CDX-5 overbalancing feature

[martzak]

Hi,

Can anyone tell me in actual numbers by how much the IP of Oceanic CDX-5 increases with depth.
On a bench the IP should be 138-142psi, so for example what is the IP supposed to be at 150ft, 200ft?

The reason I'm asking this is because my adjustable 2nd stage that breathed fine on the surface started free-flowing slightly, but enough to lose gas at a rate high enough to cut the dive short, at 175ft.

 

[DA Aquamaster]

Good question. Your observation is exactly why a technical diver would not want an "overbalanced" reg, but the concept keeps getting promoted as a good thing due to improved flow rate at depth.

Realistically, though if a reg really needs the minimal boost it would get from a 10% increase in IP, the diver needs to be using a better reg with more margin between what he needs and what it provides.

If a reg truly is over balanced, you need to know the change in pressure with depth so that you can properly tune the second stage. You would crank the IP up to a pressure equal to or slightly in excess of what it will be at your max depth, tune the second stage, then re-adjust the IP to normal.

Even if properly tuned, the down side of an "overbalanced" design is that the reg will breathe harder at shallower depths. The only way around that is to reduce or eliminate the down stream bias in the balanced downstream poppet, and that causes safety issues in the event your back up reg develops a leak in the HP seat during the dive.

 

[martzak]

Thanks for the tip. I'll make the adjustment as you prescribed. I just hope my second stage adjustment knob provides enough range to make the regulator breathable at shallow depths and 'free-flow free' in the depths.

 

[fppf]

You did not say where your diving, but if its in the great lakes I think its more likely the second stage started to freeze up at 175 feet than it started to crack due to an over balanced first stage.

Was it on the reg you where breathing or the backup? Do you have a very slow IP creep?

 

[herman]

I am glad to see you posting on this DA, I am in a learning mode about overbalancing. I was trying to find some info on "overbalancing" the other day and come to the conclusion it was more hype than anything. Assuming you are using a balanced second stage, would that not solve the tuning issues? Unless I am missing something, compensating for IP swings is what the balancing of the second stage is for so it should handle several extra PSI without a problem. I am assuming the IP does not go past the range the second stage can handle. On a more practical note, I fail to see what increasing the IP would really do for you. The second stage would flow a little more gas at full flow, but I doubt a diver could use that much gas anyway so what would it matter if you can get a few more CFM out of the reg.

 

[PhilEllis]

Actually, I don't think the CDX5 is "overbalanced". It does compensate for depth, but then again I believe all first stages do this. Basically, the "gauge" pressure goes up about 15 PSI for each atmosphere of depth.

Phil Ellis
Discount Scuba Gear at DiveSports.com - Buy Scuba Diving Equipment & Snorkeling Equipment

 

[martzak]

Straight off the Oceanic website:
"The CDX-5 high performance over-balanced first stage provides progressively greater intermediate pressure as depth and gas density increases."

So, is it 15psi with each 1atm of depth starting at the surface????
If that's the case, then on the surface the IP would be/is ~140psi, at 33ft 155psi....at 132ft IP would be 200psi...no offense, but I have some reservations about the 15psi IP increase with each additional 1atm of depth.
At any rate, I'm in the process of tuning my 2nd stage to handle higher IP.
I'll post the results.

 

[martzak]

One thing I forgot to mention was that the 2nd stage was leaking slightly on the surface when adjusted to full open indicating that the 2nd stage was just to sensitive for the 1st stage IP.
I adjusted the 2nd stage so that it breathes 'acceptably' when fully open at 140psi, which is my 1st stage IP. I didn't play around enough to see at what 'new' IP the 2nd stage would start free flowing when fully open, but I'm fairly confident it won't free flow now at depth.
I'll find out soon.

 

[fppf]

The CDX 5 is a over balanced first stage
The reason for over balancing is because as pressure increases so does the density of the gas. The human body is remarkably sensitive to changes in pressure and breathing effort. The small increase in IP is to compensate for the density increase.

It sounds like your second stage was adjusted to factory specs. It should just barely crack when the adjustment knob is fully backed off. I would not suggest that anyone adjust there regulator unless trained to do so. This is life support equipment.

I don't know what the over balance factor is, but I highly doubt its 15 PSI per 1 ATM. I have seen third party reviews where regulators are tested to 200 feet. I'm sure oceanic has tested there over balanced design far beyond the depths you would go.

I also know a few people using the CDX-5 in tech set to factory specs.

 

[martzak]

The 2nd stage I'm using is a scubamax, not oceanic, so there is really no reason the two stages are optimized for the same IP.
Adjusting IP on either 2nd or 1st stage is far from rocket science and with a little bit of reading and access to technical manuals, which I do have in my possession for both stages, almost anyone with an opposable thumb can perform the IP adjustment.
As far as the 'trained technicians' are concerned, I can give you plenty of examples of post-service regs done by 'trained technicians' that malfunctioned, usually free flowed, on the first dive.
At any rate, I'm not planning on tearing down and rebuilding any of my regs any time soon.

By the way, I was hoping the Oceanic rep would chime in here...where is he/she???