Cracking Pressure

[Fishpie]

Effort needed to start gas flow in 2nd stage.
One part of the whole breathing circle, venturi boost, exhalation effort and flow rate should also be considered.

I really don't want to criticize SL27 but CP is not really that good of an indication of a regulators performance.
To achieve the lowest cracking possible CP a reg would want:
Big flexible diaphragm.
Spring only just strong enough to hold back IP.
Soft seat.
Sharp orifice.

CP is just one indicator of a regulators performance......

 

[redacted]

Effort needed to start gas flow in 2nd stage.
One part of the whole breathing circle, venturi boost, exhalation effort and flow rate should also be considered.

I really don't want to criticize SL27 but CP is not really that good of an indication of a regulators performance.
To achieve the lowest cracking possible CP a reg would want:
Big flexible diaphragm.
Spring only just strong enough to hold back IP.
Soft seat.
Sharp orifice.

CP is just one indicator of a regulators performance......

For many 2nds, CP is the only adjustable parameter so it is a fairly good indication of the quality of the service job. As some 2nds are unable to sustain stable low cracking pressure (most can be adjusted fairly low) it is also an indicator of the quality of the engineering and manufacturing.

 

[Nemrod]

The new Argonaut Kraken has the lowest cracking effort of any regulator I have ever used except for a Phoenix AM, they are equal there. it also has the lowest exhaust effort of any regulator I have ever used. That is my initial, subjective, impressions. I may adjust that statements slightly when I get it on the bench and test these parameters.

N

 

[halocline]

What or who is SL27?

 

[LeadTurn_SD]

What or who is SL27?

I'm guessing the 12th post in this thread is what Fishpie is referring to:

http://www.scubaboard.com/forums/regulators/470594-not-lot-talk-about-oceanic-regs-why.html

Best wishes.

 

[Fishpie]

There are several very basic unbalanced cheap regs that I can tune for a very low cracking effort only to find that I have to de-tune because of case geometry.
Similarly there are performance balanced 2nd stages that really don't need to be tuned to the nth degree since they have a weaker spring and will have a low cracking pressure even if poorly tuned.

....my (poorly put) initial point was that cracking effort has much to do with how it is tuned and is only a small indicator of actual performance.

 

[halocline]

It's mostly subjective when it comes down to it. Yes there are WOB measurements but ultimately it's the divers' experience that matters. For example, WOB numbers are not that impressive on a D series reg, but once you dive with one it's a very different type of feel than a conventional 2nd stage. Same thing, only more so with doublehose regs, because of the position sensitivity, the fact that air depressurizes several inches from your mouth rather than a fraction of an inch, etc.

 

[tech_diver]

Cracking pressure is only a part of the reg's performance. You also have to consider flow rates (how many volume units of gas can flow through the mechanism once it's open.) Most reg performance today is expressed in 'work of breathing' measured in units of energy expended per volume unit of gas. This takes into account, cracking, flow rate and exhalation resistance.

 

[axxel57]

I think that it is often overlooked that Cracking Pressure and WOB are something quite different.


CP just shows the minimum effort to open the 2ndstage valve, but it doesn’t show us the inhaling effort over the course of a full breath.


That’s a pretty different thing I think.


I have been always amazed how low I could adjust the CP in almost any for example Sherwood Regulator (sometimes to 0,7 inch/h2o). But if I take a full breath, they breathe remarkably bad, probably also because the Venturi is kicking in quite late. Maybe they breathe so poor also because the lever advantage seems to be smaller than in models of other manufacturers.


One criteria for me, apart from my subjective feeling when taking a breath, to determine if a regulator breathes good or not, is to watch the CP while doing the FlowTest.


Most regulators will increase the inhaling effort while under flow to 1,5 to 2,0 inch/h2o before the Venturi Assist kicks in, lowering the breathing effort until the Venturi Override.


Halo mentioned the SP D – Serie, and in fact, a well tuned D –Serie regulator will often stay steady under 1,0 inch/h2o inhaling effort in the course of the flow test, indicating for me an excellent performance.


I think it’s a big shame that SP didn’t solve their problems with the exhale effort in the D – Serie.
They just make for all their newer models the exhaust valves bigger to look good in the ANSII tests, instead of going for a smoother breathing as they already had it with the D – Series.


BTW, does somebody here has a clue who had why the stupid idea to value the inhaling effort in the ANSII tests 50 – 50 with the exhaling effort?


I mean, does somebody here has heard of a diver who had cancelled his dive because of a too high exhaling effort?


I always found it ridiculous that nowadays almost all manufacturers are just working on a better exhaling effort ( for example with bigger exhaust valves) instead trying to reach the qualities of the D – Series concerning the inhaling process.


Up to now I have not found any ANSII test of whatever 2^nd st age with inhaling effort pattern like the D400 here.

View attachment g250-d400 wob.pdf


That is another reason why for me the SP D – Series are the best 2^nd stages ever built (haven’t tested my Pilot yet).