Upstream or downstream second stages

[CIBDiving]

I had a strange conversation the other day. I was talking to a guy in a LDS who was doing a rebuild on a reg. He said the reg I use is "Unsafe" because it's a downstream design. I didn't want to argue ( Now there's a first - I Must be getting old! ) , and I didn't ask what secound stage he recomends, But seeing as he's into that all Black cult of the koolaid, I'd sure guess it's an apeks

I'm curious, I've been inside and use the first stage but I've never taken an apeks second apart, isn't it a downstream design?? also if the linkage from the diaphram to the valve breaks what happens?

 

[Wayward Son]

Unless he's using the terms differently than I do, I was taught that the upstream design was dangerous, as an over-pressure failure of the 1st stage could force the valve closed. With a downstream design such a failure results in a free flow so you can breath at least until the gas is gone.

I was taught this in the early 1970's and still have the book we used as a manual then which discusses the problem. "The New Science of Skin & Scuba Diving", 3rd edition, reprinted 1970.

 

[CIBDiving]

I was taught this in the early 1970's and still have the book we used as a manual then which discusses the problem. "The New Science of Skin & Scuba Diving", 3rd edition, reprinted 1970.

That's exactly what I mean!
This guy pointed out that "if the linkage between the diapham and the valve broke the reg will not give air". As far as I can recall that is true for All second stages, if the diapham fails the stage quits - it sure doesn't freeflow! Does anybody know of a design that the diapham relaxing forces the valve closed??

 

[Wayward Son]

Upstream/downstream isn't about the diaphram, it's about the valve.

Upstream - the valve seats on the upstream side of the valve seat (the die towards the 1st stage, away from the 2nd). The diaphram movement causes it to tilt which allows air to flow past. A 1stage failure causing excess pressure can make it too difficult to tilt & thus cut off your air, even when you have a bunch left.

Downstream - the valve seats on the side of the valve seat towards the 2nd, way from the first. An over pressure condition from the 1st stage pushes it away from the seat & you can breath until the tank runs dry.

If the linkage breaks you may well be screwed either way, but the linkage was never part of the definition of the terms as I was taught them.

 

[CIBDiving]

Upstream/downstream isn't about the diaphram, it's about the valve.

I know the difference,or at least I thought I did untill yesterday,.

Upstream - the valve seats on the upstream side of the valve seat (the die towards the 1st stage, away from the 2nd). The diaphram movement causes it to tilt which allows air to flow past. A 1stage failure causing excess pressure can make it too difficult to tilt & thus cut off your air, even when you have a bunch left.

Or in the case of the posiedon Odin causes a ballon to inflate/deflate.

Downstream - the valve seats on the side of the valve seat towards the 2nd, way from the first. An over pressure condition from the 1st stage pushes it away from the seat & you can breath until the tank runs dry.

The unbalanced second - closed by a spring that pushes the valve closed with a given amount of force overcoming the LP pressure. The diapham linkage overcomes this spring and "pulls" the valve open.

I can see a way of designing a valve that does not need a spring to overcome the LP pressure ( a balanced design ), only enough force to close the valve against friction.

If the friction can be reduced enough a spring should not be nessesary either - the diapham relaxing would push the valve closed.

If the linkage breaks you may well be screwed either way, but the linkage was never part of the definition of the terms as I was taught them.

This is the crux of the question!
He may have only mentioned downstream and really meant 'unbalanced second stage', which mine is - a 50 year old design that is effectivly unchanged and still in production today.

I don't know if a Balanced downstream second stage will fail in free flow if the linkage breaks.

 

[Wayward Son]

Good question, I don't either. The state of the art has changed & I haven't kept up with it. Prolly wouldn't hurt me to read up on current designs.

 

[CIBDiving]

Ok, I think I see what he was getting at. My reg in Not adjustable, the upper end Apeks regs have a knob to control the cracking effort. If this is like an old ScubaPro 109 second stage, it backs the closing spring off. If you back it off enough, the reg will freeflow. It does this independant of the diapham linkage. That's the only thing that makes sence.

 

[Azza]

Balanced Diapraghm-Downstream
The operation of the balanced valve is essentially the same as the balanced piston valve, except that a flexible diaphragm is used to push the air valve open, instead of a metal piston. This valve was developed to eliminate the effects of the changes in the high pressure cylinder air on the first stage. Since the valve stem extends through the high pressure chamber into the intermediate air chamber that flows through the hose, the high pressure air from the cylinder cannot exert a closing force on the valve stem. The air pressure in the intermediate chamber acts to help balance the forces acting on the valve stem.

Unbalanced Diapraghm-Upstream
This is also called an upstream first stage, since the valve seat is located on the upstream or high pressure side of the valve. As the diver inhales, a diaphragm flexes inward, pushing on a rod which causes the valve to open. Air flow is thus provided to the diver. The valve closes when the diaphragm is returned to its relaxed position as the pressure builds up in the intermediate chamber and reaches a predetermined level.
A major disadvantage of this type of valves (as in all unbalanced first stage valves) is that the high pressure air acts directly on the valve, affecting the opening and closing of the valve.
To compensate for this effect, the valve opening is often made smaller but this in turn results in a reduced flow of air. This type of valve may be found in some older type regulators but has largely been phased out

I hope this helps a little

 

[CIBDiving]

Uh, yeah, thanks! But We were talking about second stages.