Question Skipping 1st stage Maintenance?

[elmo]

IMO, if it's not Helium transiting the main diaphragm, it's air leaking in from the outside during storage. A properly fitted environmental seal is concave when the reg is depressurized. That means the pressure in the outer chamber is below ambient.
Without the surface tension provided by seawater, a questionably designed seal like Scubapro's will leak air. When the reg is next pressurized, the seal bulges.

But that was the easier question.

Is a properly fitted environmental seal not also concave when submerged, where ambient pressure is greater than on the surface? If so, and if it leaks when depressurised in storage, could it not also leak underwater? I ask this knowing you have looked inside countless environmental seals, and I have removed the seals of and seen inside two of mine (a Mk 17 and a DS4). The Mk 17 had a bulge (which I amended by resealing under pressure.

 

[Umuntu]

Is a properly fitted environmental seal not also concave when submerged, where ambient pressure is greater than on the surface?

A properly fitted environmental diaphragm is only concave while the regulator is unpressurised. Once the regulator is pressurised the diaphragm is no longer concave.

 

[rsingler]

A properly fitted environmental diaphragm is only concave while the regulator is unpressurised. Once the regulator is pressurised the diaphragm is no longer being concave.

...because the transmitter rises to become level with the environmental cap as you pressurize the reg, the pin rises and the valve closes.

 

[Tanks A Lot]

I should have been a bit clearer in my previous explanations, especially the conclusion.

I fundamentally believe that we as humans are terrible at visualizing pressure, which very often leads to wrong conclusions. We often think of it as an object or liquid like water pushing onto something. If that something is something flexible, like a diaphragm that will bulge, in our mind, the object rolls or flows into the bulge, concentrating pressure. In our mind we follow this object or liquid and try to find the point where it is the "heaviest". At least I catch myself in a thought-process like this very often and can see that students often think along the same lines when they pose certain questions.
But that is exactly what pressure is not. Pressure is something that acts perpendicular and equally onto all surfaces in which it is contained, be it up, down, left or right.

Let me try to address a couple of points.

If I understand Roberts point correct, he believes that when starting with a bubble, which allows pressure directly to be transmitted from ambient pressure to the dry chamber, the diaphragm is flexible enough between the wall and the spring disc, over-dramatically drawn like this:

And in a sense that is absolutely correct, the diaphragm will flex somewhat in that region. However, the force does not concentrate on the flexible part of the diaphragm as I tried to outline above. Rather it will act perpendicular across the whole diaphragm, which also means it acts across the spring disc, where in a proper setup usually only the transducer and bias spring act upon. Crucially, the force does not concentrate in a single area, pressure acts equally perpendicular on the whole diaphragm!

Now as to why this actually hastens intermediate pressure rising until the bubble collapses onto the transducer:
For the sake of the argument, going forward let us assume a bubble which doubles the volume of the dry chamber. I know that this is ludicrous, but it makes the math a bit easier to follow and holds just as true for a tiny bubble. Our starting position at the surface would look as follows:

As I mentioned, the diaphragms are not the same diameter. Again, earlier I should have been much clearer on this point. In a properly setup transducer design with no bubble, the important ratio is not any ratio between the diaphragms, but rather the very top of the transducer and the bottom of the spring disc. The transducer will press onto this disc and is for calculation or engineering purposes one single part.
In short, whatever presses onto the top of the transducer, gets translated into a certain force on the bottom of the spring disc.

Lets pick some more arbitrary numbers and while the numbers are somewhat arbitrary, certain conditions must be fulfilled by them:

1. The diameter of the main diaphragm shall be our biggest number. By diameter I refer to the clearance between the brass walls. The diaphragm gets somewhat buried along its edge into this brass wall, but we mean the inner diameter, which is the yellow line in our picture. Let us say that it is 40mm in diameter.
2. The diameter of the disc spring shall be smaller than the diameter of our diaphragm. This is our green line. Let us assume it is 30mm in diameter.
3. From an engineering point of view, it would be ideal if the spring disc diameter (green line) has the exact same diameter as the very top of our pressure transducer (pink line). This isn't as trivial as it sounds, which I will get to later.

That is pretty much all we need to get us started. Let us pretend we dive with a properly setup system and the numbers from above to 10m, so that ambient pressure increases to 2bar. The outer diaphragm acts upon the top of the transducer, which transmits its force to the bottom of the spring disc, which in turn transmits it further onto the main diaphragm.
We start by calculating the area of the spring disc, which is ideally the same as the area of the top of the pressure transducer.

A_{Spring disc} = π x r_{Spring disc}²
A_{Spring disc} = π x 0.015m²
A_{Spring disc} = 0.00070686m²

We go on to calculate the force acting upon this part at 2bar, where 2bar equals 200000Pa and Force is defined as: F_Newton = P_Pascal x A_m²

F_Newton = 200000Pa x 0.00070686m²
F_Newton = 141.372N

In our properly setup system, we have a force of roughly 141N acting upon on the main diaphragm.

