conshelf XI and XXI question ?

[idive2]

I'm setting up a pony and have a couple older second stages
I can use. I tried the conshelf XI once when diving and found
that it breathes hard but seems like a pretty rugged regulator.
The XXI is a plastic housing and breathes a bit easier. Were the
conshelf XXI second stages pretty decent ? Since the XI does
fall into the vintage category I was considering just having the
XXI serviced and just hanging on to the XI for who knows maybe
something vintage someday.

 

[herman]

How well a second stage breaths is largely dependent on how it was set up. Pretty much any second stage can be tuned to breath very badly and conversely most any second stage can be tuned to do a good job, granted some are better than others but it's unlikely you could tell the difference in a XXI and a XI second if both were tuned the same. If you pop the covers off both regs you will find internally they are pretty much the same, just different material. The one issue I have had with a plastic one is I dropped the second stage from tank valve level onto a solid surface and broke loose an internal part, distroying the second stage. I have heard of others having the same issue. This would not have happened to a metal version.

 

[captain]

Mechanically and operationally they the same other than material of construction. As Herman said it's all about set up and possibly diaphragm material, silicone vs neoprene.

 

[Paladin]

I like the Conshelf XI. I have two of them and I think they breathe rather easily. I don't like the plastic second stages of the newer regs. All my single hose regs have all metal seconds.

 

[John C. Ratliff]

The Conshelf XXI (it's called the Conshelf 21 [part #1043-00] in the 1993 catalog) is a pretty good regulator from what I've heard, though I have not used it. The advantage of plastic is that it does not corrode. It also has a large exhaust (see below), but the openings in the front cover may make it free-flow a bit in current.

The Conshelf XI is an earlier generation, and can be made to breath very easily for inhalation as it does have a balanced first stage. But the second stage has the earlier, smaller exhaust. This is what kept it from being accepted by the US Navy. It wasn't until the AMF Voit MR12 came out that the exhaust became large enough to pass the US Navy requirements. Here's a graph of the later Conshelf XII, which had the modified exhaust and did meet the US Navy specs:

These specs were the reason that the single hose regulators did not meet the US Navy standards in the mid- to late-1960s. However, we in the US Air Force used them as our mission equipment.

If you'll look at the two regulators that you are comparing, the XXI has a larger exhaust than does the XI. The Conshelf XI was out in the 1970 catelog, and I have one with the smaller exhaust.

I actually like the original Calypso, with the exhaust in the diaphragm (same size) over the Conshelf/Calypso second generation, as it breathed easier and drier (no turn in the exhaust flow). If you have a Conshelf XI or the early Calypso second stages, you can improve the exhalation performance by switching the exhaust mushroom valves for a silicone one. While silicone was not made for these regulator valves, I've found that the silicone mushroom valve on an older Scubapro Shotgun snorkel are the right size, and work well. I use one in my original Calypso regulator, but now have a Shotgun snorkel without an exhalation valve.

The third generation Calypso and Conshelf, in the 1973 catalog, had a notation "NAVY APPROVED." This generation of the Conshelf is still in use by the US Navy.

As these regulators from AMF Voit (the MR12) and US Divers Co. came out, the double hose regulators began to be phased out.

SeaRat

 

[idive2]

Thanks to all for the info and comments. Love those old catalog pictures
on the regs. Yes, I can see where the cover on the conshelf 21 could invite
a free flow in current, the diaphragm is in clear view from the cover. In calm
waters though sounds like a decent performing reg. In current and for durabilty
seems the XI may fair better.