Freeflowing Reg

[Lemonade]

...So why would regs free-flow at depth?
--Marek

That I do not know.

 

[Kestrell]

OK, at the risk of getting just a little bit off-topic... I understand why a regulator free-flows... it's because of the pressure difference between the diaphragm and mouthpiece, cause by even the couple inches of depth difference when the mouthpiece is pointed up.

But why (in contrast to Morpheus) have just about all of my free-flows happened on the surface? Since water pressure increases linearly, the delta-pressure should always be the same at all depths.

My guess is that, on the surface, free-flows usually get started when the reg is dropped into the water diaphragm-down -- the smack against the surface pops the valve open, and it just goes from there. Yes?

--Marek

You answered your own question. Yes this is probably the case.

 

[Kestrell]

I have and M1 and it has done this before. I just dialed back the adjustment knob a bit and it hasn't happened since.

 

[lairdb]

OK, I see what you're saying (I think)... the two inches of air are still practically at 1 atm; while the water two inches down would be at significantly higher pressure. (My rough estimate is that the difference would be just over one ounce per sq in... does that sound right? Enough to pop the diaphragm?)

I believe that to be correct. The measurement "cracking pressure" (pressure differential needed to begin opening the valve) is measured in inches of water and a typical factory spec might be "1 1/4 to 1 3/4 inches".

And it also seems to make sense now that in a flooded reg, there should be no measurable difference in water pressure just above the diaphragm (inside the housing) versus outside -- assuming water can move freely in and out of the housing. So why would regs free-flow at depth?

I'm theorizing here, but either:
- the purge button gets bumped, or
- a really strong current (natural, or induced by movement) has the same effect,
which causes the chamber to be full of air, leading back to the original situation.

Both situations are probably exacerbated by the clever designs in moden regs that use the airflow to help keep the valve open ("venturi assist", etc.)

 

[eponym]

And it also seems to make sense now that in a flooded reg, there should be no measurable difference in water pressure just above the diaphragm (inside the housing) versus outside -- assuming water can move freely in and out of the housing. So why would regs free-flow at depth?

I've seen it happen in cold water (near freezing), because air flowing out of the tiny orifice expands and therefore cools. That chills the mechanism further, down to freezing, and frozen water can hold it open. More likely to occur if you take the reg out of your mouth, as it's no longer being warmed by your exhales.

If your shop has any junked and disassembled second-stages, have a look at one. You'll be surprised how simple the mechanism is.

Fin on,
Bryan

 

[hecker-the-wrecker]

If the reg was brand new, untouched out of the box, you might want to have it tuned by your lds, my brother in law had the same problem with his reg. He tried to use his adjustment knob, nothing helped.(continued to freeflow) The tech at the dive shop told him it's common for regs to do that "out of the box". After the tune, it has worked perfectly.(no freeflow) hope this helps!!! His reg is an atomic.

 

[Hallmac]

If the reg was brand new, untouched out of the box, you might want to have it tuned by your lds, my brother in law had the same problem with his reg. He tried to use his adjustment knob, nothing helped.(continued to freeflow) The tech at the dive shop told him it's common for regs to do that "out of the box". After the tune, it has worked perfectly.(no freeflow) hope this helps!!! His reg is an atomic.

Seat wear and seat break in also play a part in it.

Hall

 

[rescuediver009]

So why would regs free-flow at depth?

--Marek

There is usually a little bit of bubbles trapped inside the reg housing. Bubbles always want to go up. As soon as a reg is tilted in a method that facilitates this, the bubbles create a bit of a vacuum effect taking the diaohragm with them. Then the reg is so finely tuned that it locks in that position.

At the surface it is because the bubbles trapped on the outside of the purge are force up but th water pressure again causing a freeflow.

 

[DA Aquamaster]

And it also seems to make sense now that in a flooded reg, there should be no measurable difference in water pressure just above the diaphragm (inside the housing) versus outside -- assuming water can move freely in and out of the housing. So why would regs free-flow at depth?

