Question to Sherwood Brut Specialists

[BRT]

It's pretty straightforward to work the numbers.
First, when an uninformed tech gets lube in the piston head filter, flow usually doesn't completely stop, but just slows down. When flow drops from the specified 26ml/min down to say, 6 ml/min (as I've seen many times in eBay regs), the depth compensation doesn't stop. It just slows down.

So let's make some assumptions...
If the volume of the ambient chamber is 5ml, it will take 5/26 minutes, or 12 seconds to double in pressure during descent. That means you'd need to descend from the surface to 33 feet in 12 sec (>150 ft/min) to overcome the ability of the old Sherwoods to keep up.
So if your clogged filter was down to 6 ml/min, descending at 25 ft/min would still be okay.
If you descended at 60 ft/min, your relative IP would begin to lag.

In the worst case scenario, a completely clogged filter with a perfect bleed valve would lose 15psi of relative pressure every 33 feet.
In other words, at 132 feet, the ambient chamber would still be at surface pressure (15 psi), while ambient would be 75 psi.
Thus, your operating IP would be 135-60psi, or 75 psi.

Most balanced second stages will barely register an increase in cracking effort at 100 psi. An old center balanced second like the D400, or my new TFX, won't even notice it. At 75psi IP, cracking effort begins to rise for barrel regs, but not for center balanced valves.

In fact my new TFX, initially tuned to 0.9" cracking effort, will still crack at only 1.4" at 20 psi IP!!!

So with a non-functioning filter, I'd barely notice a problem at max recreational depth.

Of course, there's more to the issue. It's not just cracking effort, but dynamic flow. At 75 psi static IP, there might be a bigger drop in delivered IP during a breath.

But the point is, you're not gonna die due to your tech's excessive lube clogging the filter, even at max recreational depth. And since the bleed valve is never perfect, the reg will flood and the ambient chamber will not stay at surface pressure, but will instead lag just a bit behind actual ambient during descent. So your cracking effort and dynamic IP will be even better.

Yes, you'll have to service your reg for flooding after the dive, but absent unusual circumstances, you'll make it back to the boat alive.

It all depends upon the second stage. In the worst case - an unbalanced second, everything is IP dependent. Here there might be a problem. I measured my old Cyklons in this regard, and they're illustrative.
The Cyklon's cracking effort will rise about 0.2" WC for every 10 psi IP drop. With a Cyklon tuned to 1.0" cracking effort at 160psi IP, cracking effort rises to 2.5" WC by the time IP drops to 100 psi. Attached to a Sherwood with a clogged filter that could be problematic. But balanced seconds are not similarly troubled.

This is how it actually is, speaking from experience.

 

[johndiver999]

It's pretty straightforward to work the numbers.
First, when an uninformed tech gets lube in the piston head filter, flow usually doesn't completely stop, but just slows down. When flow drops from the specified 26ml/min down to say, 6 ml/min (as I've seen many times in eBay regs), the depth compensation doesn't stop. It just slows down.

So let's make some assumptions...
If the volume of the ambient chamber is 5ml, it will take 5/26 minutes, or 12 seconds to double in pressure during descent. That means you'd need to descend from the surface to 33 feet in 12 sec (>150 ft/min) to overcome the ability of the old Sherwoods to keep up.
So if your clogged filter was down to 6 ml/min, descending at 25 ft/min would still be okay.
If you descended at 60 ft/min, your relative IP would begin to lag.

In the worst case scenario, a completely clogged filter with a perfect bleed valve would lose 15psi of relative pressure every 33 feet.
In other words, at 132 feet, the ambient chamber would still be at surface pressure (15 psi), while ambient would be 75 psi.
Thus, your operating IP would be 135-60psi, or 75 psi.

Most balanced second stages will barely register an increase in cracking effort at 100 psi. An old center balanced second like the D400, or my new TFX, won't even notice it. At 75psi IP, cracking effort begins to rise for barrel regs, but not for center balanced valves.

In fact my new TFX, initially tuned to 0.9" cracking effort, will still crack at only 1.4" at 20 psi IP!!!

So with a non-functioning filter, I'd barely notice a problem at max recreational depth.

Of course, there's more to the issue. It's not just cracking effort, but dynamic flow. At 75 psi static IP, there might be a bigger drop in delivered IP during a breath.

But the point is, you're not gonna die due to your tech's excessive lube clogging the filter, even at max recreational depth. And since the bleed valve is never perfect, the reg will flood and the ambient chamber will not stay at surface pressure, but will instead lag just a bit behind actual ambient during descent. So your cracking effort and dynamic IP will be even better.

Yes, you'll have to service your reg for flooding after the dive, but absent unusual circumstances, you'll make it back to the boat alive.

It all depends upon the second stage. In the worst case - an unbalanced second, everything is IP dependent. Here there might be a problem. I measured my old Cyklons in this regard, and they're illustrative.
The Cyklon's cracking effort will rise about 0.2" WC for every 10 psi IP drop. With a Cyklon tuned to 1.0" cracking effort at 160psi IP, cracking effort rises to 2.5" WC by the time IP drops to 100 psi. attached to a Sherwood with a clogged filter that could be problematic. But balanced seconds are not similarly troubled.

why does the first stage flood if it stays above ambient? see bolded section

 

[rsingler]

why does the first stage flood if it stays above ambient? see bolded section

It doesn't! That's the beauty of the Sherwood design. They have been among the most inventive of all the manufacturers, and have been making regulators longer than Scubapro.

When the bleed filter inside the piston head is working properly, 26 ml/min of IP is leaking into the ambient chamber all the time. It keeps the chamber dry. And unless you descend at faster than 150 ft/min, the ambient chamber is never less than actual ambient. That's why uninformed buddies were always telling Sherwood divers that "your regulator is leaking!" Well, yes it was, but it was supposed to, and the leak is insignificant in gas loss over a 90 min dive. The excess gas bled out past the one-way rubber bleed valve as a tiny stream of bubbles, because ambient chamber pressure was always equal to ambient due to the calibrated leak in the piston head. As the diver descended, the edges of the bleed valve would be pressed against their land by increasing ambient pressure (compared with the chamber) until the chamber caught up (which was near-instantaneously).

So the little top hat bleed valve on the outside was never very stressed, and the edges of the valve did fine at keeping out seawater.

But when the bleed filter gets clogged by excess grease at service, sometimes the bleed air doesn't keep up with increasing ambient at depth. If the difference between ambient chamber pressure and the actual ambient pressure gets large enough, a few drops of seawater will leak past the edges of the rubber top hat and begin to corrode the reg (as well as equalizing the pressure and restoring reg function for that dive).

 

[Bob DBF]

It doesn't! That's the beauty of the Sherwood design. They have been among the most inventive of all the manufacturers, and have been making regulators longer than Scubapro.

For those that haven't heard about Sherwood. From 1930 was one of the formost US companies in design and manufacture of high pressure gas valves, regulators and associated equipment. They are still in business.

In the 1950s, as the fledgling scuba industry developed, they turned to Sherwood to manufacture parts, and even entire regulators for companies that started the scuba industry in the US. This includes valves and manifolds with other companies logos stamped on them.

In 1958, engineers from Sherwood Manufacturing modified their piston regulator for underwater. Although there was a patent, Sherwood did not keep it proprietary, as the more companies used it, the more manufacturing business they received. Several other manufacturers adopted the piston design over the other widely used diaphragm design, and now it is the predominant type of scuba regulator available.

I believe they started their scuba company in te early '70's. They were the first, and probably only company to make a 4000# service pressure yoke regulator in 1980 or so. The dry bleed system to environmentally seal the regulator was designed in the late 1970s.