A little advice on 2nd stages please

[halocline]

The maximum flow rate for 2nd stages must be measured at a constant IP to have any significance. Since IP is much higher at depth, it stands to reason that the maximum air flow (measured in surface cubic feet) through the valve at depth would be higher as a result. It's the first stage that really gets stressed at depth because it must supply far more air to the 2nd stage, at a higher pressure, without any increase in it's supply pressure. The 2nd stage is getting more air from the 1st stage as depth increases.

I don't believe that flow rates have anything to do with cracking effort anyways, except someone who is starved for air could easily perceive that as increased cracking effort. Cracking effort is the pressure differential required to open the 2nd stage valve and initiate air flow.

There could be any number of reasons that Diver0001 felt more resistance with his conshelf at extreme depth (60 meters or 200 ft is far beyond recreational depth) and my guess would be 1st stage issues. It's possible that the first stage experienced a much larger IP drop on inhalation than it would have on the surface, and that consequently lowered pressure in relation to ambient going to the 2nd stage. That's a guess, but it would certainly increase cracking effort. Plus, anyone breathing air at 60 meters is going to be seriously narced, with a resulting change of perception. I know I would not trust my own powers of analysis at that kind of depth on air.

 

[LeadTurn_SD]

1) Air and nitrox do get "thicker" at depth, but you have to get to about 600' before the effect is pronounced. You'll notice the increased viscosity in your lungs before you'll note any change in the performance of the second stage.

2) all second stages have a moderate degree of venturi effect - that's why if you drop a free flowing reg, it keeps right on free flowing. Adjustable flow vanes just let you increase or inhibit the effect in some circumstances

3) The valve does have to move more gas at depth, but that's simply because you breathe more cubic feet with each breathe at depth - if you breathe .6 cfm at the surface, you'll be breathing 3 cfm at 132 ft (5 times more as you are at 5 ATA). So if you take 5 breathes per minute, that's only .12 cu ft per breath at the surface, but it's .6 cu ft per breath at 132 ft, and both breaths will have the same amount of inhalation time. Consequently, at 132 ft, in that fixed period of time, the reg has to move 5 times the gas that it does at the surface. The thing is that a reg like the Conshelf XIV can move about 30 cfm.

So...if I am at 132 ft and inhale that expected .6 cubic feet needed for one full breath in 5 seconds, that's equivalent to a flow rate of only 7.2 cfm, leaving a reserve flow from the second stage of about 23 cfm. However, if I get panicked and inhale that same .6 cubic foot needed for a single full breath at 132 ft in only 1 second, then it requires a flow rate of 36 cfm and I'll start out breathing the reg and feel starved for gas - increasing the panic response. That's why panic = really, really bad underwater, especially at depth.

Great post! Especially the final paragraph.

It sums up, from a regulator performance standpoint, why heavy exertion and/or undue "excitement" needs to be avoided at depth. Most of the time we tend to believe that it is "impossible" to over-breath a properly-functioning modern reg (from a purely mechanical standpoint), but at depth that may not be entirely true.

Best wishes.

 

[axxel57]

Hi folks

Back from thinking.......

The reason I have been pretty confident about what I wrote, is the fact that I thought that the increasing cracking effort is what I see everyday in my workshop when I do the flow tests with the regulators. They all show the same patterns. With the increasing air flow through the 1st and the 2nd stage on my Magnehelic I can watch

---------- Post Merged at 12:15 PM ---------- Previous Post was at 11:34 AM ----------

Sorry, don't know why my computer already sent this, so again

Hi folks

Back from thinking.......