The difference in pressure across the diaphragm is what normally open a valve in an air filled second stage. In a flooded second stage this does not happne as the pressure is equal on both sides of the dipahragm. But anything that moves the diaphragm can cause the valve to open. This could be a strong current pressing on the purge cover, a rapid movement of the reg, something bumping the reg, etc. Once the air starts flowing, venturi effects can keep them going.

Or, it could be an adjustment issue with the reg. A second stage involves a balance of air pressure and spring forces, so if the second stage is slightly out of adjustment a slight leak can occur as the spring force is not quite enough to prevent the intermediate pressure air from slipping past the orifice and seat.

A very, very small freeflow over the course of a dive will eventually fill an unused second stage with air (your octo for example) and as some air eventualy leaves the second stage, it can open the valve further and sustain a slight freeflow.

With your primary, a slight freeflow will occur if the cracking effort is lower than the difference in height between the center of the diaphragm and the upper edge of the exhaust valve in any given orientation. The technical term for this is "case geometry fault".

If this difference in height is, for example, .7 inches in a normal swimming position and the cracking effort of the reg is only .5 or .6 inches of water, then the difference in pressure that develops as air naturally slips out the top of the exhaust valve will be sufficient to open the valve slightly and continue to feed the freeflow out the exahust valve.

The solution is to increase the cracking effort slightly so that the reg is stable and does not freeflow. On a non-adjustable second stage of normal design you also have to adjust the reg so that it will not freeflow in a worst case scenario with the reg pointed straight down. In that case the difference in height may be as much as 1.3 inches or so and the cracking effort would need to be at least 1.3+ inches to prevent a freeflow when the diver is face down. With an adjustable reg, then diver can crank up the cracking effort during the dive as needed to prevent a freeflow in different attitudes. This allows a lower cracking effort to be used in a normal swimming position (.7 fr example) yet enables the diver to crank it up (to perhaps 1.4-1.6 inches) if they are in a face down position, for example in a rapid face down decent where the descent rate also essentially amounts to a current pushing against the diaphragm.

To make it worse, a new second stage seat will acquire a set over time. It will acquire his set while on the shelf over a period of days or weeks or in a matter on hours if the reg is pressurized. So the tech has to anticipate for this and set the orifice in the second stage slightly farther into the second stage and more firmly against the seat than is needed to get a desired cracking effort. The orifice is normally set in about 12th turn more than is needed for a given cracking effort. Sometimes the tech gets it wrong when finely tuning a reg and the reg will freeflow after a few weeks or a few dives.

Ideally the tech will be able to either let the reg sit for a week or two on the shelf before making the final adjustments or will be able to leave the reg pressurized overnight before making the final adjustments. In many cases however, this is just not practical for a tech or dive shop to do.

The alternative is to set the cracking effort higher than is optimum for peak performance to ensure no freeflow will develop. The reg will breathe harder, but it won't free flow after the seat acquires a set.

About the only time I do this though is if someone brings a reg in for a fast service just before leaving on a dive trip (really bad planning on their part). If it absolutely can't free flow (as it would screw up the trip) they are going to get a guarenteed reliable but heavier than optimum breathing regulator. I'll lighten it up for them after they get back as the seat will be broken in by then.

Adjustable regs are a real advantage as they can be fine tuned by the diver to correct for case geometry fault effects and any seating effect that occurs. And they make the tech's job a lot easier.

 

[whitedragon13]

On surface you have water on one side of the diaphragm, air - on the other, the difference in pressure of water vs. air is what pops it open. In contrast if you fill you reg with water and submerge it even with diaphragm down it should not open, since the delta is negligible.

Right on the money. In fact, you can use a bucket of water when adjusting regulators...it shouldn't take "inches" to cause a freeflow on the surface...is should only take "milimeters" for a well tuned reg...well, maybe centimeters.