DIY SPEC Boot

[rsingler]

First, halocline was absolutely right - I looked at the wrong invoice. Tribolube 66 IS cheaper - $176/lb vs $210/lb for the Tribolube 77. Now I'll feel a little better about trying it. Will probably try both silicone and Trib on two Mk10s.

Second, to answer couv, your description of the geometry of the SPEC boot was an ahah! moment for me. I'm now seeing the concept as a second chamber that transmits pressure via the ambient holes to the inside chamber. That flexing of the softer roof with essentially no holes means that all the water movement toward the chamber(s) takes place in the SPEC boot, OUTSIDE the regulstor. Ingenious! Gosh, I wish I had one.

I will have to rethink this whole thing. My idea just basically kept the mess down, while still allowing water into the environmental chamber. This is a whole 'nother concept. I'm really sorry it didn't catch on. Engineering-wise it is terrific.

Hope the group out there can help me think of a way to duplicate this. I'm still a fan of high flow simple pistons, and for the price of a little mess, this was a great concept. I didn't know enough back in the 80's to try and find one when I bought my first Mk10. Darn!

Actually, a softer version that's removable after a few dives may be even better: the water is outside, so you remove the boot, wipe it down, syringe a new second thin layer of goop into the groove, stretch the boot into place and wipe off the excess. Now a three-year annual begins to make sense.

But what am I missing? Why is the boot concave outside (depressurized)? The knife edge has come off the seat, so environmental chamber is SMALLER, right? And any bubbles in the goop are now back to normal size. All that should make the boot bulge.

---------- Post added March 6th, 2013 at 08:48 AM ----------

Never mind. I was so excited to see this info this morning that I replied on my phone lying in bed before I engaged my brain. When the piston comes off the seat, the environmental chamber obviously gets bigger, and the SPEC boot sucks in. Duh.

OK, now to figure out how to design a boot. I'm thinking two inner tubes, just slightly different diameters. One is cut in a ~1/2" strip to form the boot. The smaller one is cut into two tiny rings, and I'll try to glue one to each edge of the wide outer ring. That will form the shoulders for the space inside the boot.
But I don't think I can do it and have it stand up to much use.

What stretchy thing is shaped like the boot that we could use?

 

[Couv]

I would start with your original idea. Simply a rubber band made from a inner tube. If it's thin and flexible enough, it should deform enough to transmit ambient pressure to the grease through the chamber holes.

If you're just going to use inexpensive silicone grease, I would not bother with a boot at all.

BTW you can purchase Tribolube or Christolube in 2oz tubes rather than spending $175 on a lb can.

http://www.divegearexpress.com/hardware/orings.shtml#1614

http://www.divers-supply.com/Christo-Lube-2Oz-P259.aspx

 

[rsingler]

First of two posts --
I called a molding company and gave them a rough description of the SPEC boot:
$6000 for the mold
$12 apiece for the first 100, in silicone rubber.
That would be $72 each for the first 100.

Decided to try to do it myself:
[INLINE=left]http://www.scubaboard.com/forums/attachments/do-yourself-diy/150261d1363581992t-diy-spec-boot-mold1.jpg[/INLINE]Made a mold from two pieces of PVC pipe with slightly smaller diameters, and filled with silicone rubber catalyzed mix used to make molds for plaster casting.




Result? Silicone set poorly covered on all sides by plastic. Thought the catalyst would harden it without air, but apparently wrong. In any case, the ring that came out was very delicate, and disintegrated with mild handling.
[INLINE=left]http://www.scubaboard.com/forums/attachments/do-yourself-diy/150262d1363581993t-diy-spec-boot-mold2.jpg[/INLINE]
Decided I'd have to follow couv's recommendation and just try it with a bicycle tire.

 

[rsingler]

Second of two posts: the bicycle tire.

[INLINE=left]http://www.scubaboard.com/forums/asset.php?fid=142026&uid=421016&d=1363581993[/INLINE]First off, even with silicone, this is why not using a boot at all gets out of control. If you fill the environmental chamber completely, to prevent junk from accumulating and protect the innards, here is what happens when you pressurize the rig.




Although some of it retracts back into the body when you depressurize, that sort of ooze will occur regularly without a boot if you have more than just general lubrication amounts of grease in the environmental chamber.
While there are decades of experience with rinsing out the chamber, I still think that the SPEC boot was the perfect way to get more than a year between annuals, OR dive without concern in very silty water, OR eliminate the frozen piston in cold water. Engineering-wise, I think the SPEC boot was perfect. Just messy for the LDS techs. Well, that's not a problem for me.

