1491447637884_9465">Alan/ Cliff,</div><div class="qtdSeparateBR"
id="yui_3_16_0_ym19_1_1491447637884_9473"><br /></div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1491447637884_9511" dir="ltr">That's interesting- but I don't understand why it's better than a good working second stage? This is what I did-</div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1491447637884_9544" dir="ltr"><br /></div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1491447637884_9820" dir="ltr">Three trolling motors (one vertical and port/starboard pushers) had clear vinyl tubing connected to the tube/pipe where the wires come out. </div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1491447637884_9918" dir="ltr">Each clear piece of tubing ran to the "inlet" side of a small water filter housing (the kind with a clear plastic removable sight glass). </div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1491447637884_9925" dir="ltr">The water filter housing served as a junction box so each motor could be removed individually.
</div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1491447637884_10049" dir="ltr">The tubing them came out of the "outlet" side of the filter housing and went to a "T" fitting. </div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1491447637884_9926" dir="ltr">One piece of tubing carrying the wires went from the "T" to a penetrator in the hull. </div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1491447637884_9927" dir="ltr">The other piece of tubing went downward to a set of two "T" fittings that served as a manifold connecting all 3 pieces of tubing together. </div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1491447637884_9961" dir="ltr">Finally one Large piece of tubing went to a scuba second stage mounted low on the hull (connected where the mouthpiece usually goes).</div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1491447637884_9847" dir="ltr"><br /></div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1491447637884_9861" dir="ltr">When the sub descended, air
was "inhaled" into the motors. When the sub ascended, the air in the motors "Exhaled" back out of the reg. Very simple.</div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1491447637884_9917" dir="ltr"><br /></div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1491447637884_10013" dir="ltr">The only reason I went to oil was that it seemed to be all the rage at that time. Having used both methods, I have to say that I like the air better. It was easy to set up and take apart without a lot of mess and best of all- no worries about a leak.</div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1491447637884_10021" dir="ltr"><br /></div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1491447637884_10020" dir="ltr">Greg C<br /><br /></div><div class="yahoo_quoted" id="yui_3_16_0_ym19_1_1491447637884_9557" style="display: block;"> <div id="yui_3_16_0_ym19_1_1491447637884_9556" style="font-family: verdana, helvetica, sans-serif; font-size: 13px;"> <div
id="yui_3_16_0_ym19_1_1491447637884_9555" style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 16px;"> <div id="yui_3_16_0_ym19_1_1491447637884_9554" dir="ltr"> <font id="yui_3_16_0_ym19_1_1491447637884_9553" face="Arial" size="2"> </font><hr size="1" id="yui_3_16_0_ym19_1_1491447637884_9552" /><font id="yui_3_16_0_ym19_1_1491447637884_9553" face="Arial" size="2"> <b><span style="font-weight: bold;">From:</span></b> Alan via Personal_Submersibles <<a href="mailto:personal_submersibles@psubs.org">personal_submersibles@psubs.org</a>><br /> <b><span style="font-weight: bold;">To:</span></b> Personal Submersibles General Discussion <<a href="mailto:personal_submersibles@psubs.org">personal_submersibles@psubs.org</a>> <br /> <b><span style="font-weight: bold;">Sent:</span></b> Wednesday, April 5, 2017 8:58 PM<br /> <b><span style="font-weight: bold;">Subject:</span></b> Re: [PSUBS-MAILIST] Oil Compensator<br /> </font>
</div> <div class="y_msg_container" id="yui_3_16_0_ym1"></div></div></div></div></div></div></blockquote></blockquote></div></body></html>