Why Piston vs. Diaphragm???

[SDAnderson]

Sherwood 1st stage and the dry air bleed systerm that by design allows it adjust to ambient pressure but keep all the water gunk out. It's a piston. Wow what a great design. Then it's got the cheapest parts as far as maintence $8 for the whole kit. It comes in unbalanced (brut model) and balanced magnum through-maximums models.

By Contrast the most others require a parts kit around $30.

Of course, as soon as the bleeder screw gets wet the system is AFU. The bleeder system works very well until it doesn't, then water is definitely (no ifs, ands or buts) going to get inside the first stage. That means a new bleeder screw (~$25) at a minimum, if you don't catch the problem quickly you'll need a new orifice, maybe new washers, wait a little longer and the spring will bite it, then the piston and the cap...sorry, bad attitude on my part but I spent nearly two hours today trying to drag an SRB5600 back from the dead for exactly this reason. Damn thing blew saltwater all the way down the hoses to the second stages (new hose, new orifice, two new springs, new poppet) and fried a computer transmitter.

 

[keyshunter]

Of course, as soon as the bleeder screw gets wet the system is AFU. The bleeder system works very well until it doesn't, then water is definitely (no ifs, ands or buts) going to get inside the first stage. That means a new bleeder screw (~$25) at a minimum, if you don't catch the problem quickly you'll need a new orifice, maybe new washers, wait a little longer and the spring will bite it, then the piston and the cap...sorry, bad attitude on my part but I spent nearly two hours today trying to drag an SRB5600 back from the dead for exactly this reason. Damn thing blew saltwater all the way down the hoses to the second stages (new hose, new orifice, two new springs, new poppet) and fried a computer transmitter.

Would you mind saying just how all of that water got to the bleeder screw, which ordinarily is dry at both ends? And how this is the fault of the regulator?

 

[cerich]

Would you mind saying just how all of that water got to the bleeder screw, which ordinarily is dry at both ends? And how this is the fault of the regulator?

dust cap left off. user fault

 

[Scott L]

dust cap left off. user fault

Maybe they should call it a water cap instead.

 

[shogan]

So that I can learn something what keeps water from doing the same thing in all the other regs that allow water to come into the 1st stage so as to adjust for ambient pressure. Somethings not making sense.

 

[cerich]

So that I can learn something what keeps water from doing the same thing in all the other regs that allow water to come into the 1st stage so as to adjust for ambient pressure. Somethings not making sense.

the water will still get in other regs but they don't have the part Sherwood uses. Most other regs will just blow thru the water when put on a tank. The Sherwood has very fine tolerances to work properly, a little water can cause it to corrode and stop working correctly.

 

[rcs9250]

I have used my share of regs in a lot of different conditions and tank pressures and have found all to be acceptable. Shallow and deep. The exception is some octos that are just crap right off the drawing board. Aqualung recently redesigned there compact octo which is now called the ABS. It's predecessor as they say was like breathing through a straw. Most looked figured that if an out of air diver was using it their adrenalin would allow them to suck a golf ball through a garden hose.
The thing that I see as a big negative in some low end regs is the effort required to operate the purge button. Some of them just plain suck. In an effort to keep manufacturing costs low the make it a one piece composite that is supposed to be soft for the purge function. Often times not the case. Or maybe after time our ozone buddy stiffens things up. Most middle to high end regs have the additional parts that make purging very easy.
I will say that the Aqualung legend for one is the one piece design but works very well. I do have some regs in my teaching regs box that are quite stiff.

 

[SDAnderson]

Would you mind saying just how all of that water got to the bleeder screw, which ordinarily is dry at both ends? And how this is the fault of the regulator?

So that I can learn something what keeps water from doing the same thing in all the other regs that allow water to come into the 1st stage so as to adjust for ambient pressure. Somethings not making sense.

Hope this helps, much of it has already been covered in this thread (and a million others) but for those that just tuned in, here's an attempt to explain. Sorry if it's too simplistic...

