Regulator Geeks 3: Reg Performance at Max Safe Gas Density

[RyanT]

@rsingler, quick question for you. In your graph where you tweaked the Venturi/cracking of the Deep6, I noticed that the curves start at different places (even for the regs all tuned to crack at 0.8"). Shouldn't all the regs tuned to crack at 0.8" start at that spot? Or am I missing something? Thanks!

 

[lexvil]

I find the sort of switch found on HOG, Deep 6, Zeagle, Apeks etc. these act to block the flow from the barrel, Sherwood SR’s do the same sort of thing but as a combined operation with the spring pressure knob (it works but not as well as the separate adjustments) I find this type much better in a hot tuned reg than the SP viva.

 

[rsingler]

@rsingler, quick question for you. In your graph where you tweaked the Venturi/cracking of the Deep6, I noticed that the curves start at different places (even for the regs all tuned to crack at 0.8"). Shouldn't all the regs tuned to crack at 0.8" start at that spot? Or am I missing something? Thanks!

The first point on my curves is at 2 SCFM.
A LOT happens between 0 & 2 SCFM. Some regs immediately drop to 0.5", some rise and then come back down as Venturi kicks in. Lever friction plays a role. "Stiction" plays a role. Early turbulence plays a role.
Including that segment makes the graphs very difficult to decipher, and doesn't add to the discussion. My rotameter only starts registering at 0.5 SCFM, so there is measurement inaccuracy as well. I've ordered a low-flow rotameter to start sorting out those early variables, but that's a different discussion.
To answer your question, changing Venturi vanes and cracking effort changes the entry point of the reg onto the high flow part of the curve at 2 SCFM.

 

[RyanT]

Got it, makes sense, thanks!

 

[rsingler]

I think that some exhaust resistance is a very good thing.

I don't disagree with you, @Angelo Farina . If Dave Shaw had had a chest mounted rebreather, he might have survived. But that ignores his choice of sorb, and gas density at depth of >10 g/l.
And for recreational diving to the 121' I measured, it would have to be a really long vigorous swim in current before fatigue from exhalation work of breathing killed me from hypercarbia/CO2 retention.
However, I don't see an advantage at recreational depths to working harder than you have to. Positive pressure (PEEP) helps control pulmonary edema in the ICU, but I'm not sure expiratory resistance does nearly as well. As you hinted, I think breathing technique is more the key.
My biggest concern open-circuit is prolonging my dive. I have a slightly lower SAC with an easy breathing reg.

 

[Open Ocean Diver]

The first point on my curves is at 2 SCFM.
A LOT happens between 0 & 2 SCFM. Some regs immediately drop to 0.5", some rise and then come back down as Venturi kicks in. Lever friction plays a role. "Stiction" plays a role. Early turbulence plays a role.
Including that segment makes the graphs very difficult to decipher, and doesn't add to the discussion. My rotameter only starts registering at 0.5 SCFM, so there's measurement inaccuracy as well. I've ordered a low-flow rotameter to start sorting out those early variables, but that's a different discussion.
To answer your question, changing Venturi vanes and cracking effort changes the entry point of the reg onto the high flow part of the curve at 2 SCFM.

What is the scale of your low-flow rotameter that you ordered?

 

[Luis H]

The flow impedance of the simple mushroom exhaust valve is mostly proportional to the flow rate. There is no such thing as cracking effort (unless the valve is stuck or sticky) or anything similar to the inhalation curve.

Therefore, the resistance mostly increases linearly with flow and it becomes insignificant a very low flow.

The smaller valves only play a role during very high flow rates… Yes, I understand that is what the ANSTI test is all about high flow rate conditions, but my point is for that for normal breaths the “medium size” mushroom valves in a Scubapro 109 tend to be perfectly adequate (not the same with the real small early 108 valves).


BTW, I mentioned terms like “mostly proportional” and “resistance mostly increases linearly”, because there is some non-linearity in the design of the modern mushroom valves. Their geometry with the circumferential bend and the outside lip, is design to create a good seal, but they are also designed to elastically buckle and collapse when they start to flex. That design helps them move out of the way when there is some flow.

The design of the modern mushroom valve is very different to the early valves that were just a flat disc and just relied on material stiffness to keep them in place.


You can see that the exhaust portion of the ANSTI curve is not following a pure sine wave (like the driving flow function), but it is still proportional to the flow.

 

[rsingler]

You can see that the exhaust portion of the ANSTI curve is not following a pure sine wave (like the driving flow function), but it is still proportional to the flow.

Thank you, Luis!
Once more, an engineer reminds us that the ANSTI loops that give manufacturers their bragging rights bear little relation to diving reality.

All the new work on reducing exhalation WOB is of zero benefit on 90% of dives. And @Angelo Farina 's chrome beauties are prettier than my D420, I'll admit.

 

[Open Ocean Diver]

The flow impedance to the simple mushroom exhaust valve is mostly proportional to the flow rate. There is no such think as cracking effort (unless the valve is stuck or sticky) or anything similar to the inhalation curve.

Therefore, the resistance mostly increases linearly with flow and it becomes insignificant a very low flow.

The smaller valves only play a role during very high flow rates… Yes, I understand that is what the ANSTI test is all about high flow rate conditions, but my point is for that for normal breaths the “medium size” mushroom valves in a Scubapro 109 tend to be perfectly adequate (not the same with the real small early 108 valves).


BTW, I mentioned terms like “mostly proportional” and “resistance mostly increases linearly”, because there is some non-linearity in the design of the modern mushroom valves. Their geometry with the circumferential bend and the outside lip, is design to create a good seal, but they are also designed to elastically buckle and collapse when they start to flex. That design helps them move out of the way when there is some flow.

The design of the modern mushroom valve is very different to the early valves that were just a flat disc and just relied on material stiffness to keep them in place.


You can see that the exhaust portion of the ANSTI curve is not following a pure sine wave (like the driving flow function), but it is still proportional to the flow.

And I can say it works beautifully when you have to give up your breakfast during a morning dive.

 

[Geo7]

Oh, no I can't believe I missed the whole show (was monitoring the regulator forum)...

My question is whether there is a trend towards flatter curves in newer regs (S620ti, S270, D400, Atomic) as opposed to the original high performers like the classic G250? Did those perhaps cross over into positive flow in a more aggressive manner (something I like)? "Well behaved" regs with hardly any crossover into positive venturi flow leave me wanting more. For the most part I can achieve that by tuning hot.