I then took a bicycle tire with an ID about the size of the groove, thinking that it would fit tightly.
[INLINE=left]http://www.scubaboard.com/forums/asset.php?fid=142020&uid=421016&d=1363581988[/INLINE]

But with even a little grease on it, it would not span the gap, but instead just contracted completely into the groove. Working with the thin rubber became impossible.


[INLINE=left]http://www.scubaboard.com/forums/asset.php?fid=142023&uid=421016&d=1363581991[/INLINE]So I chose a second inner tube just slightly smaller than the OD of the entire regulator. Important to not accept a 1.75-2.3 universal inner tube, for it's still too small. The one I found was from a 29" balloon tire.
[INLINE=left]http://www.scubaboard.com/forums/asset.php?fid=142027&uid=421016&d=1363581995[/INLINE]


As you'll see from the nearby 1" stone disc, I abraded a groove for the tube to pass in the plastic bushing that goes between the yoke retainer bolt and the body.



[INLINE=right]http://www.scubaboard.com/forums/asset.php?fid=142022&uid=421016&d=1363581990[/INLINE]After sliding it into place, I was able to lift it gently with an aluminum stylet and fill the groove with Tribolube 66 using a small tube and syringe (the body had previously been completely filled with Tribolube, with no or at least very few small bubbles.
With the boot full, I secured one edge in place with a zip tie. The other side would be very hard to secure, due to the nearly HP ports and the yoke retaining bolt. I'm hoping the thick rubber and adjacent zip tie will be enough to keep it in place.

[INLINE=left]http://www.scubaboard.com/forums/asset.php?fid=142018&uid=421016&d=1363581986[/INLINE]If you'll compare the photo at left with the one from an earlier post of an actual SPEC boot, when reasonably filled and unpressurized, the bicycle boot is concave, just like the SPEC boot.

[INLINE=left]http://www.scubaboard.com/forums/asset.php?fid=142021&uid=421016&d=1363581989[/INLINE]When you add IP pressure and the piston drops onto the seat, that little bit of grease that pops out is contained within the boot:

[INLINE=left]http://www.scubaboard.com/forums/asset.php?fid=142019&uid=421016&d=1363581987[/INLINE]When you depressurize the rig, the grease is sucked back into the chamber, and the boot reverts to its original concave shape.






So far I've noticed that the boot can still be shifted with firm pressure, due to a thin layer of grease between it and the body. Therefore, this might be more susceptible to dislodgement if the tank is picked up by the yoke (not that we ever do that). If so, the mess might make this whole project a waste of time.

I'll go dive it and see how it stands up. Meanwhile, I'm still looking for original SPEC boots and hoping they haven't gone hard in all the years since they were made.

 

[halocline]

Why don't you cut the innertube to fit in the groove instead of trying to span it? All it needs to do is to contain excess grease from leaking out the ambient ports.

 

[rsingler]

Respectfully, I strongly disagree. Think of the SPEC boot and this DIY thing as a diaphragm above the piston. The grease (and bubbles) in the boot groove and environmental chamber must get compressed by the "diaphragm" surrounding them to whatever pressure exists at depth. As you saw in the first photo, 1/4" of grease pops out of a fully filled environmental chamber when it is pressurized. That grease will quickly ooze out of a tightly fitting boot.
Now, for the first dive, you may be right. Piston movement is much less during breathing than during initial seating upon pressurization, so the rubber inner tube can probably deform enough to transmit sea water pressure to the underlying grease.
But can the inner tube deform 1/4" into the chamber (the opposite of what you saw in photo #1)??? I think not.
That's important, because after the first dive, when the piston comes off the seat as the reg is depressurized, the grease is going to want to retract into the chamber the same amount as oozed out. There's doubtless going to be a tiny leak that will allow air under the boot during the hour/day/month that the reg sits unpressurized, so air will now be in the chamber with the grease. And remember that the original SPEC boot even had one pinhole in it.

What happens the next time the reg is pressurized? Some air bubbles out, some doesn't. At some point, you will have several bubbles of air in the grease (or may have already if you didn't do a complete chamber fill).
The question to then ask is, will a tightly fitting inner tube in the groove deform enough into the holes as sea water pressure increases to transmit all that pressure to the chamber? If the rubber seals firmly against the holes, as you'd expect it to under pressure, then the pressure in the chamber will be sea pressure MINUS whatever counterforce is provided by the elasticity of the rubber.