Let us do exactly the same, but this time with our big bubble. Remember, in this scenario the ambient pressure acts directly onto the main diaphragm, until the bubble collapsed onto the transducer. The moment just before the outer diaphragm touches the transducer would like like this:

A_{Main diaphragm} = π x r_{Main diaphragm}²
A_{Spring disc} = π x 0.020m²
A_{Spring disc} = 0.00125664m²

F_Newton = 200000Pa x 0.00125664m²
F_Newton = 251.327N

As we can see, the main diaphragm is exposed to a much larger force, 251N vs. 141N in the properly setup system. The reason for it is rather simple. It has a much larger surface area for the pressure to act upon than the spring disc does.
And because a larger force acting upon the diaphragm translates directly into an increase in intermediate pressure, intermediate pressure actually hastens at rising until the bubble collapsed and the main diaphragm is not directly exposed to ambient pressure anymore.

Now it must be said that for all intends and purposes the person that is correct is here @lowwall. I have deliberately picked vastly exaggerated numbers to show the difference. For real world scenarios the effect is virtually negligible.

I also vastly simplified the mathematics. In reality its not the entire transducer area at the top involved in the transmission of the force. This would require a limitless stretchable environmental seal, which is nonsense. At its edge it will be ever so slightly less stretchable, the further the transducer has to move in a given cycle to equalize depth changes. That's one reason that manufacturers like to draw the edge of this diaphragm so squiggly (Brown bit below).
Furthermore, the spring disc oftentimes is a cup (Red bit below), which transmit a bit of force onto the main diaphragm along it's cupped edge. But I must admit that taking all this into account is way beyond my mathematical capabilities. Borrowing from the MK17 EVO cutaway:

Robert raised a very interesting point about the origin of the excess gas and as he correctly points out there are only two ways gas can get inside the dry chamber.

1. From the environmental seal: During storage the main diaphragm pulls the environmental seal towards it, creating an area of less than ambient pressure. If the environmental seal wasn't sound, gas could creep into the dry chamber, equalizing the area with its surrounding during storage. The trouble I always had with this explanation, is that whatever path the gas took, would almost certainly be taken by water during diving. In fact, the pressure differentials during a dive on either side of the environmental seal are vastly bigger than they are during storage. As I have encountered very few flooded dry chambers and if water was present, it usually was literally flooded, I think this is not the path the gas will take.
2. From the main diaphragm: This part is an actual bitch to engineer. The outer diaphragm is really simple, as it isn't exposed to any actual great forces onto most of its body. The transducer fills in the gap underneath it almost entirely, which gives it a great place to rest upon. Quite literally you can make environmental seals successfully out of a plethora of materials yourself. However, the main diaphragm is an entirely different beast. It has to be flexible to a fair amount, very sturdy (See @CG43 explanation of a pressure differential of 20bar to 1bar at 100 meters for example, and at the same time impermeable by gases. And the last part is very hard to engineer while satisfying the other two conditions. Very often when I did encounter a bulge, it was from technical divers that used TRIMIX. Robert mentioned above how hard it is to contain helium reliably which is absolutely true.

The problem I have with explanation number 2 is that not all bulges I got into the workshop had been from technical divers. Regardless, I find the somewhat permeable membrane explanation the most likely but must admit I do not know for sure.

Edit: I guess the "There are new posts button, would you like to read them" button is there for a good reason, I should have pressed it, apologies...

 

[Pearlman]

I haven’t fully absorbed the complicated theoretical discussion you folks are having but I found this interesting presentation that makes some claims that befuddle me:

The intermediate pressure (IP) increases to a depth of about 200m.
At depths below 200m, the intermediate pressure starts to drop, and can drop to zero at depths below 300m

Edit: Also this -

At a depth of approx. 180 - 200 m, the transfer of hydrostatic pressure to the working diaphragm stops functioning properly, the plastic transmitter stops and starts to deform. starts to deform.

At depths below 200m, medium-pressure starts to drop significantly, and at depths below 300m, it can drop to values close to zero.

The regulator gradually stops supplying gas

Regulators and IP for Deep Dives

 

[Mobulai]

Well this is certainly a higher education and will keep me entertained during my flights

 

[Still Kicking]

I wish I could understand what you’re talking about here, but I’ll try to help if I can. First, all the rings, bushings, and o-rings you are talking about, I assume what you are referring to is the HP piston o-ring with a plastic bushing on either side, is that correct? And there is a spring on the HP side of that arrangement that keeps it in place. Right? The way to get that installed is with that installation tool that’s in your photo, the stepped side. You put the inner bushing, then the lubed o-ring (010 85-90 duro), then the outer bushing on the installation tool, then push it in from the HP side and give it a little twist, remove the tool. The seat gets installed on the seat retainer, with an o-ring. Not in the space where you just installed the bushings. When you’re ready to install the piston, turn the installation tool over and use the ‘hollow’ end to hold the bushings/HP o-ring in place, then install the piston (use a bullet!) from the ambient side, of course with the main spring in place. The hollow end of the tool allows for the piston bullet to fully go through.

