Why would one increase cracking pressure to spec on a primary second stage?

[Wiznutaggie]

Just went through and serviced my regs (Atomic) for the first time, and a question occurred to me. Once you have the orifice set, why would you ever turn in the spring pad to increase cracking pressure to the 1.x"-1.x" range that is "spec" on your primary second stage? I can understand de-tuning the octo slightly, but why the primary? After setting the orifice, I've got my reg sitting around the 0.5" range on the manometer and just wondering why I would basically double the breathing effort to spec.

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[formernuke]

I'm new to servicing myself but my understanding is to prevent unwanted free flow.

 

[JohnN]

Just went through and serviced my regs (Atomic) for the first time, and a question occurred to me. Once you have the orifice set, why would you ever turn in the spring pad to increase cracking pressure to the 1.x"-1.x" range that is "spec" on your primary second stage? I can understand de-tuning the octo slightly, but why the primary? After setting the orifice, I've got my reg sitting around the 0.5" range on the manometer and just wondering why I would basically double the breathing effort to spec.

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When I took the DRIS HOG regulator service class (which required using a Magnahelic to confirm the cracking pressure) I asked the instructors if this was how they tuned their personal regulators. . .

Of course, their answer was no for their personal gear, but absolutely if they were doing this as part of a paid regulator service.

So unless you're doing this for hire, tune your second however you wish. As long as it doesn't free-flow, you're good

 

[rsingler]

Search for the term "Case Geometry Fault" here on ScubaBoard. That will answer your question.

In the face down position, the top edge of your exhaust valve is higher in the water column than your diaphragm by 0.75-1.0". Ambient water pressure is thus pushing on the lever by that amount. Since that amount is greater than the 0.5" at which your valve seals, your reg will dribble a bit of freeflow every time you look down.
In addition, though it is less likely with Atomic and its seat-saver feature, as the seat takes a "set" (gets indented), your cracking effort will drop another 0.1-0.3". Pretty soon, any Scubapro or Aqualung regulator tuned like yours will freeflow all the time.

 

[johndiver999]

Search for the term "Case Geometry Fault" here on ScubaBoard. That will answer your question.

In the face down position, the top edge of your exhaust valve is higher in the water column than your diaphragm by 0.75-1.0". Ambient water pressure is thus pushing on the lever by that amount. Since that amount is greater than the 0.5" at which your valve seals, your reg will dribble a bit of freeflow every time you look down.
In addition, though it is less likely with Atomic and its seat-saver feature, as the seat takes a "set" (gets indented), your cracking effort will drop another 0.1-0.3". Pretty soon, any Scubapro or Aqualung regulator tuned like yours will freeflow all the time.

Yes, for me, it is a very uncomfortable feeling to have a regulator that is always at the verge of a freeflow. It makes me insecure, if it starts blowing air or falls out of my mouth, I could lose a lot of air quick. The adjustment knob goes a long way toward addressing this issue, but if it was a choice, I would rather have a slightly tighter reg that won't make any unexpected moves on its own.

 

[BoltSnap]

why would you ever turn in the spring pad to increase cracking pressure to the 1.x"-1.x" range that is "spec" on your primary second stage?

Because of the "instability" of the second stage that can happen, and will happen, with cracking pressure that low. It will start by bubbling every now and then and then develop to bubbling most of the time when looking down or any time you turn your head or face into the current. Way back I started to tune it down more than what the specs said and I learned that specs are a good thing to prevent issues when least expected or desired. For customers or my own, I may tune it down to the lowest spec with the knob all the way out but never less than what the specs say.

 

[Wiznutaggie]

Search for the term "Case Geometry Fault" here on ScubaBoard. That will answer your question.

In the face down position, the top edge of your exhaust valve is higher in the water column than your diaphragm by 0.75-1.0". Ambient water pressure is thus pushing on the lever by that amount. Since that amount is greater than the 0.5" at which your valve seals, your reg will dribble a bit of freeflow every time you look down.
In addition, though it is less likely with Atomic and its seat-saver feature, as the seat takes a "set" (gets indented), your cracking effort will drop another 0.1-0.3". Pretty soon, any Scubapro or Aqualung regulator tuned like yours will freeflow all the time.

Thank you! I understand now! I'm curious then why using a manometer/magnehelic gauge is the way people set up their regs. Wouldn't a more accurate way of doing it be to submerge the reg in a exhaust valve high position and tweak the spring pad just to the point of stopping freeflow after setting the orifice?

 

[rsingler]

Yes. But you're wet. I'm dry and sipping my beer.

Seriously, 0.1" one way or the other is not discernable. Magnehelic is quick with an inline adjuster. You learn pretty quickly which regs tolerate a "hot tune" and which don't.

Nice manometer, by the way!

 

[Tracy]

For mine, minimum to prevent freeflow. For customers, tune to spec.
I like your manometer.

 

[Nemrod]

Some people will set their regulators that have the adjustment knob such that the regulator will just barely, almost inaudibly, flow when the knob is turned fully out. Turning the knob inward increases cracking effort and stops the free flow. James