That's the beauty of the SPEC boot as designed. The area of the boot is much larger than the area of the holes, and the roof of the boot flexes easily. So whether there are a few air bubbles in there or not, the boot easily flexes 0.5cc or more of volume to move grease back in until all the air and bubbles are compressed to sea pressure. A tight boot doesn't provide a reservoir of grease, and can't deform enough to transmit the full amount of sea pressure to the environmental chamber.

To the extent that environmental chamber pressure is less than sea pressure, the IP transmitted to the 2nd stage will fall, maybe to the point of significantly increased WOB with a balanced 2nd stage, and maybe much worse with an unbalanced 2nd.

Of course, to the extent that the diaphragm leaks, then all this is moot, as sea water passes into the environmental chamber directly to pressurize the compartment, eliminating all of the benefit in the first place of having an environmental chamber that doesn't need rinsing and can't freeze.
Then you have all the mess that the LDS techs probably complained about in the first place when you rinse out a little sea water, a little silt, a little grease.

 

[halocline]

I don't think that the stock boots are as flexible as an inner tube. They do have ridges along the sides so they trap some grease outside the holes, but not much.

There's a lot of excess grease when you first pressurize, but on subsequent pressurizations there's hardly any. It's already been squeezed out. I don't think you'd have to worry about air getting in. A little water is definitely going to get in eventually with the stock boots, but there's so much grease in the chamber that it keeps things clean and freeze-resistant, as long as you get any air pockets out before you first pressurize.

Maybe you should try it both ways on two MK10s, go for a few dives, and then see how they look? I'd be curious.

 

[Six2Life]

Anyone know if the dimensions of a boot for a hog or atomic are close? I'd guess the hog boots would probably be easier to get.

 

[rsingler]

Well, as requested earlier, here's the DIY SPEC boot after three dives.
I tried to not wipe off the housing until three dives were done. It wasn't messy, as I was careful where I laid my gear afterward. You can see the ooze of a little grease from the unsecured side of the boot. I lifted the edge to inspect, and there was no water inside the boot (I didn't put a pinhole in the boot - just left one edge not ziptied).

Upon depressurization of the rig after the third dive, the boot was looking pretty concave, so just to make sure I had enough grease to fluctuate with future piston movement, I slipped my syringe and tube underneath the un-ziptied side and added 0.75cc of Tribolube. Now it's still slightly concave, but less than in the photo.

I'm still afraid to try the other suggestion of diving with a piece of inner tube flat and tight inside the slot, as opposed to suspended from the edges with a space inside. I still think a tight boot will cause a differential in IP as I descend. I'm trying to figure out a way to get 2400 psi inside my reg inside my GMC chamber to test my theory. I may pressurize the reg via the HP port. It won't allow enough flow to test breathing fluctuation, but it should show whether or not IP tracks upward with depth.
If not, we'll know the "boot in the groove" is unsafe, and will possibly result in insufficient IP with depth.

From a theory standpoint, though, it may not be a significant safety issue. Let's say the thick inner tube is so good a barrier, that it only allows 25% of sea water pressure to reach the ambient chamber.
Normally, with a nominal IP of 125 at the surface, the delivered IP will increase commensurate with the added sea water pressure. Since sea water is adding 43 PSI at 100 feet, the IP should be 125 + 43 or 168 PSI. If the inner tube blocks 75% of that, the second stage will only be seeing 136 PSI. Since that's still above sea water pressure (43 PSI) you should get air from your second stage, and a balanced poppet design shouldn't increase work of breathing much.
I figure it's probably like a second stage at an end-of-tank scenario. 136/168 is 81%, which seems like a work of breathing that you'd get when your 250 PSI empty tank near the surface is yielding only 100 PSI from your second stage. That's about the way it always used to be with a MK2 coupled to a downstream demand second stage with an empty tank.

But I'm still afraid to try it.
I'll post again if I can get 2400 PSI contained inside my regulator without blowing the lid off my 250 PSI GMC chamber.

 

[Couv]

Please be very careful is you decide to try to pressurize the reg while in a test chamber. You'll need a metal, rather than rubber/plastic, dust cap with a secured o-ring. If it develops a leak your chamber could quickly explode; so if it does not already have one, a pressure relief valve is a must. I would also fill it with water.

A safer alternative might be to sacrifice a cheap IP gauge by drilling a hole in the case and attach it to the regulator. Mount the regulator on a pony tank and take it on a dive. You can experiment with it easily if you have a small (6-13 cu ft) tank while you breathe off your back gas.