Cleaning Sherwood First Stage Laser Drilled Flow Control

[KeithG]

...

As for the consequence of low bleed rate, if you had a VERY rapid descent, with a bleed rate of 7 cc/min, and an IP chamber volume of 3 cc, then it might take 25 sec for IP to recover. In other words, if you instantaneously descended to 99 ft (4 atm = 59 psi), your delivered IP to your second stage might temporarily drop to 80 psi.
A G250 would barely notice the difference, and the same is probably true of an Oasis.
And it would recover in 25 sec, EXCEPT that there is a little risk of water entering via the rubber bleed plug as you suck IP down further with a breath.
...

Are you sure that is how it works?

It was explained to me that the bleed was from the IP chamber to the ambient chamber.

If the bleed is blocked, then as you descend ambient water WILL enter the ambient chamber since the small amount of air in the ambient chamber gets compressed to ambient pressure by water entering the chamber past the flimsy one-way valve. But the ambient chamber will always be at ambient pressure.

If the bleed is working then the ambient chamber is just very very very slightly above ambient pressure and the over pressure in the ambient chamber is released via the tiny trickle of bubbles from the one-way valve. This keeps the water from forcing its way backwards past the one-way valve and keeps the ambient chamber dry.

I do not understand how a faulty bleed valve can affect IP.

 

[grf88]

Are you sure that is how it works?

It was explained to me that the bleed was from the IP chamber to the ambient chamber.

If the bleed is blocked, then as you descend ambient water WILL enter the ambient chamber since the small amount of air in the ambient chamber gets compressed to ambient pressure by water entering the chamber past the flimsy one-way valve. But the ambient chamber will always be at ambient pressure.

If the bleed is working then the ambient chamber is just very very very slightly above ambient pressure and the over pressure in the ambient chamber is released via the tiny trickle of bubbles from the one-way valve. This keeps the water from forcing its way backwards past the one-way valve and keeps the ambient chamber dry.

I do not understand how a faulty bleed valve can affect IP.

Without the bleed the reg acts like a non-balanced reg.

 

[rsingler]

Here's the beauty of the Sherwood system:
Normally, ambient pressure at depth enters the holes of the ambient chamber (think: Mk 10/25) and pushes on the back side of the piston, opposite the IP chamber.
As pressure (depth) increases, it gets easier for the spring to push along with ambient pressure to open the valve. The valve finally seals again at the sum of spring pressure and ambient pressure. That way, the RELATIVE IP is always the set amount (~135psi) above ambient.

But the Sherwood is dry. There are no holes to allow sea water pressure to push against the back of the piston (as long as the bleed plug is working). Instead, as the reg sits there, a tiny amount of gas (16-30 cc or so, depending upon model) leaks through the piston head from the IP side into what looks like the ambient chamber on any other reg. But the piston head side of this is a blind chamber, serving only to add pressure on the head of the piston. The second stages do not feed from this chamber like they do on a Mk 10. Instead, this chamber serves only as the IP chamber, and air leaks from here into the standard ambient chamber and out the bleed plug. If sea water pressure is higher at depth, the bleed plug is held closed. The pressure in the blind chamber increases as HP air bleeds through at ~20 cc/min until the pressure in the blind chamber exceeds seawater pressure plus set IP and the piston is forced to close the valve. At that point the bleed plug feathered edges unseat and the reg bubbles again.

This happens continuously (hence folks telling a Sherwood diver that his "first stage is leaking"). However, when you dive, the bleed plug is pressed closed against the passage from the ambient chamber to the sea. So this 'true' ambient chamber stays dry.

If the bleed plug is not bleeding, it is most likely that the calibrated leak in the piston head is gunked up, and the reg briefly acts like a commercial "sealed" regulator. In other words, its relative IP drops as ambient pressure increases. Its IP is briefly "fixed". And @giffenk , you are abolutely correct. With a loss of bleed air into the ambient chamber, sea water will indeed push past the edges of the bleed plug into the ambient chamber. But whatever resistance to water ingress that the bleed plug provides until the ambient chamber reaches equilibrium will mean a lower relative IP. That was the source of my comment about a drop in IP during a rapid descent if you have minimal bleed air flow. But the bleed plug is sturdier than it looks. Leaks past the feathered edges are usually from corrosion disrupting the flat land where the rubber seals. I'd bet a new plug and land could withstand at least an atmosphere of excess pressure from the outside and stay dry for awhile. But eventually it would indeed equalize from sea water leaking into the ambient chamber, and IP would recover.

