Referring to the "scum balls" and free flows:
In our most overloaded classes, I'll assist an instructor with 8-10 students. On our checkout trips, the most an instructor ever has is 6 students (plus at least one DM). While there are certainly entries in which no ball-induced free flows have been present, I can recall one or more cases for every weekend class or checkout trip I've worked. Quite often, more than one student has a dribbling free flow. (It should go without saying that the rate would necessarily be higher if not for those entries in which the octo is not retained by the ball.)
Whenever one presents, I'll point it out and solve it with the student (by popping the octo out, letting it and the ball flood, and putting it back together without the air column), and the next time they see it, most students will solve it themselves. Still, why would you use a system with such an in-built flaw? (Then again, I'll change an O-ring that just looks old, while others may be content to dive with a slowly leaking one. I may just have higher standards. :biggrin
As for the free flowing, it's just simple physics. If the ball and mouthpiece form a seal (as is intended), and the internal space is filled with air, you will necessarily have a pressure difference across the diaphragm equal to the height of the air column from the diaphragm to the hole at the top of the ball. If you detune the cracking pressure on the octo enough to prevent a dribbling free flow given that pressure difference, the octo will breathe like a balled-up sock. If you don't detune the octo to the point it breathes like a balled-up sock, the pressure difference will be great enough to cause a free flow.
That doesn't surprise me. It's not like they *always* cause free flows. I'm not going to pull an alleged statistic out of thin air, but I can at least give an idea.
In our most overloaded classes, I'll assist an instructor with 8-10 students. On our checkout trips, the most an instructor ever has is 6 students (plus at least one DM). While there are certainly entries in which no ball-induced free flows have been present, I can recall one or more cases for every weekend class or checkout trip I've worked. Quite often, more than one student has a dribbling free flow. (It should go without saying that the rate would necessarily be higher if not for those entries in which the octo is not retained by the ball.)
Whenever one presents, I'll point it out and solve it with the student (by popping the octo out, letting it and the ball flood, and putting it back together without the air column), and the next time they see it, most students will solve it themselves. Still, why would you use a system with such an in-built flaw? (Then again, I'll change an O-ring that just looks old, while others may be content to dive with a slowly leaking one. I may just have higher standards. :biggrin
As for the free flowing, it's just simple physics. If the ball and mouthpiece form a seal (as is intended), and the internal space is filled with air, you will necessarily have a pressure difference across the diaphragm equal to the height of the air column from the diaphragm to the hole at the top of the ball. If you detune the cracking pressure on the octo enough to prevent a dribbling free flow given that pressure difference, the octo will breathe like a balled-up sock. If you don't detune the octo to the point it breathes like a balled-up sock, the pressure difference will be great enough to cause a free flow.
