Understanding the Intermediate Pressure Gauge

[Leisure Pro Brett]

Comment:
I am assuming that you are referring about the Brut. I am not familiar with that first stage… actually the truth is that I am not that familiar with any of the Sherwood, I have only serviced a few. I don’t know the geometry of the Brut, but if they wanted to reuse most of the parts, one way to do it is to replace the Belleville spring with some solid spacers. Is that what they are doing? If that is the case they would require the same O-ring to isolate the HP chamber from the LP chamber, but in this case it would be a static O-ring (still the same type of gland seal, but it doesn’t move).


The balanced design does contain a dynamic O-ring with high differential pressure and it seems a lot more complicated than the Mk-5, but IMO it really isn’t that complicated. All the O-rings work in a very clean environment and if the tolerances on the O-ring gap are design properly it is possible to design against O-ring extrusion.

IMO what is far more critical is the controlled leak or dry vent as they call it. That is a very clever idea and when it works, it works great (and it seems like it works most of the time). But if it stops leaking the ambient pressure does not compensate at depth and diving below about 60ft it get to be like sucking molasses with a straw (I haven’t personally experienced it but I have witnessed it).

I have serviced just a few Sherwood and when I opened the environmental chamber they were all immaculate. The dry vent (I prefer to call it the controlled leak) does a great job at keeping any contaminants and corrosion out of the chamber. Therefore, all the O-rings are sliding on a clean surface.

Even if the ambient chamber flooded, the two O-rings exposed to water are the LP O-rings similar to the Mk-2. The HP dynamic O-ring is always in a clean internal environment (as opposed to the one in a Mk-5) and the pressure differential is a bit lower (HP to IP, versus HP to ambient), but truly this last point is very minor.

Luis,

You got it in reference to the Brut vs. the rest of the sherwood line (everything minus the sr1), the difference in first stages is basically just the orifice, number of ports and external finish. For the Brut orifice they just have a solid thicker head where the belleville washers would normally be in their balanced regs. You can actually swap out the orifice, add the bellevilles and now you have a balanced Brut.

In reference to the # of dynamic o-rings, true there are 3 off the top of my head, but they are at least in theory in a nice clean reg. The 2 on the main piston and the one on the orifice are in the sealed ambient chamber which should be nice and clean. The other side of the orifice is the reg inlet which (at least theoretically) should also be very clean and away from the elements. As for extrusion, the only one that would experience that is the ring on the orifice which has IP on one side and HP on the other. They have a backup washer to help prevent that and keep things moving easily and while I have seen my fair share of extruded o-rings in piston regs, none of them were in a sherwood.

The controlled leak is a cool system and while it adds it's fair share of complexity it seems to work without issue most of the time and when it's doing it's job it helps keep the rest of the reg clean and functioning normally. Of course if it stops working the reg's WOB will probably go way up, you can while underwater just pop that little black knob off and the reg should go back to normal breathing.

Too bad I haven't had the chance to work on some of these older regs mentioned here. Most of the time when I see them it's someone who dug it up in the garage and it has broken pieces that have most likely no hope of actually being replaced unless you find a sacrificial reg somewhere.