The second issue causing zero bleed could be a blockage of the passage from the ambient chamber to the bleed plug and the sea, but this is unlikely, since the passage is ~2mm in diameter. If that were to happen, the IP would probably creep, but I'd have to look at the schematics again to make sure I've got that right. I think this is correct because the ambient chamber is usually in the 14.7-90 psi zone (1-6 atm). Meanwhile, the IP is a minimum of 40 psi over that, so if the laser hole is working but the bleed is plugged, then the ambient chamber pressure increases, simulating deeper diving, and the IP should increase correspondingly, just from air bleed without a depth change, and that should look like creep.

Unfortunately, it will not just become an unbalanced reg if the bleed air stops. Being "unbalanced" is a different thing relating to changing tank pressure and IP, not changing ambient pressure and IP.

 

[KeithG]

Here's the beauty of the Sherwood system:
Normally, ambient pressure at depth enters the holes of the ambient chamber (think: Mk 10/25) and pushes on the back side of the piston, opposite the IP chamber.
As pressure (depth) increases, it gets easier for the spring to push along with ambient pressure to open the valve. The valve finally seals again at the sum of spring pressure and ambient pressure. That way, the RELATIVE IP is always the set amount (~135psi) above ambient.

But the Sherwood is dry. There are no holes to allow sea water pressure to push against the back of the piston (as long as the bleed plug is working). Instead, as the reg sits there, a tiny amount of gas (16-30 cc or so, depending upon model) leaks through the piston head from the IP side into what looks like the ambient chamber on any other reg. But the piston head side of this is a blind chamber, serving only to add pressure on the head of the piston. The second stages do not feed from this chamber like they do on a Mk 10. Instead, this chamber serves only as the IP chamber, and air leaks from here into the standard ambient chamber and out the bleed plug. If sea water pressure is higher at depth, the bleed plug is held closed. The pressure in the blind chamber increases as HP air bleeds through at ~20 cc/min until the pressure in the blind chamber exceeds seawater pressure plus set IP and the piston is forced to close the valve. At that point the bleed plug feathered edges unseat and the reg bubbles again.

This happens continuously (hence folks telling a Sherwood diver that his "first stage is leaking"). However, when you dive, the bleed plug is pressed closed against the passage from the ambient chamber to the sea. So this 'true' ambient chamber stays dry.

If the bleed plug is not bleeding, it is most likely that the calibrated leak in the piston head is gunked up, and the reg briefly acts like a commercial "sealed" regulator. In other words, its relative IP drops as ambient pressure increases. It's IP is briefly "fixed". And @giffenk , you are abolutely correct. With a loss of bleed air into the ambient chamber, sea water will indeed push past the edges of the bleed plug into the ambient chamber. But whatever resistance to water ingress that the bleed plug provides until the ambient chamber reaches equilibrium will mean a lower relative IP. That was the source of my comment about a drop in IP during a rapid descent if you have minimal bleed air flow. But the bleed plug is sturdier than it looks. Leaks past the feathered edges are usually from corrosion disrupting the flat land where the rubber seals. I'd bet a new plug and land could withstand at least an atmosphere of excess pressure from the outside and stay dry for awhile. But eventually it would indeed equalize from sea water leaking into the ambient chamber, and IP would recover.

The second issue causing zero bleed could be a blockage of the passage from the ambient chamber to the bleed plug and the sea, but this is unlikely, since the passage is ~2mm in diameter. If that were to happen, the IP would probably creep, but I'd have to look at the schematics again to make sure I've got that right. I think this is correct because the ambient chamber is usually in the 14.7-90 psi zone (1-6 atm). Meanwhile, the IP is a minimum of 40 psi over that, so if the laser hole is working but the bleed is plugged, then the ambient chamber pressure increases, simulating deeper diving, and the IP should increase correspondingly, just from air bleed without a depth change, and that should look like creep.

Unfortunately, it will not just become an unbalanced reg if the bleed air stops. Being "unbalanced" is a different thing relating to changing tank pressure and IP, not changing ambient pressure and IP.

Seems I am working off a slightly different assumption about the effectiveness of the one-way valve. I used the word flimsy on purpose.

I know the one-way valve is good to at least 3 feet since I dunk my unpressurized regs all the time and they do not get water ingress to the ambient chamber. I would be surprised if it was water tight much deeper than 15 feet with a blocked bleed. Ask any camera owner how deep you have to go to flood your housing...Water pressure is impressive.

Gut feel, no evidence other than we do know they leak at some depth. Hence my assumption that the ambient chamber is always very close to real ambient pressure.

