recomended depth limits for unbalanced regs

[stonedfish]

Hi to all. Can anyone give advice on what depth an unbalanced 1st stage will make a substantial difference in breathing (air volume and effort)? And at what depth it would be dangerous or unreliable? I dive to recreational depths using an unbalanced reg in warm water but would like to know how near the edge of my regs performance I get.

I am using a Sherwood brut 1st stage with a Scuba Pro G250 primary 2nd stage and Sherwood Brut octo.

Thanks in advance for any inputs.


Happy diving,

Stonedfish

[firediver293]

I've had my G250 to 100' without any problem. I have used it on a MK15 and an MK20.

I'm not a techie (yet). May take the plunge one day. But for rec diving I have never had a problem. I also have 2 G200's and a R190 that I have had that far down as well without issue. Although I like the 250 for the adjustability while diving.

Good luck.

[DA Aquamaster]

The issue of balanced versus unbalanced does not directly affect the depth at which the reg can no longer provide adequate air.

A balanced reg will maintain a more constant intermediate pressure at 3000 psi to 300 psi where an unbalanced piston reg will experience a 10-12 psi drop in imtermediate pressure as the tank pressure falls from 3000 psi to 300 psi. This will increase the cracking effort slightly as tank pressure falls. Conversely, an unbalanced diaphragm reg will actually increase its IP as tank pressure falls so oddly enough cracking effort is actually higher at higher tank pressures.

The determining factor with both unbalanced designs is actually the flow rate of the reg. In general an unbalanced piston first stage tends to have a rather low flow rate due to the way the design operates.

This is not neccesarily a problem with an unbalanced diaphragm reg however and many unbalanced diaphragm regs are very similar in overall design and flow rate to their balanced siblings. Some however are fairly low performance designs, balanced or not.

Using an adjustable second stage like the G250 will allow you to adjust the spring tension on the orifice to compensate for the changing downstream force thoughout the dive that occurs with an unbalanced first stage so you should be able to maintain minimum cracking effort through out the dive.

[rescuediver009]

I have no idea what the g250 does for it but I know that the brut is intended as a training regulator and not intended for use at depth. It is the only sherwood first stage that lacks the moving orifice. Never tried one at depth but Iknow that they are not too friendly even at the intermediate levels ( 50ft. ish). This may differ for your case with the scubapro through.

[DA Aquamaster]

If a Sherwood Brut does not dliver enough air at depth with its own second stage, a G250 is not going to make any difference as the problem most likely lies with the first stage's inability to deliver adequate air to the second stage.

[LUBOLD8431]

Well, to give some perspective here, the Oceanic SP4 unbalanced piston was very close to meeting the US Navy standards for 198 ft.
So, for recreational scuba diving to 130 ft, I dont think you would notice anything...

[stonedfish]

Thanks for your inputs michael, DA and rescuediver.

I replaced my Sherwood Brut primary with the SP G250 thinking that it would improve breathing effort. As it turned out, I could not tell the difference in breathing effort between it and my Brut octo when I tested at 60 fsw, even though the octo was using the non prefered low pressure port (tank pressure around 2000psi then). Both were serviced just before the dive and the technician may have tuned them to similar cracking effort. At low tank pressures after a dive, the difference in air volume delivered between the prefered and non preferred low pressure port is very noticeable when I depress their purge.

I have tried to overbreath the reg through the G250 at 60fsw but had no problem. I have gone down momentarily to 100fsw on the G250 with no problem as well.

I would like to know however if the regs performance will decline dramatically at a certain depth and at what depth that would be. If it turns out that the reg will not be able to safely deliver enough air at 150 feet, I will replace my first stage with a SP mk 20. I hold on to my Brut 1st stage because of the environmental sealing and reliability. But I have read from uk's Diver test that some regs will suddenly change performance at a certain depth. (http://www.divernet.com/gear/regs398/regtst10.htm). The aqualung micra and impulse did so near 50 meters even though both had balanced first stages. Their breathing effort jumped from good above 50meters to unacceptable at that depth.

I was wondering if the Brut has the same problem at an even shalower depth.

Thanks again for your replies.



stonedfish

[stonedfish]

thanks Lubold.

A question for DA, Which regulator models have unbalanced diapghragm 1st stages?



stonedfish

[paolov]

i have been using an unbalaced first stage and posed that questions before and as the posts here, it really doesn't matter much for recreational diving (130ft).

my 1st stage would be an old oceanic unbalanced 1st stage and have not experienced problems with it. I had it in 1997 and had it serviced once, just for the heck of having it serviced - no problems encountered. However i no longer use it as i have sort of retired the first stage.

the 2nd stage reg with the oceanic is now used as an octopus. my primary is a Viper, and the viper gives a better flow than the reg i was using with the oceanic.

i don't know if scubaworld still have them vipers on discounted sale.

[DA Aquamaster]

thanks Lubold.

A question for DA, Which regulator models have unbalanced diapghragm 1st stages?

I can't give much of an answer on that one - there are lots of regs out there.

If I remember correctly, Sherwood is one company though that offers balanced and unbalanced regs with very similar designs.

In many balanced diaphragm regs the only difference is a through hole in the soft seat carrier and a couple o-rings around the carrier to balance the tank air pressure on the seat. In those cases, the overall flow performance of the reg is essentially the same, the IP is just more stable.

In other balanced diaphragm designs, the balancing of the seat carrier allows the use of a much larger orifice, without the otherwise large change in IP that would result with an unbalanced design, and the flow rate can be greatly improved