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:
View attachment 876979
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:
View attachment 876981
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:
- 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.
- 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.
- 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 disc2
A
Spring disc = π x 0.015m
2
A
Spring disc = 0.00070686m
2
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 0.00070686m
2
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:
View attachment 876983
A
Main diaphragm = π x r
Main diaphragm2
A
Spring disc = π x 0.020m
2
A
Spring disc = 0.00125664m
2
F
Newton = 200000Pa x 0.00125664m
2
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:
View attachment 876985
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.
- 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.
- 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.