The reason I have been pretty confident about what I wrote, is the fact that I thought that the increasing cracking effort is what I see everyday in my workshop when I do the flow tests with the regulators.
They all show the same patterns. With the increasing air flow through the 1st and the 2nd stage on my Magnehelic I can watch always an increasing breathing resistance which is higher than the iniitial cracking effort. This goes on until the end of the test at 12.5 SCFM or until the Venturi Assist kicks in and lowers again the breathing resistance or even overrides the 2nd stage mechanism.
I got irritated when I realized that I cannot simulate the increasing IP at increasing dephts. This is why I had to overthink my comments.
I came now to the conclusion that this fact doesn't matter, because if the Flow Module shows this type of reaction from 2nd stages (concerning the dynamic IP also from 1st stages), it will show the same patterns at 33 feet depht because the increasing IP is corelating with the ambient pressure.
The mistake in thinking from my part was, as I see it now, that I was thinking I watched the cracking effort instead of the inhallation effort, what probably is something quite different.
The cracking effort describes the minimum effort to open the valve, but it doesn't say anything about how it is it doing.
I have sometimes been surprised how bad some 2nd stages are breathing although they have an exellent cracking effort number.
With the discussion we had her now, I think I understand now better that cracking effort is one aspect, the inhallation effort, which describes the total effort of the DA Aquamaster's five second inhallation, another.
The inhallation effort probably depends on factors like the stiffness of the spring in the 2nd stage, the lever advantage and others.
In the end I think I was right saying that 2nd stages without or only with little Venturi assist will breath harder at depht, regardless if the cracking effort is increasing or not ( probably Halocline is right that this is not changing, I'm not sure).
I'm sure though that the valve has to open more the more air it has to flow, so the lever has to open more the valve, by that increasing the spring pressure in the 2nd stage, which causes an increase in the inhallation effort.
Why at higher IP still the valve still has to open more comes maybe from the fact that the airstream going through the aspirator (injector) is not laminar but turbulant, it has to go around the LP seat to reach the aspirator before entering the 2nd stage.
Hope my thoughts are understandable. Thanks for reading

 

[halocline]

I'm sure though that the valve has to open more the more air it has to flow, so the lever has to open more the valve, by that increasing the spring pressure in the 2nd stage, which causes an increase in the inhallation effort.
Why at higher IP still the valve still has to open more comes maybe from the fact that the airstream going through the aspirator (injector) is not laminar but turbulant, it has to go around the LP seat to reach the aspirator before entering the 2nd stage.
Hope my thoughts are understandable. Thanks for reading

You seem to be confusing increased volume at ambient with increased volume measured in surface cubic feet. The air in the 2nd stage is at exactly the same volume regardless of depth, only denser, as you get deeper. The pressure differential across the 2nd stage seat is the same regardless of depth, assuming IP function (drop and recovery) remains the same. I realize that measured in SCFM the flow is 4 times as much at 4 atm, but the contained volume is the same, it's just that the pressure is 4 atm instead of 1 atm. I don't see how it affects the 2nd stage valve at all, except if increased density of the air caused more friction and slowed flow in that way. I have read that this is not a significant factor until well beyond typical scuba depths.

The first stage is an entirely different matter, as I explained in an earlier post, no need to re-hash it. I would be confident that increased inhalation resistance in the 2nd stage at depth is due to a greater IP drop in the first stage. If that happens, then your pressure differential in the 2nd stage is no longer equal, and the air force countering the spring in the 2nd stage is proportionally lower, hence increased resistance. This effect would be amplified in unbalanced downstream 2nd stages, because they are strictly mechanical spring vs air pressure (IP). Balanced 2nds, which capture IP and use it to counteract the downstream air force, would react less because the captured IP (upstream air pressure) would be lowered along with the downstream force.

Someone did a test to see how long a free-flowing regulator would take to empty a tank, on the surface, and then at depth. Surprisingly to me (at the time) the tank at depth emptied much more quickly. This was not the case (as expected) with simply opening a tank valve on the surface and at depth. The reason the regulator can empty the tank much quicker at depth is because the first stage is putting out air at a much higher IP, which results in far greater flow to the 2nd stage. If I remember, the tank with the regulator at depth emptied much more quickly than the tank with simply a wide open valve. (I think that's what happened) Try explaining that one! The valve, if it's a limiting factor, should keep the regulator from flowing any more than valve flow capacity. The only explanation I can come up with a venturi effect in the first stage that causes enough of a pressure drop to increase flow through the tank valve.