Then take the bullet out, drop the spring in place, and install the seat retainer with the seat already installed. All this info (or similar, I might not have all the details in the right order, I don’t work on MK25s very often) is in the service procedure manual, which is everywhere on the internet.

The only thing that causes IP creep in a MK25 is a bad seal between the piston edge and the conical seat. Meaning, either a worn seat or worn piston edge, or some minute bit of debris in there. If the seat is mis-aligned, I’d think you’d have major HP leakage, not a little IP creep. But, again, I don’t know what you were describing.

A couple other things, the silicone bushing (s) that go on the piston shaft near the base have probably varied a little from generation to generation, and I don’t really know exactly what the ‘evo’ includes, but I believe it’s all various efforts to prevent icing on the piston and main spring. If you dive in cold water, that stuff has use but in warm water it’s basically meaningless.

Removing the old HP piston o-ring/ bushing arrangement should be easy to do without using a sharp o-ring pick. I think you can push it out from the ambient side with a wooden dowel. I know I’ve never had any sort of problem getting that stuff out cleanly. In fact, this whole bushing arrangement surrounding the HP o-ring was instituted starting with the MK15; SP initially claimed it tightened tolerances around the piston and as such prevented extrusion of the HP o-ring. I’m sure that’s accurate, but I suspect that the bigger reason was to prevent careless technicians from scratching the journal that the HP o-ring on the earlier balanced piston regs (MK5/10). Removing that o-ring without damaging anything is tricky. I use a double hook o-ring pick and bury the end of the pick in the o-ring itself. This of course destroys the o-ring but you are there to replace it.

You make it sound so straightforward LOL.

 

[CG43]

We start by calculating the area of the spring disc, which is ideally the same as the area of the top of the pressure transducer.

ASpring disc = π x rSpring disc2
ASpring disc = π x 0.015m2
ASpring disc = 0.00070686m2

We go on to calculate the force acting upon this part at 2bar, where 2bar equals 200000Pa and Force is defined as: FNewton = PPascal x Am2

FNewton = 200000Pa x 0.00070686m2
FNewton = 141.372N

In our properly setup system, we have a force of roughly 141N acting upon on the main diaphragm.

Sorry, you have omitted some area between the spring disc and membrane outside the spring disk with 2 bar pressure .
To calculate this force, we need the effective diaphragm diameter.
In the case of a diaphragm clamped at the edge, most of the forces usually go to the central reinforcing plates, but part of the forces go to the clamping at the outer edge. This force does not contribute to the control of the 1st stage. Since the effective diameter for both sides of the diaphragm, i.e. also for the IP, is the same, there is no problem.
The clamped diaphragm behaves like a low-friction piston with a slightly smaller diameter. Calculating this diameter is difficult. As a 1st. approximation you can take the mean diameter between the clampings, as a second approximation equal area clamping-eff.dia. and eff.dia.-zenterplates .

Now there are three possibilities:

eff.dia. environment membrane. less than eff.dia 1.stage membrane = undercompensated. For example, environment dia. = zero (O2 constantflow)

eff.dia. environment membrane. same as eff.dia 1.stage membrane
Result = constant IP

eff.dia. environment membrane. greater than eff.dia 1.stage membrane
Overcompensated reg.

 

[rsingler]

At depths below 200m, the intermediate pressure starts to drop, and can drop to zero at depths below 300m

When the force on the transmitter is greater than the strength of the plastic, the tip collapses until the transmitter plate rests on the adjuster. If the environmental seal stretches enough to maintain the seal, then additional ambient pressure is no longer added to spring tension in setting IP.
Since "breathed IP" is absolute IP minus ambient pressure, if absolute IP no longer rises at greater depths due to the failure of the transmitter, then the difference between it and ambient falls as you descend, until it is less than the second stage can support.
If the environmental seal fails, the IP will recover, as full ambient pressure is added to the spring's contribution.
Scubapro's current transmitter design is superior, with a metal tip that is surrounded by a heavy plastic end, compared with the hollow plastic tube that Apeks uses. And since I've now learned that that bubble doesn't affect IP response, I can live with the fact that they make leaky, if pliable, environmental seals...

 

[lowwall]

Can we now introduce Mares' current environmental seal system? They call it Twin Balanced Piston. That's more marketing fluff than useful description, but it does have some theoretical advantages over the standard design. Perhaps the largest is that all the parts are metal except for the conventional flexible diaphragm that forms the environmental seal. This should allow for a more precise and consistent transfer of force to the pin pad.

Here's a crop of the schematic:

The main diaphragm is replaced with machined metal disc and hollow column (9) that is sealed by an o-ring (10). The outside of the column guides the spring (12) which is preloaded by the regulating nut (22) which is how you adjust IP. The inside of the column accepts the post of the part that replaces the transducer which Mares calls the upper piston (21).

You can see all the parts and how they are assembled in this video (starting at 41 seconds)