A little advice on 2nd stages please

[halocline]

As far as I can remember they don't have venturi flaps, any kind of manual adjustment and the poppets are not balanced. I also think that the springs must have been a little stiffer because the diaphragms were larger, which I assume is to compensate for the stiffer spring and keep work-of-breathing as low as possible. Moreover I've been down there with such regulators so I have some personal experience to draw upon. They're not *awful* at depth. Don't get me wrong, but they're *relatively* stiffer than modern regs in my experience.

R..

The things that you're describing, venturi assist and cracking effort, don't have anything to do with depth, except for the fact that I believe the venturi effect might increase at depth because more air is being sent through the regulator, and it's the airflow in the 2nd stage that creates the venturi effect. Just because there's no adjustment doesn't mean there's no venturi assist. So there's really no reason that this 2nd stage (or any, really) would have increased breathing effort at depth, unless the reg simply could not flow the volume of air necessary, and that would much more likely be a 1st stage issue.

The spring tension would not be stiffer due to a larger or stiffer diaphragm. The spring holds the poppet closed, so it's tension is based on the amount of force (IP times seat size, basically) trying to push it open. It really has nothing to do with the diaphragm at all. If in fact the diaphragm were significantly less flexible, there would be increased cracking effort, but that would be at least as apparent on land where the pressure on the diaphragm is close to nil.

I'm not disputing the fact that you have experienced what you perceived as increased breathing effort at depth with your regulator, I'm just saying that there is nothing in the design of this 2nd stage that would cause actual performance issues as depth increases. I'm sure you realize that 'balanced' has nothing to do with depth compensation, but refers solely to how the regulator handles changes in supply pressure; for a 2nd stage, that means changes in IP over ambient.

 

[axxel57]

As far as I understand it, a 2nd stage without Venturi Assist will breath harder the deeper you go. The reason is that with the increasing demand of air in depht the lever has to open more the valve, so the spring compression in the 2nd stage is also increasing and by that increasing the cracking effort.
The reason why nowadays I think all manufacturers are using the feature of the Venturi Assist in their primary 2nds is to neutralize this mechanical 'problem'.
As far as I know does this US Diver model not have a Venturi Assist, so I would expect it to breath stiiffer with increasing depht.

 

[Diver0001]

When I say 'cleaning up', you can see how clean they are anyway :

Niiice

So what's the serial number?

R..

 

[halocline]

As far as I understand it, a 2nd stage without Venturi Assist will breath harder the deeper you go. The reason is that with the increasing demand of air in depht the lever has to open more the valve, so the spring compression in the 2nd stage is also increasing and by that increasing the cracking effort.
The reason why nowadays I think all manufacturers are using the feature of the Venturi Assist in their primary 2nds is to neutralize this mechanical 'problem'.
As far as I know does this US Diver model not have a Venturi Assist, so I would expect it to breath stiiffer with increasing depht.

I'm quite sure that the conshelf 2nd stages, and basically all 2nd stages, have some degree of venturi assist. It's just that some are not adjustable. The venturi assist is very important in WOB and directing airflow towards the diver. It's a basic part of the type of air movement that exists in 2nd stages.

The increased 'amount' (number of molecules) of air flowing past the 2nd stage valve at depth does not increase the cracking effort. Remember that you have the same 'volume' of air, it's just at ambient pressure. Because IP increases with depth and remains constant over ambient pressure, the pressure differential in the 2nd stage is the same regardless of depth. Thus, the mechanics in the 2nd stage are basically the same regardless of depth. The only difference would be if air had enough of a viscosity so that the increased density affected flow. As DA Aquamaster pointed out, that would occur only at very extreme depths far beyond open circuit diving.

The first stage is where the extra work is done at increasing depth, because it needs to provide consistent air flow at an increasingly high pressure, without any increase in it's supply pressure.

 

[analbeard]

Niiice

So what's the serial number?

R..

Completely forgot that! They are:

H2300533
H2300586

I know they're not old enough to be collectors items, and if I were to sell/use either they'd obviously need a service first.

 

[Paladin]

How much do you want for one, including shipping?

 

[OldNSalty]

That is a fantastic example of a 14. Why would you sell either? Most people dive with a primary and octo-you've got the perfect set up. I have 14s as my primary and octo...but not in that shape externally. Not even close....

 

[axxel57]

Halocline, I think you are overlooking one thing concerning the increasing craccking effort in depht.
You are right that whatever depht we have the volume of air will be ambient and so the same.
But to get the double or triple or whatever more amount of air molecules into the 2nd stage and your body in the same timeframe of a normal breath, the lever of the 2nd stage has to open much more than on a breath at sea level.
By doing so the spring in the 2nd stage is compressed much more than it is while delivering air at sea level, increasing the cracking effort until the Venturi Assist kicks in.

 

[OldNSalty]

I'm not that type of engineer so forgive such a silly question but why would you need a larger opening at depth to get the same amount of air if the air is in fact compressed?

 

[axxel57]

I would think the air entering the 2nd stage is compressed to the level of the Intermediate Pressure anyway, but if the valve let a certain amount of molecules through to produce an equivelant of pressure as the ambient pressure in one normal breath, it needs to open a bit more to let the double amount of moleciules through the valve in the same time.