 

[rsingler]

@giffenk , I think you're right. Perhaps the bigger question is, "How low a bleed rate is acceptable?"
Obviously they've spec'ed it for a reason at 13cc/min minimum. I'm guessing, that to avoid the potential for leaks into the ambient chamber, 13cc/min equates to a descent rate.

If we suppose that the ambient chamber is 5.2cc, and the bleed plug will handle only half an atmosphere before leaking, instead of my posited 1 atm, then if you descend from 0 to 33 feet in 24 seconds, the ambient pressure will rise 14.7 psi, and the "dry chamber" will add 5.2cc in the same amount of time. Since the pressure of the bleed air at such a slow rate should be about the same as ambient, then you've doubled your ambient pressure in those 24 seconds. I think the ambient chamber is smaller than this, so we've got some wiggle room. In any case, at this descent rate, there will be zero net stress on the bleed valve. Descent at twice this rate might add our 1/2 atm pressure against the bleed plug, temporarily.

Unfortunately, the same thing won't apply from 5 to 6 atmospheres of depth, as the bleed air is pressurizing an ambient chamber at a higher absolute pressure. Still, 13 cc/min seems to give a fair amount of leeway on descent rates. But 7cc/min bleed rate like our OP noted would mandate a very slow descent rate to avoid flooding the ambient chamber, if the little bleed plug doesn't seal well. If the plug works, then a lower bleed rate seems safe enough. At the worst, it just means that the reg needs disassembly and corrosion cleaning on a more frequent basis.

A fun discussion!

 

[KeithG]

@giffenk , I think you're right. Perhaps the bigger question is, "How low a bleed rate is acceptable?"
Obviously they've spec'ed it for a reason at 13cc/min minimum. I'm guessing, that to avoid the potential for leaks into the ambient chamber, 13cc/min equates to a descent rate.

If we suppose that the ambient chamber is 5.2cc, and the bleed plug will handle only half an atmosphere before leaking, instead of my posited 1 atm, then if you descend from 0 to 33 feet in 24 seconds, the ambient pressure will rise 14.7 psi, and the "dry chamber" will add 5.2cc in the same amount of time. Since the pressure of the bleed air at such a slow rate should be about the same as ambient, then you've doubled your ambient pressure in those 24 seconds. I think the ambient chamber is smaller than this, so we've got some wiggle room. In any case, at this descent rate, there will be zero net stress on the bleed valve. Descent at twice this rate might add our 1/2 atm pressure against the bleed plug, temporarily.

Unfortunately, the same thing won't apply from 5 to 6 atmospheres of depth, as the bleed air is pressurizing an ambient chamber at a higher absolute pressure. Still, 13 cc/min seems to give a fair amount of leeway on descent rates. But 7cc/min bleed rate like our OP noted would mandate a very slow descent rate to avoid flooding the ambient chamber, if the little bleed plug doesn't seal well. If the plug works, then a lower bleed rate seems safe enough. At the worst, it just means that the reg needs disassembly and corrosion cleaning on a more frequent basis.

A fun discussion!

Well I know from personal experience that a reg with a completely blocked bleed has no noticeable performance difference from a working one for low effort warm water dives down to around 90 feet.

Several years back during annual service my LDS decided to "pack" the ambient chamber with lube. And not surprisingly, the bleed stopped working as the gob of lube eventually fouled the piston. I randomly noticed mid week of a dive trip that I had no longer had any bubbles...The reg did not display any performance symptoms that I could detect.

 

[rsingler]

Well I know from personal experience that a reg with a completely blocked bleed has no noticeable performance difference from a working one for low effort warm water dives down to around 90 feet.

Several years back during annual service my LDS decided to "pack" the ambient chamber with lube.

Holy crow, @giffenk! I hope they bought you a new piston! That's awful. As you doubtless know, that's exactly the wrong thing to do with a dry bleed Sherwood. Minimal Christolube is the rule with them, just to protect that bleed filter/laser hole.

Well, that occurrence raises an interesting question: with the ambient chamber packed, what was the performance of the bleed plug in transmitting ambient pressure to the head of the piston? I would imagine that the ingress of sea water was significantly inhibited. And the plug is so thick at its center, that I don't imagine it flexed much to transmit ambient pressure to the goo. See this ancient thread for a long argument about ambient pressure transmission with packed regs: DIY SPEC Boot

So my guess would be that pressure below the plug lagged behind ambient as you descended, and maybe didn't recover at all if the goo sealed the edges of the bleed plug.