Most regulators have open ambient chambers - changes in pressure are compensated for by allowing water into the ambient chamber. Works great until you dive in cold water, when the air whooshing through the regulator tends to freeze the water, causing a free-flow and all manner of unwanted excitement.

Sealing the ambient chamber keeps the water out but you still have to let the pressure in - and that is tricky. The ambient chamber in a diaphragm regulator can be sealed by simply adding a second diaphragm over the ambient chamber and a transfer pin to carry the load from the second to the primary diaphragm. Piston regulators aren't so easy, however. Why isn't as important as understanding that there are a couple of methods in use: pack the chamber full of something soft that won't freeze or allowing a controlled flow of air to purge the chamber.

Sherwood uses a controlled flow of air, what they call the Dry Bleed System. In this system, the pressure inside the ambient chamber is regulated by the force that the water exerts but they keep water from actually penetrating the chamber by creating just enough of a positive pressure to allow some of the air to dribble out. It's a razor-thin balancing act that is controlled by the bleeder screw which has a tiny hole in it, a tiny filter and the one-way bleed valve. If water does get inside a Sherwood ambient chamber, bad things can happen quickly. It doesn't take more than a dot of water to trash the whole system: plug that bleeder screw or filter enough to stop the outward air flow and the water pressure will push it's way backwards through the port, through the filter and screw, and into the ambient chamber.

So, how does water get at that bleeder screw? Probably the most common way is a dunking in the rinse tank with the dust/water cap off, which allows water in through the inlet. The second most common way is by taking an unpressurized regulator underwater which allows water in through the one-way bleed valve, which isn't so one-way if it isn't pressurized. This can happen in a dunk tank (even in a foot or two of water) or if the regulator is on a stage or pony bottle.

Anybody that would like to learn more can pick up a copy of Scuba Regulator Savvy by Peter Wolfinger or Scuba Regulator Maintenance and Repair by Vance Harlow.

 

[keyshunter]

dust cap left off. user fault

Actually Chris,
I suspected as much. I would not have asked the questions, except that I got the impression that the poster felt that this was somehow the fault of the reg.

Interestingly, the knock on the older Sherwood firsts was that when the bleed orifice (which was located in the piston) was blocked, the piston needed to be replaced. When Sherwood redesigned the first so that the bleed orifice was in a separate screw, there is occasionally the case where someone gets water inside and does nothing to clean it out, causing the screw to bite fast, thus making it very difficult, if not impossible, to remove. If it can't be removed, the body is toast. I suppose that's progress. FWIW I still prefer the newer design with the replaceable filter.

 

[cerich]

Actually Chris,
I suspected as much. I would not have asked the questions, except that I got the impression that the poster felt that this was somehow the fault of the reg.

Interestingly, the knock on the older Sherwood firsts was that when the bleed orifice (which was located in the piston) was blocked, the piston needed to be replaced. When Sherwood redesigned the first so that the bleed orifice was in a separate screw, there is occasionally the case where someone gets water inside and does nothing to clean it out, causing the screw to bite fast, thus making it very difficult, if not impossible, to remove. If it can't be removed, the body is toast. I suppose that's progress. FWIW I still prefer the newer design with the replaceable filter.

i found the orifice on the piston to be in a place where a small amount of water was less likely to bugger up the orifice compared to the new design. I like the design, so much that at one time back in the 90's I had a personal fleet of them! In Canada where I was diving and teaching they were bullet proof even when ice diving with -40 air temps. As I started to do deeper and deeper stuff on air (this was the early 90's folks, I do NOT recommend you try that, we know better now) I found they weren't quite up to what I was doing but they remained my recreational regs until 1997 or so.

I have seen that Sherwood is introducing a whole new line of regs, I'm excited to see them! The truth is the new millennium left the Sherwood designs at a disadvantage compared to other brands performance wise, and it will be great if Sherwood gets back in the game!