While on the subject of venturi assist, all 2nd stages will have some, and my guess is that they all have a pretty good amount, some more than others, but no 2nd stages have little or none. It's simply too important a part of how regulators feel to be missing, divers would not tolerate it. That's a guess.....but the presence or absence of venturi adjustment is not relevant; many, many non-adjustable 2nds have loads of venturi assist. If, as you are claiming, 'more' air is moving through the 2nd stage at depth, then the venturi effect would be greater, as it's air speed (and density, I suspect) that determines the venturi assist in a given flow path.

So I'm not disputing that there are some regulators that feel stiffer when used in really deep situations, I'm just convinced it's a first stage issue, specifically greater IP drop in an attempt to keep up with the increasing demand of producing higher IP from the same supply.

Sorry for the lengthy discourse.....I just can't stop geeking out at times.

 

[Luis H]

What Halocline is saying is correct about the first and second stages.

The second stage is for all practical purpose a constant volume device. For the most part, it works with the same volume independent of depth. The air density (and viscosity) in the second stage is what changes, not the volume.

Don’t get confused with the “equivalent” surface air volume. We tend to always talk about the air consumption as a function of the equivalent surface volume.

On the other hand, the first stage has to handle more volume (since the source pressure doesn’t change with depth, only with consumption) and therefore the IP response will change with depth. A lot more volume has to pass through the same first stage valve.
I have been wanting to measure IP and IP fluctuation (IP dip) as a function of depth, but I haven’t gotten around to it. I need to set up an IP gauge that I can take diving.



The venturi effect is affected by the air density. The denser air will have more venturi effect.

I have a tested a balanced single stage double hose regulator with a finely tuned venturi flow (I designed an adjustable venturi flow for that regulator with a precise metering adjustment). The way I had it adjusted, the regulator worked fine until around 60 ft. As I went deeper the venturi was increasingly stronger. If I took deep or strong breath, a lot of extra air was wasted out the exhaust. The harder I breath the more excess air was supplied. I had to sip my air to diminish the amount of wasted air.

BTW, Emilio Gagnan introduced the first regulator with venturi flow in the single stage Mistral in the 1950’s (single stage regulators were not introduced until the 50’s). Since then most regulators have some form of venturi flow. For certain, all single hose second stages (that I am aware of) since the late 60’s have some form of venturi flow.

That good looking US Divers second stage has a very effective vane that direct the air flow from the demand valve to the right side of the mouthpiece. This flow vane creates an effective venturi flow.

 

[axxel57]

Hi folks, your comments are highly appreciated!
Give me some time to answer.......

 

[axxel57]

Hi folks, Halocline, Luis in particular, thanks for your comments.

Of course it makes sence that the dropping dynamic IP during higher air demand is responsable for the increasing inhallation effort also during diving at depht. Don't know why I overlooked this especially for me easy to see effect.
Also the explanation why a 2nd stage at depht not necessarely has to open more although flowing greater amounts of air molicules sounds logic.

The idea that the lever movement (how far it opens) in one breath on the surface is exact the same at 200'feet with a much higher air flow though 'feels' wrong.
I've never been good in Physics, so for the moment I guesss I have been wrong in my assumption and have been mistaken by my daily work on the flow bench where of course more air flow means more lever movement. Thanks for showing me my mistake in thinking.

There is though another point I have to find out.

Usually the IP drop at test between a full tank and 'empty' tank is 15 - 20 PSI and translates in a 0.4 to 0.6 inch/water increase in cracking effort with for example an MKII - R295 (unballanced 1st & 2nd).
That means a 2nd stage calibrated for 1.0 inch/h2o at full tank starts the flow test ( I'm testing at 'worst case scenario') at 1.4 1.6 inch/h2o.
The drop of the dynamic IP during the flow test seems to be usually about 1/4 to 1/3 less then the drop from full tank to low pressure tank, but it translates as I watch it in a 0.7 - 1.0 inch/h2o increase in inhallation effort, what according to the explained mechanics should not be I think.
So I got quite some things to think about and watch and will start to put in my data base also the figure 'maximum inhalling effort at flow test' to see if there is in the long run really a discrepancy between the effect of decreasing static IP on the increasing cracking effort and the decreasing dynamic IP ( during flow test) on the increasing inhallation effort.
Anyway, thanks again for the discussion.