In that case, 90 ft = ~45 psi of ambient pressure lag, and that says that your balanced second stage handled an IP of 90 (135 - 45) just fine. No surprise there. Sherwood has excellent (if underappreciated) designs.

Cool story! Sad story - no wonder so many of us work on our own gear. In my case, it was the year my LDS told me I "had a hole in my diaphragm", which translates into their having poked a hole in my diaphragm while removing the rubber case on a D-350 reg. A rookie maneuver that I first learned about as a D-series problem from the great folks here on SB! I started taking care of my own gear after that, after some great training by Rudy Mola in FL.

 

[tmassey]

Let's see if I can bring this thread back to topic...

So, I bought a gallon of Klean Strip Phosphoric Prep and Etch. Sure, food-grade stuff would be great, but as has been pointed out, they're only $17 to replace and easy to get, so I'll chance whatever they put in the bottle... After all, I'm not eating the stuff!

10oz hot water (microwaved for 2 minutes) + 1oz Phosphoric Acid + small squirt of Dawn soap. (No antifreeze: didn't have time to stop by an auto parts store too, and I don't have any.)

I started with just the three with no light. 10 minutes of ultrasonic cleaning: one now has light! 10 more minutes of ultrasonic cleaning: all three have light! An improvement!

So now I'm going to do all 6 for 10 more minutes. At that point I'll test all six of them in a regulator (properly tightened but still no filters) and see what type of flow I get. But that will have to wait until tomorrow.

Progress! Thank you for your suggestions. Here's hoping for better results!

 

[tmassey]

OK, so this is an adventure! Make sure you have some coffee, it's a long one!

When we last spoke, I had 6 clean flow control screws. They first had 10 minutes in ultrasonic with ~3% phosphoric acid and a little Dawn (less than 1/4 tsp in 10oz). Light check. Those that were still clogged had another 10 minutes. When I was done, I could see light through all of them. Great! So time to test.

A few notes about testing. I tested the first one with the first stage end cap tightened properly. That gave me an IP of 125psi (these are Blizzards and tuned for icewater). I then re-tested with the end cap hand tight, which left it about 1/16" from fully screwed down. Turns out that this doesn't make a difference at all: same IP, same flow results. Good to know: it's a lot faster testing without using a big wrench...

Also, hitting the purge button on the reg for, say, a full second or so causes the bubbles to completely stop. It takes a good 5+ seconds for the bubbles to start again. Interesting: it seems that in normal operation that one-way valve will need to keep the water out for at least a little bit.

Back to the results. Of the six, one performed well below spec (7cc/min), three performed slightly below spec (17cc/min), one performed within spec (25cc/min) and one was actually past spec (35cc/min).

I decided to work on the three slightly below spec They were all within 16-18cc/min and would give me an opportunity to see how more cleaning affected them. I ran them through 10 minutes of ultrasonic in the phosphoric acid again. Pulled them out and rinsed them. Put then back into the reg: no bubbles on two, and very weak bubbles on the third (well below what it was before and didn't even bother to measure)!

What was the difference? Well, for one thing, I had quickied out on the rinse. So, I redid them, this time going through my full O2-style process (which I did last night). Another thing: I hadn't dried them. Last night they air-dried. Today, there might have been a droplet in them. I wouldn't normally think of that as a problem, but that is a *really* tiny hole to try to push air *and* water through... So I blew them out with tank air.

Somewhat better: one was at 17 cc/min, one at 20 c/min, and one was clogged, and I couldn't see light through it (which I could when the rinse was done). So maybe there's a reason Sherwood put a filter upstream of these?

For the two clogged ones, I whipped up a new batch of cleaning solution, this time 1:5 (or ~6% phosphoric acid) and less than 1/4 tsp Dawn to 10oz solution. Another 10 minutes, then full O2 rinse process, then quick blow dry. I can see light.

After testing: better results! One was 20cc/min and one was 22cc/min. Nice!

So I now have six unclogged flow elements! The process seems to have worked. But it's really picky. Seeing as I had six of them, it was well worth the effort. If I only had one? I'd be tempted to simply buy a replacement -- especially once I had bought a spare one on the shelf that I could use immediately and not have to wait on the replacement.

For the future, I'd go with: 1:5 Klean Strip to Hot water (10oz total), <1/4 tsp Dawn. 10 minutes ultrasonic. Look for light. Repeat if no light. Full O2-style rinse process. And it does work!

Hopefully I can simply keep these guys bubbling and won't have to go through this for a while!

 

[ams511]

Thank you for documenting your trials and tribulations trying to clean the part. It may be helpful to future DIYers.