Regulator Free Flow Question

[diver3523]

Hello All:

I wanted to put a question out to the body of knowledge on the board here, as its proved invaluable in dozens of other instances. This concerns my recent purchase and use of a Scubapro R390/MK16 setup. When I first took the setup to the pool for testing after purchase, I encountered free-flow every time the 2nd stage was inverted (mouthpiece up). It also occured once when the reg was in my mouth. I had the flow set to "Dive" on the adjustment tab.

I took the mouthpiece into my LDS, where I asked them to take a look at it (I was thinking this free-flow was unusual and wasn't supposed to happen). The tech at the shop "played with the settings and couldn't get anything to happen." I'm assuming that meant he couldn't simulate the free-flow that I experienced in the pool. I also consulted with another LDS where the resident tech mentioned that this free flow was not only normal, but "a matter of physics" in terms of the way the 2nd stage is designed and based on the settings of my specific setup.

So, here is my question: is this free flow I experienced normal? Should it occur with the setup I have? Could someone explain the "physics" of why this is happening? Is this free-flow specific to setting the flow adjustment to "Dive" vs. "Pre-Dive" on the 2nd stage?

Thanks to all!

 

[akscubainst]

Just about anytime you drop them purge first with the VIVA set to Dive it will freeflow it's normal. The Diaphram floats up and pushes the lever causing the freeflow. Just turn it over or plug the mouthpiece hole with your thumg.

In fact if you put the reg in air and set the VIVA to DIVE and push the purge button it should by specification, freeflow.

IMO, Set the VIVA to predive and leave it there, it eliminates alot of freeflow and the VIVA doesn't really do that much anyway. I would be surprised if on an R380 you see a big difference in the Dive vs predive setting.

Dave

 

[redacted]

Sounds like your reg is well tuned. For good breathing, the pressure it take to start airflow, called cracking pressure, should be about 1 inch of water. That means that when you submerge your 2nd with the mouthpiece pointed up, when it gets to be about 1 inch deep the air should start flowing. With your VIVA switch in the max position, the airflow is directed right at the mouthpiece with notheing slowing it down. This actually creates a slight vacuum in the area of the diaphram resulting in the strong freeflow. With some high performance regs it may feel like it is pushing air into your lungs. Moving the VIVA to the min position putes a deflector in the way of this flow and prevents (discourages) the strong freeflow. If you can slowly submerge your 2nd with the mouthpiece pointed upwards without initiating airflow before the water pours into the mouthpiece, then cracking pressure would be around 2 inches of water which would be uncomfortably hard breathing.

 

[DA Aquamaster]

A freeflow occurring with the reg in your mouth while face down or inverted is due to a concept called "case geometry fault". Essentially, the difference between the average depth of the diaphragm (ie the center of the diaphragm) and the highest point of the exhaust valve changes as the second stage rotates in different attitudes.

When you (and the second stage) are face down for example, you have a worst case scenario where the exhaust valve is over an inch shallower than the diaphragm. So since the air in the second stage case is pressurized to equal the pressure at the diaphragm when the poppet closes, (ie: 1.0 to as much as 1.4 inches more water pressure than is acting on the exhaust valve) air will leak out the exhaust valve until the pressure in the second stage is equal to the pressure at the upper ends of the exhaust valve.

However this will cause the pressure to drop in the second stage which in turn causes the diaphragm to be forced up by the greater pressure beneath it. If the cracking effort for the second stage poopet is set too low (less than about 1.0 to 1.4" of water) the movement of the diaphragm will be sufficient to open the poppet slightly. This will cause air to flow into the second stage and provide air for a continuous freeflow out the exhaust valve. Very annoying and hard on the SAC rate.

So while most poppet designs can produce cracking efforts as low as .8 to 1.0 inches of water, most standard regulator case designs cannot use this low cracking effort as they will freeflow when face down.

With an adjustable reg like the G250 or S600, cracking effort can be adjusted in the water by the diver to enable them to use lower cracking efforts in positions where the difference in depth between the center of the diaphragm and exhaust valve is less.

But with a non adjustable reg like the R190 or R380/R390, the cracking effort has to be set adequately high to prevent this from occurring in the worst case situation. Consequently, on the R190 and R390 minimum allowable cracking effort this is 1.2" of water.

This was the big advange of the D400. It had it's exhaust valve coaxially mounted in the center of the diaphragm so the greatest difference in depth was only the radius of the exhasut valve, approximately .5". So cracking efforts as low as about .6" were possible with no need to adjust the reg for different positions.

The unbalanced first stage on the Mk 2 can also contribute to the problem. If the reg is tuned with a supply pressure less than the maxium that may be encountered by the diver, the reg may freeflow slightly at higher tank pressures. I always used to have problems with my Mk 3 for this reason as the shop would tune it on a shop air tank at less than full pressure and it would then freflow slightly when attached to a full tank. It the impetus for me to learn how to tune ny own regulators.

 

[diver3523]

Hey Guys,

Thanks for the helpful info. It is comforting to know that what is happening is expected.

Safe Diving!!