What causes shuddering at low tank psi?

[kidspot]

Yesterday I got a little distracted while shooting my first lift bag and by the time I'd finished my safety stop my tank was down to 250psi - I noticed that when it dropped below about 350psi the 2nd stage reg started shuddering - it still delivered air, just shuddered about every fifth breath. I am using a Kirby Morgan Superflo 2nd stage on a DiveRite 1st stage with a 7 ft. hose.

Would adjusting the 2nd stage knob all the way out have fixed that? (I didn't think about that till after the dive)

It breathes better than any other reg I've tried so far (3 Scubapros, 1 DiveRite, 1 Abbysmal, and 1 Atomic), just curious what causes this effect.

Tim

 

[Diver0001]

Yesterday I got a little distracted while shooting my first lift bag and by the time I'd finished my safety stop my tank was down to 250psi - I noticed that when it dropped below about 350psi the 2nd stage reg started shuddering - it still delivered air, just shuddered about every fifth breath. I am using a Kirby Morgan Superflo 2nd stage on a DiveRite 1st stage with a 7 ft. hose.

Would adjusting the 2nd stage knob all the way out have fixed that? (I didn't think about that till after the dive)

It breathes better than any other reg I've tried so far (3 Scubapros, 1 DiveRite, 1 Abbysmal, and 1 Atomic), just curious what causes this effect.

Tim

Hi Tim,

Does the 2nd stage make a distinct shuddering or snorting noise or is it something else?

Generally speaking the shudder is caused by a vibration in the valve assembly (either in the first or second stage--hence my question) that causes the valve to open and close quickly. In your case you might be getting it because of teh combination of the long-hose combined with the pressure drop in the first stage.

As you breathe in the resulting pressure drop in the in 2nd stage hose could be causing the 2nd stage to close, which causes the pressure to build in the hose, which causes the 2nd stage to open which causes the pressure in the hose to drop and so on.... If you think about that happening 40 times a second then you get shudder.

What you could do about it is to turn off the venturi when tank pressure is low and see if that helps. Failing that you might be able to make it stop by shortening up the hose or swapping out the first stage with one that doesn't experience the same kind of pressure drop.

It's kind of like one of the mobiles you hang above a baby's bed, change one thing and the whole system is affected. Making it stop will take some experimentation.

R..

 

[kidspot]

Hmmm - as far as I could tell it felt like the 2nd stage was vibrating very slowly. Kind of reminiscent of a subwoofer that has feedback. Shudder seems like the most accurate description of it - 40 times a second seems to be about right also (per your description)

Does that clarify it enough? I'll try to remember more details as the day goes by, but it's off to work for now.

Thanks
Tim