Why would a reg become difficult to breathe from?

[MrChen]

Last week on a charter, my high pressure SPG hose failed prior to doing the first dive. The crew handed me what looked like a well maintained rental reg set. I believe they were AquaLung regs (could be wrong), and they looked pretty new (no signs of wear or tear). They weren't anything fancy, just a knob to set pre-dive and dive. Dive 1 to 91 feet, zero issues. Dive 2, descending down the rope, at about 40 ft, I suddenly had to really work to get a breath. I did this two times and did get air, but it wasn't sustainable. Think of a large sucking sound to breath. I had the knob on dive and rotated it forward to pre-dive. Just like that, I could breathe again. Since it wasn't my gear and the dive depth was going to be 90 to 100, I opted to end my dive and headed back to the surface. I didn't panic and had a pony bottle. If the reg would have failed or continued breathing hard, I would have just switched to my pony. I've never had my personal regs do this. Could this be caused by the knob not being fully to the dive position, maybe it was bumped slightly from being turned full to dive? I didn't try to turn the knob back to dive once I was able to breath normally.

An additional thought, what if this reg was used by a student and they ran into the same issue? I could totally see them panicking. I did let the crew know why I aborted the dive and reminded them when I handed the reg back to them.

 

[EdC]

Did it breathe ok at shallower depths on the way down? Is it possible your tank wasn’t fully open, sounds like that may be it?

 

[scubadada]

The predive/dive lever simply adjusts the venturi assist. It should not have a profound effect and should generally make breathing easier in the dive position. I can't imagine what was wrong with your regulator, maybe someone else will know.

 

[MrChen]

Did it breathe ok at shallower depths on the way down? Is it possible your tank wasn’t fully open, sounds like that may be it?

My tank was fully open. It was breathing fine until suddenly I had to forcible suck air. I instantly stopped on the rope when this happened. Literally all I did was turn the knob forward and was breathing again. I verified after returning to the boat that forward was pre-dive. I was moving down the rope pretty fast. There were divers coming up and I crawled right past them, head down, feet towards the surface. There was a pretty healthy current. Around 15ft, my mask almost got knocked off by another divers' bubbles, so I wanted to get through them pretty quickly. 20 feet past them is when I ran into the problem.

 

[MrChen]

Here's my dive profile. You can see where the descent slows to green, dark green, part of that is probably my body changing orientation. My perdix 2 told me I had a 3 min SS obligation, so I followed it.

 

[rhwestfall]

Is there a regulator doctor/master/whisperer in the house?

@rsingler

 

[Mobulai]

would you happen (I know, asking for a lot here) to know if it was a donwstream or a blanced reg?
I have a tiny suspition something tiny is stuck somewhere inside that reg

 

[rhwestfall]

Is the dive/pre-dive adjustment on that reg just an adjustable venturi vane (induces turbulence in the flow path), or does it do more (mechanical)?

 

[Mobulai]

Is the dive/pre-dive adjustment on that reg just an adjustable venturi vane (induces turbulence in the flow path), or does it do more?

are you thinking about the one that has both the poppet spring adj and venturi in one?

 

[rhwestfall]

Stuff like the Pilot, Air1, D300/350/400....

Don't know if others new stuff might do things mechanically to the regulator's tune...