Sherwood dry bleed contaminated piston

[underH2Oman]

If you decide to get new piston make sure you get the correct one. Sherwood changed the regulator design in the early 2000's and the depth of the groove that the spring sits in was increased. You will not be able to get the IP high enough with that piston. The piston you need was hard to find for a time as Sherwood ran out of stock. They did produce more so you should be able to get the piston you are needing.

 

[fmerkel]

Yes, I noticed that the design had changed a bit.

 

[chillyinCanada]

You folk need our Scubaboard member rsingler in here! He's da man on Sherwood regs! You might have to send him a pm though. He doesn't read or contribute as often as he used to do. He says he doesn't mind so pm him up!

 

[axxel57]

Whow, what a discussion.

If I get it right, then you guys overlook one important principle of that design.

The sinter filter in the piston is not only bleeding air into the main chamber and out of the one way valve into the water to prevent water entering into the main chamber.

More important is its function for the adaption to depth which usually the ambient water is doing.
So the amount of bubbles getting through the filter plays an important role.

One of the points of that design I don't like is that even with a perfect working filter the air stream cannot follow up the demand when you are descending fast, means the breathing resistance is increasing a while until the filter has let through enough air to compensate for the increasing ambient water pressure.

Of course you guys could also dive with a complete clogged filter if you don't mind breathing harder the deeper you go and having probably the main spring chamber flooded each time.

Cleaning the bottom of a boat for sure is no problem, but if you want to dive deep I would reconsider.

 

[BRT]

Whow, what a discussion.

If I get it right, then you guys don't really understand the principle of that design.

The sinter filter in the piston is not only bleeding air into the main chamber and out of the one way valve into the water to prevent water entering into the main chamber.

More important is its function for the adaption to depth which usually the ambient water is doing.
So the amount of bubbles getting through the filter plays an important role.

One of the points of that design I don't like is that even with a perfect working filter the air stream cannot follow up the demand when you are descending fast, means the breathing resistance is increasing a while until the filter has let through enough air to compensate for the increasing ambient water pressure.

Of course you guys could also dive with a complete clogged filter if you don't mind breathing harder the deeper you go and having probably the main spring chamber flooded each time.

Cleaning the bottom of a boat for sure is no problem, but if you want to dive deep I would reconsider.

They seem to work fine descending.

 

[Bob DBF]

One of the points of that design I don't like is that even with a perfect working filter the air stream cannot follow up the demand when you are descending fast, means the breathing resistance is increasing a while until the filter has let through enough air to compensate for the increasing ambient water pressure.

Of course you guys could also dive with a complete clogged filter if you don't mind breathing harder the deeper you go and having probably the main spring chamber flooded each time.

These must be more like a theoretical problems as my experience seems to differ considerably. On numerous fast descents to the 100 to 130' range I have not noticed any change in regulator breathing resistance in my present reg or in the one I was diving from '80 something 'till after the turn of the century. If I had I would have had the reg checked or changed manufacturers. I had to dive the older Sherwood for a while when the filter went out and found no difference in performance, only the lack of bubbles. For it's reliable and faithful service, even when broken, I now use it on my pony bottle.

From my recollection, in a closed hydraulic system, which is what a regulator is working in, a pressure change is felt instantaneously throughout the system. Due to friction and design features, say dry bleed, the response will be slightly less than instantaneous, but measured in milliseconds rather than seconds. I don't know about anyone else's breathing, but even in the range of a second or so, and I don't it would be that long, it would never be noticed.


Bob
-------------------------------------------
Loves Sherwood regs

 

[rsingler]

Hi guys! Not much to add after these last three posts. Axxel had it mostly right. The bleed function is critical to regulator compensation for depth. Internal reg volume change with depth and with breathing is much less than 1cc, so response to VERY rapid descent should be within two seconds (not quite milliseconds, but close). For any normal diving, not even noticeable.
Getting even minimal bleed valve flow with cleaning/grinding/ultrasound makes the reg perfectly safe for boat bottom cleaning. No depth compensation really needed; just enuf air to keep reg dry inside.
But posts commenting on need for new piston had it right.
If you send me the model #, I may have an old one in the shop. Sorry of its already been mentioned, but I didn't read every post.

Cheers!
Rob

 

[axxel57]

I'm sure that normal descending doesn't cause any increase in breathing resistance, otherwise these regulators wouldn't be on the market anymore for long time.

But I'm also sure that in a 'bungee jump' from surface to let's say 130 feet one could feel the delay in compensation if, for example, in stress.

If Rob is right with his 2 seconds for also this depth range, then surely it would not matter, but I have my doubts.

And it should be clear that a clogged filter causes a 'reduction' of IP of 15psi each 33' feet increasing depth and therefore has to deteriorate its performance with increasing depth ( means the IP stays at let's say 140psi instead of increasing by 15psi each 33' feet depth, and so cannot maintain constant lung volume).

 

[rsingler]

Here's the math:
Spec bleed is ?26 cc/min? (sorry, working from memory), or .43 cc/sec. Less than one cc volume change therefore requires ~2 sec.
As for the relative IP drop with depth with a clogged bleed filter, keep in mind that a good quality balanced second will stay easy to breath well below 80psi. At 15psi per atm, we could almost hit axxel's 140 feet with 80 psi left in relative IP if you tune to 140 psi for your static IP.
In other words, with a balanced second, you can probably dive once with a clogged valve and not notice it. At least until the water forced its way through the rubber one way valve at 5atm, the reg filled with water, and you died.
Yeah, probably better to keep a little bleed going.

Rob

 

[fmerkel]

Thank you for jumping in Rob. If you look at the first post it describes my predicament > saltwater intrusion into the ambient chamber, probably been sitting that way a long time. Threw the piston into the US bath with everything else. Bleed not working. I've tried a whole lot of different things to 'clean' it. Nothing works, at least so far.

What if I drilled a tiny hole through the plug? I have some very small drill bits, though I'm not at all sure how I would chuck them. Too much flow?

If that's an absurd notion the piston is from an Oasis, around 1995. Can't find any real ID marks. The piston has the deep groove for the spring on the underside of the piston head. Earlier (other?) models seem to be flatter on the underside.