 

[axxel57]

You seem to be confusing increased volume at ambient with increased volume measured in surface cubic feet. The air in the 2nd stage is at exactly the same volume regardless of depth, only denser, as you get deeper. The pressure differential across the 2nd stage seat is the same regardless of depth, assuming IP function (drop and recovery) remains the same. I realize that measured in SCFM the flow is 4 times as much at 4 atm, but the contained volume is the same, it's just that the pressure is 4 atm instead of 1 atm. I don't see how it affects the 2nd stage valve at all, except if increased density of the air caused more friction and slowed flow in that way. I have read that this is not a significant factor until well beyond typical scuba depths.

What Halocline is saying is correct about the first and second stages.

The second stage is for all practical purpose a constant volume device. For the most part, it works with the same volume independent of depth. The air density (and viscosity) in the second stage is what changes, not the volume.

Don’t get confused with the “equivalent” surface air volume. We tend to always talk about the air consumption as a function of the equivalent surface volume.

This issue has bothered me a while.

As I mentioned I have always been pretty bad in physics, but the idea that the valve of any given 2nd stage is only opening to a certain degree to produce a constant volume in the 2^nd stage and airways regardless the depth under water felt 'wrong' to me.

But when Luis is confirming a theory of Halo and others concerning regulator mechanics, normally I stop arguing .

This was also now the case for a couple of months.

Recently I came back to this thread and thought it over again and I think I have good arguments why you guys might be wrong concerning this issue.

This is why…………

If the valve of any 1^st stage is flowing a certain amount of air (air molecules) in a certain time frame to produce a certain IP, everybody understands that, if the supply pressure is the same, the same valve has to open more if it wants to flow in the same time frame a higher amount of air.

U/W we need a mechanism which helps us to receive the more air from the 1^st & 2^nd stage the deeper we dive to secure a constant volume in our natural airways.

This mechanism works using the increasing water pressure in depth helping the main spring to stay longer (further) open to let more air getting through the valve of the 1^st stage in about the same time as in shallower water depths. It’ a mechanical adaption to depth.

Each 33’ feet (10m) more depth produces an IP around 15psi (1bar) higher.

The second stage valve produces on the surface exact the 15psi (1bar) ambient pressure we need here. In 33’ feet (10m) depth the ambient pressure is doubling, so the 2^nd stage valve has to flow the double amount of air molecules in the same time frame.

If any valve is opening to a certain extent (the lever opens up to a certain angle) to produce 15psi (1bar) and it wants to flow the needed double amount of 30psi (2bar) in the same lever position, then the supply pressure must double to allow the double amount of air molecules through the same opening of the valve in the same time frame.

THAT IS NOT WHAT IS HAPPENING!

The supply pressure (IP) for the 2^nd stage valve is only increasing by about 10% (not 100% ) from around 150psi (to make calculating easy) to 165psi (from 10bars to 11bars) Intermediate Pressure.

How should any 2nd stage valve flow the double amount (in this example) of air in the same lever position using the same time as on the surface, if the density of the supply pressure has not doubled but increased by only 10%?

For me that means that any 2^nd stage valve has to open a bit more the deeper we dive to flow the needed amount of air in the same time into the 2^nd stage and the human airways.

That also means that the spring force in the the2^nd stage is increasing, by that increasing the inhalation effort in depth if not a Venturi Assist is neutralizing this process.

So for the moment I stay with my claim that 2nds without Venturi Assist will breathe harder the deeper one go.

The increasing inhaling effort during a breath is not only because the 1^st stage cannot keep up the initial IP, but also due to the mechanical resistance in the 2^nd stage (if there is no Venturi).

[FONT=&amp]Halo, Luis, what is the mistake in my thinking?[/FONT]

 

[halocline]

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.

 

[redacted]

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.