A little advice on 2nd stages please

[axxel57]

The second stage flows the same volume of air regardless of depth, it's just that the air is at higher pressure. So when you say "double the amount" that's not an accurate statement. It's more air molecules, sure, but gas is measured in volume and pressure. Since the pressure increases as depth increases, the volume of air passing through the 2nd stage and into our lungs remains the same. Why can't you understand that?

If you are talking about an unbalanced downstream 2nd stage, the spring pressure does not increase with depth. It's a mechanical spring; it doesn't change with depth. What does change is the ambient pressure on the downstream side of the seat; that's how 2nd stages compensate for depth. With a balanced 2nd stage, IP in the balance chamber also goes up with depth, but I'm sure you understand that. Those two pressure increases are what counter the increase in IP pushing the 2nd stage valve open, resulting in a net zero increase in countering forces at the 2nd stage seat as depth increases.

The density of the air increases, and with that there is an increasing friction coefficient. I don't know what that is quantitatively, but I've read that it's insignificant until far past the depth at which air becomes O2 toxic, meaning divers will be using trimix, which has lower friction than air. But, eventually, at some extreme depth, I guess there would be enough of a friction build up to increase WOB in some measurable way.

All 2nd stages have some venturi assist. It's probably impossible to not have any given the fact that you are moving air quickly through a restricted passage into a higher volume area. This will cause a pressure drop in the surrounding chamber, which aids in the movement of air. I'm not an expert in aerodynamics by any means, but it's very difficult for me to imagine a conventional 2nd stage that has no venturi assist. Just because there's no vane or adjustment does not mean there's no assist.

Halocline, thanks for answering

Of course flows the 2nd the same volume of air regardless the depth, just the air is heavier and denser (has a higher pressure) and we get this denser volume in our lungs. Don't worry, I understand this, and I think I also understand [FONT=&amp]how[/FONT] this is happening.

Again, you guys claim, that if you take a normal breath on the surface, the valve (lever) will open up to a certain extent (angle) to flow the needed amount of air to produce a certain lung volume.


Not difficult to understand
.

But you claim that at for example at 33feet (10m) the valve has to open exact only to the same extent like on the surface and that would be the case also in 330feet depth because the adaption to depth, which is done using the water pressure in the 1^st and on the 2^nd stage which will make it unnecessary for the 2^nd stage valve to open more to keep a constant volume in our lungs.


I still think that is wrong.


The valve has to open more the deeper we dive, because the supply pressure, and this is here the key (in this case the IP and not the tank pressure) is not getting in the same proportion denser (it is less) - the intermediate pressure (IP) is not increasing in the same proportion - as the ambient pressure is increasing. So the valve has to open more to compensate the missing pressure or density to flow the same volume in the same time frame of one normal breath.


In 33feet(10m) the ambient pressure has doubled in comparison with the ambient pressure on the surface, so we need a supply pressure double as high………….


The question is if a valve can flow in the same lever position the double amount of air molecules (density) when the supply pressure is increasing only by 15psi(1bar). Obviously is there a correlation between the valve output and the increasing IP with increasing depth.


If the supply pressure (IP) would be 15psi(1bar) , not 150psi(10bars)and then the IP would increase each 33feet(10m) by 15psi(1bar), the IP increase would be in the same proportion as the ambient pressure increases, so the lever would open always to the same angle and not more. But we start at 150psi(10bars) supply pressure (IP).


There is a difference between the absolute increase of pressure with increase of depth, and the pressure changing rate which we feel when we start to descent and have to compensate our ears.


In the first 33feet(10m) we have to compensate much more often our ears than in for example 100feet.


To ‘flow’ the same 15psi(1bar) per 33feet(10m) into our middle ear we need a much higher effort (we have to open the ‘valve’ more often or we would have to open it more) than later in greater depths.


In my example comparing the surface pressure and the one in 33feet (10m) ‘double amount of air’ seems to me quite accurate. Speaking about greater depth, the pressure changing rate changes and then we are of course not talking of always ‘double amount of air’.


This is also why I think that of course the spring pressure is changing (increasing) in the 2^nd stage.


The mechanical resistance is normally getting higher the more a spring is compressed.


Maybe it is getting clearer what I mean, if we just concentrate our thinking on our octopus during the dive without breathing from it..
If you go down to 33feet in 20 seconds, the 2^nd stage valve (lever) will open up to a certain extent until the octopus has in its inner the air pressure of 30psi.
If you would use a slide to rush in the same 20 seconds down to let’s say 330feet, according to your logic the compensation would take place with the same lever position, because you think that the increasing adaption to depth (15psi (1bar) every 33feet (10m) on part of the 1^st and the 2^nd stage produces the same pressure changing rate as the increasing depth. But as I wrote in my last post that is not the case.


Of course can a normal regulator supply pressure(IP) produce the needed ambient pressure with a constant lever position, but to be able to do that in always the same time frame, the supply pressure has to increase not only absolute, but also in the same proportion as the ambient pressure is increasing to the pressure on the surface. That’s not the case.


So the lever (valve) has to open more in this time frame to do the compensation.


Two of the mayor objects in conducting flow tests is to check the lever height and so secure optimal poppet – orifice separation and to check if and how the venturi assist in the 2ndstages is working.
I can assure you that not all 2nds have a venturi, not even all modern ones, or if they have, it might show only at flow rates of 20 – 25SCFM. I usually test only to 12,5SCFM.
I understand that increasing air density with increasing depth might trigger the venturi in some of 2nds earlier, but that will not help them in recreational depths.
And I am pretty sure that there are old 2nds, which have not even from their design the possibility to flow a laminar air stream to produce any kind of venturi in flow rates needed for recreational depth ranges.


I will leave this for now, and I will not provoke you again to answer me, I know now what you mean when you stay silent.

I tried to show my arguments from different angles, if I cannot convince anybody of my view, then there is not much more I can do.


If I should be wrong in my thinking, then hopefully someone can explain me that in words I can understand.


Anyway, thanks for the discussion!

---------- Post added May 19th, 2013 at 01:33 PM ----------

When something does not seem right, it is often helpful to look at things from another perspective. In this case, consider what is going on from the demand side (the lungs) rather than the supply side. Human lung capacity averages about 6 liters and that does not change with depth. The amount of that capacity a healthy adult uses (tidal volume) with each resting breath is about .5 liters. That will change with exercise and some other conditions like nervousness. So, consider your gas usage from that perspective and see what you get.

I'm not sure I understand where you want to guide me........

 

[halocline]

The proportion of IP to ambient doesn't matter, only the amount of pressure drop, and that stays the same. That's precisely what depth compensation is. If the proportions mattered, 2nd stages would work wildly different at different depths, there's no way scuba systems as they are currently constructed could work. A pressure drop of 135 PSI will result in the same 'amount' of downstream pressure regardless is it's 150 going to 15 or 500 going to 365 or 3000 going to 2865. The only thing that changes is the density of the air which increases friction in the line, but the effects of that in normal sport diving depths are negligible to my knowledge.

I think you are seriously over-complicating the concepts here.

Regarding venturi assist, any time you have air moving in the manner of how it flows through a scuba 2nd stage, you will create lower pressure surrounding the air movement, which helps the diaphragm push on the lever. All 2nd stages have some venturi assist, whether or not you feel that you can measure it with scuba shop equipment.

BTW, thank you for keeping a cordial tone during this discussion, I'll try to do the same.