Thanks a lot, Luis!
Again straight on the spot.
You are right, I was writing about cracking effort instead of inhaling resistance or WOB.
And my ‘translation’ of lower IP into inch/h2O is only really valid if I use a static IP as a reference.
It would be very tricky to track the effects of lower, dynamic, IP on the inhaling resistance (inch/h2O) over the full inhaling process with the overlaying (with greater depth higher air flow) Venturi Effect of most 2nds.
I’m not an engineer, I just service and repair Scuba Regulators for the last thirty years, but I think that the efforts of the engineers of most of the manufacturers over the decades to reduce the dynamic IP drop in 1sts by means of high flow ports for example, indicate that the Venturi alone is not considered to neutralize the lower IP ( and so the missing Down Stream Power on the LP seat) sufficient during the inhaling process, to secure a smooth and easy inhaling also in greater depths.
Maybe those efforts to design 1sts with an as low as possible dynamic IP drop are just about the reliability and stability of the Venturi, which might show different values if the dynamic IP differs a lot from the static IP, but I still believe that the lower dynamic IP has a direct effect on the easiness and smoothness of the inhaling process ( as a static IP drop would have more clearly), even with the (over)compensating effect of the Venturi.
I will do some tests on my Flow Bench, using also the ‘normal’ port of the MK17 versus the High Flow port, checking which patterns the Magnehelic shows me concerning ‘inhaling resistance’ during flow ( up to 15 SCFM).
Maybe so I can track the effects of the dynamic IP drop and ‘isolate’ them from the effects of the Venturi.
Anyway, great pleasure again to read your explanations, I personally have benefited from your posts over the years more than from anybody others posts.
I wish I would be able to explain technically complex matters as clear as you do, even in my mother language.
It is relatively easy to analyze and predict flow when it is in a steady state configuration. For example, designing a venturi jet pump is not that hard because it normally operates with constant flow.
The air flow in the breathing cycle of a human is not at all constant. We tend to simulate it using a sinusoidal function. We also do that because a sine wave is very predictable, repetitive and for anyone that has taken basic calculus, it is easy to get its derivative and the integral (or the area under the curve).
The sinusoidal function is not only easy to do math with, it is also fairly easy for a test machine to replicate. A reciprocating piston or bellows can be used to replicate a sinusoidal breathing machine.
For regulator design and test purpose the sine wave function is also close enough simulation to the actual human breathing cycle. We don’t breathe in a perfect sinusoidal function, but it is close enough for this purpose. I had this conversation with one of the research technician/ scientist at the Navy Experimental Diving Unit in Panama City, Fl.
For venturi flow to develop it requires some air velocity to create the dynamic effect. When our breathing cycle is in the form of a sine wave, we are only producing any significant flow velocity in the middle of the suction cycle. We start and finish the cycle with very low (approaching zero) flow velocity.
When the flow is constantly changing, it is a lot harder to predict and design.
BTW, I also find it interesting all the talk about "high flow ports", etc. I see the labeling of a high flow port as an easy marketing gimmick. Yes, it is easy to make a demonstration of in steady state flow, but my “educated guess” is that they make no difference to WOB in a sinusoidal testing machine (never mind human perception). The high flow ports is a complete debate that I don't what to get into at this point, but they sound good.
A point of observation is that the volumetric flow rate requirements for a second stage are always the same at any depth. Yes, the gas density will change, but your lung volume is basically the same at any depth and therefore the second stage is a constant volume device. Well, not constant (due to the breathing cycle) but the maximum flow volume required is the same at any depth.
The first stage is different. It does need to deal with higher upstream volume to accommodate the higher output density gas at depth.
It is very desirable to limit the dynamic IP fluctuation that is fed to the second stage. This is especially true with a basic downstream demand valve (not pneumatically balanced). Limiting fluctuating parameters like IP makes it much easier to design and adjust a second stage.
Since the flow is constantly changing due to the breathing cycle, having a relatively constant IP to work with helps a lot. Again, this is another subject that I could write a lot more about, but not at this time.
Most of us don’t have a sinusoidal breathing machine test simulators, so the best we can do is a comparative tests using constant flow (steady state). A steady state flow air test is fairly easy to set up.
Most of us also don’t have test pressure chamber; so again, the best we can do is trying to simulate the flow effects of higher density air (gas) by doing what is considered a “scaling simulation”. It this case it would be a Reynolds Number scaling. It is possible to scale the flow impedance with higher flow velocity to simulate the higher gas density, but again this is just a steady state flow simulation that is best done for comparative side by side measurements.
For more information I recommend Googling: sinusoidal functions, steady-state flow, Reynolds Number, Reynolds Number scaling, etc.
Sorry about the lengthy post. I hope my explanations are clear.
And I am sorry about the minor derailing of the thread, but after 37 pages of talking about some new… something…
Is it time to move on to something like a debate between piston and diaphragm first stages…
I am just kidding...
