Beginners course servicing Regs and other Stuff

[Afishianado]

So you are saying that the intermediate pressure measured relative to the ambient pressure will be dropping with depth?

The delta from the IP to the ambient pressure will decrease with depth, yes. However it isn't because the IP is decreasing, it is only because the ambient pressure is increasing.

The intermediate pressure delivered by a balanced 1st stage regulator doesn't change unless the tank pressure drops so low that you are OOG. In an unbalanced 1st stage the IP drops during the dive as your tank pressure drops, but not because of depth.

This is the whole reason for balanced 1st stages.

A better way to think of it is to say that with depth...the ambient pressure increases but the IP remains the same (in a balanced 1st stage). So, in an unbalanced 2nd stage it gets slightly more difficult to breath as you go deeper since the ambient pressure resists the 2nd stage opening more as you go deeper. In order to breath you have to overcome the 2nd stage spring pressure( Also called cracking pressure or inhalation effort) plus the increase in ambient pressure. Also, the delta here between the IP and Ambient pressure does affect the actual airflow delivered to you. The larger the delta the faster the air flows through the hose and into you. So, the deeper you are the more you have to suck to get the same air volume.

Many regulator manufacturers add a venturi assist to their second stages to counteract some of these effects. Some of these venturi assists are slightly adjustable by the diver with +/- levers. Not the knobs on higher end user adjustable regulators, that's a different aspect all together and I am already starting to ramble...

In a Balanced 2nd stage, the increasing ambient pressure is accounted for so you only have to overcome the spring pressure regardless of depth.

This is the whole reason for balanced 2nd stages

This is essentially the marketing version of what is happening, when I want to sell you a balanced 2nd stage regulator. It's accurate enough for this discussion.

Generally speaking, most people don't really feel the difference for shallow dives. Once you start going deeper and have been diving for a while then it starts to matter more.

 

[Six2Life]

Afishianado, unless I am grossly misunderstanding what you wrote, it looks like you are incorrect concerning depth compensation. All properly functioning 1st stages depth compensate, meaning the absolute pressure delivered is directly proprotional to the ambient pressure. Let say for example a reg is set to an IP of 130psig (guage) on the surface it is actually producing 145psia (absolute). 130psi guage + 15psi(1atm) = 145psia (absolute). If you take the same 1st stage to 100ft it will still put out 130psig relative to ambient pressure which is now 60psi (3 + 1 atm), so the absolute pressure delivered will be 190psi. If you took an IP guage with you to 100ft and plugged it in, it would still read 130. If you ran a super long hose from your first stage at 100ft to an IPguage on the surface it would read 175. (190 - 1atm ambient on the surface)

 

[axxel57]

Afishianado, looks like you don't understand one important feature of first (and 2nd) stages (anyway which brand).

If the first stage s wouldn't adapt to depht, we couldn't secure a constant lung volume while diving.

The water chamber allows water to enter and help the main spring to keep the piston (or push pin) away from the HP seat (or orifice) to stay a bit longer open allowing more air to pass and pressure to build up. This is the reason why we have more and more intermediate pressure in our system while diving deeper.

 

[ianr33]

A better way to think of it is to say that with depth...the ambient pressure increases but the IP remains the same (in a balanced 1st stage)..

That is incorrect.

 

[elan]

The delta from the IP to the ambient pressure will decrease with depth, yes. However it isn't because the IP is decreasing, it is only because the ambient pressure is increasing.
...

You said you work on regs, which make do you mostly work on? Are they used for open circuit?

 

[herman]

Don't mean to dog pile fish but you are off on this one. The delta between ambient and IP does not change (PSIG) (unless were are talking about an "overbalanced" reg in which case the delta actually increases- hence the "overbalanced BS") but the absolute value (PSIA) of it does. Bounching between PSIA and PSIG can get confusing.

Axxel57, what you are saying about the SWs would be true IF the exhaust port of the SW reg would keep water from entering the spring chamber, problem is it will not. As water seeps in, it returns the spring chamber to ambient, all be it more slowly than it would with a common piston reg.

 

[Afishianado]

You know what I hate about these kinds of internet discussions? Losing....:blush:

Sorry guys...not enough sleep the last few days I guess... I was connecting my thoughts with something from work. I'm usually smarter than this.

 

[NotSure]

Thanks for the interesting discussion. I’m not sure if I have this straight, just trying to understand. I did a quick drawing of the pressure forces as I see them acting on the piston in a balanced first stage and if you look at the forces acting on the piston both in the open and fully closed position from a static perspective I see no reason why intermediate pressure would necessarily rise with depth. It looks to me like it’s not a simple sum of the forces problem, it’s the dynamics of the piston and spring that determine the intermediate pressure. It’s not that a fixed intermediate pressure wont close the valve at any depth; it’s that a higher ambient pressure causes the rate at which the valve closes to slow allowing intermediate pressure to build.

 

[herman]

You are likely missing one force, ambient pressure. There are 2 forces working against the spring side of the piston (balanced or nonbalanced makes no difference) and one - IP- working against the other side of the piston. The equation is- spring (a constant) + ambient (14.7psia at the surface) =IP at equilibrium. As you descend ambient pressure increases so to maintain equlinerium, IP (PSIA) must also increase. However, the differential pressure between ambient and the IP (PSIG) will remain the same (except in regs that are "overbalanced" but that is another story). PSIA= PSIG +ambient

Think of it another way, if the absolute IP pressure (PSIA) of the reg did not increase, at a given depth, the IP inside the hose would be equal to the ambient pressure resulting in zero differential pressure. Using very rough numbers, for every foot of depth, the ambient pressure increases by 1/2 PSI so if a mythical reg that did not depth compensate (sealed spring chamber) was using an IP of 50 psig, at 100 ft the IP and ambient would be both be 50 PSIA or 0 PSIG between them and you would get no air from the reg

 

[axxel57]

Herman, SW calls the rubber seal at their 1st stages 'One Way Bleed Valve' (3106-6) and that has it's reason.
I can assure you that no water is supposed to enter the main spring chamber, and if it does, it's a faillure.
If the surface below this seal is not perfectly clean or the flow restrictor is blocked, water might enter causing massive corrosion, because there is no way the water can exit the spring chamber unless you remove the rubber seal, and even then you don't get out the water completely, because the orifice is too small. You have to remove the cap to get this done. So rinsing the 1st stage is virtually impossible.
The reason SW designed this type of valve was to avoid problems with water in the main spring chamber as sealed membran regulator designs do as well.
The compensation for depht is a bit slow because the air needs quite some time in the beginning to fill the main spring chamber to ambient pressure, this is why SW (as far as I remember) is warning their customer to descend very fast. That might result for a short while in a bit heavy breathing. Thanks for reading.