Pressure drop of first stage

[hmoffatt]

Hi,
I'm looking at the specs for Oceanic regs and the first stage data includes pressure drop. For the high end FDX-10 it is 9 PSI while for the next highest model CDX-5 they quote 20 PSI.. what's this mean? I didn't see it in other manufacturers' data?

Apart from materials that's the only difference?
Trying to decide if a Delta4+FDX-10 is worth the money over GT3+CDX-5?

thanks
Hamish

 

[Hallmac]

Hi,
I'm looking at the specs for Oceanic regs and the first stage data includes pressure drop. For the high end FDX-10 it is 9 PSI while for the next highest model CDX-5 they quote 20 PSI.. what's this mean?

The pressure drop is the amount of intermediate pressure lost during a breathing cycle.
It relates to how well a regulator can perform.

For example, let's say you have an intermediate pressure of 138psi. As the second stage opens a pressure drop is observed. The peak of demand and highest flow rate is the pressure drop.


A 9 psi drop means during a breathing cycle with the highest demand the intermediate pressure will drop to 129psi. A unit with 20psi will drop to 118psi.

The lower the pressure drop the higher the performance. The regulator is able to provide more air consistently. High numbers indicate a poor performance.

Extended ranges, alternative gasses, and heavy work loads all require a regulator with high flow capacity and the ability to maintain the delivery rates.

I hope that helps.

 

[redacted]

In this case, I suspect it really means they want you to buy the FDX-10 rather than the CDX-5. While less pressure drop is probably better, all else being equal, it may well be an entirely meaningless parameter for practical purposes. The work of breathing (WOB is probably a much more meaningful parameter and even that needs to be used with caution as there are ways of achieving low WOB numbers that make the regulator uncomfortable to use.

Plus pressure drop measurements are very dependent on the location and the responsiveness of the measuring instrument. So the parameter may well not be validly comparable across different regs.

 

[PhilEllis]

Hi,
I'm looking at the specs for Oceanic regs and the first stage data includes pressure drop. For the high end FDX-10 it is 9 PSI while for the next highest model CDX-5 they quote 20 PSI.. what's this mean? I didn't see it in other manufacturers' data?

Apart from materials that's the only difference?
Trying to decide if a Delta4+FDX-10 is worth the money over GT3+CDX-5?

thanks
Hamish

I am not sure that a diver would ever recognize the performance difference between these two regulators, or for that matter, any two random regulators of any brand, with the same difference in pressure drop during the breathing cycle.

The balance chamber of the FDX-10 is larger than the balance chamber of the CDX-5 first stage. In addition, the effective surface area of the pressure diaphragm is larger on the FDX-10. This is likely to explain the difference in pressure drop for these two regulators. Pressure drop would also be highly affected by the volume of air being flowed through the regulator at the time of the measurement. The more the "flow demand" being exerted on the regulator, the larger the pressure drop.

I have tested many of both the CDX-5 and the FDX-10 regulators on an ambient flow bench. To be honest, I typically see a pressure drop of about 10-12 PSI on both at a flow rate of 10 CFM.

If you are concerned about noticeable performance, I am not sure you could tell the difference between these two products. Obviously, there are cosmetic and styling differences between the two. However, Oceanic clearly had reasons for enlarging the balance chamber on the FDX-10, they had reasons to enlarge the effective surface area of the diaphragm on the FDX-10, and the WOB (a combined results of many features in the first and second stage) for the Delta 4.1/FDX-10 is clearly lower. So, I am sure they would comfortably say that the Delta 4.1/FDX-10 is clearly the "higher performer".

Phil Ellis
www.divesports.com

 

[Hallmac]

I would agree Phil,

Recreational divers and modest Tec-divers will be very hard pressed to tell any difference. I think the only way you would ever tell the difference is at the far end of the scale. I don't think I will ever be that deep, at least not planning to at this point.

Marketing, such a game!

 

[captain]

Of all the reasons to select one regulator over another IP pressure drop wouldn't be in the equation for me. My 50 year old Aqua Master double hose regulator IP will only drop about 10 psi on a hard inhalation. I wouldn't expect a 20 psi drop on any regulator old or new.

 

[hmoffatt]

Thanks all,

It sounds like I can safely save my money with regard to the first stage. How about the Delta4 versus GT3, appears that the differences are a slight WOB improvement, pre-dive switch and a different mouthpiece... not worth it? ($200 Australian difference in prices for the 1st/2nd stage combination)

 

[Jimmer]

I would say save your money and get the GT3. I have used Delta 4's and own four GT3's. I've used them deep and cold in the Great Lakes and deeper and warm in the tropics. Never had a concern with them at all.

Jim

 

[halocline]

A while back I started a thread about the relationship between IP drop and flow. I got curious when my MK10 and MK15 both experienced a larger IP drop than my MK5 while using the same second stage, same IP gauge, and even same LP inflator hose for the IP gauge. It got to the point where I talked with Peter Wolfinger, the guy that wrote Reg Savvy and was a SP engineer.

Anyhow, I never did get a definitive answer why my regs acted like that, when the MK10 is supposed to have higher flow than the MK5 and the MK15 clearly is a higher flow reg. If you are using the same 2nd stage purge to create the demand, theoretically there should be an inverse relationship between IP drop and flow.

Eventually I tried to really tax the 1st stages by putting 2 balanced/adjustables on them and going full purge (they have really strong purges) on both at the same time. I found that the MK15 initially dropped 15-20 and then leveled off at about 7-10 PSI under IP. The others dropped a full 15-20 under that kind of demand.

In the end I explained the results by guessing that there must be some variance in venturi effect which could lower the pressure in the LP inflator hose in the presence of increased flow through a different LP port. IOW, I don't think it's possible to really accurately measure the IP drop in dynamic flow situations accurately with the set up I have.

BTW, you can really tell a difference between the IP drop with different MK20 pistons. I was working on a really creep-ridden MK20 last summer and eventually tried all three piston styles. The MK25 composite piston has a much smaller IP drop than the other two in the same reg.

Of course none of this makes any difference in dive performance. For example, a relatively med-high performance 1st stage with a maximum flow rate of 150 SCFM would theoretically empty an AL80 in around 30 secs, much faster than the tank valve itself. Second stage flow rates probably max out at about 50 SCFM, and I couldn't imagine any diver, even the most hyperventilating panic-stricken one, drawing more than 10 or 15 SCFM.

 

[PhilEllis]

As AWAP said earlier, I think it might be slightly dubious to even make qualitative comparisons between published data for IP drop on a particular regulator design, and as others have stated, certainly makes little sense when you rate all of the things that would make one choose one regulator over another.

While I don't know how the various manufacturers actually measure IP drop, I do know that we would expect a graph of IP drop to be correlated to the volume of flow at which that drop in pressure was measured. Stating that a particular regulator has an IP drop of "x", without specifying the flow in cfm at the time that drop was measured, seems useless. One persons "sharp" inhalation is quite different from another's "sharp" inhalation. On our regulator test sheets, I simply record the IP pressure at a flow of 10 CFM, which I think is some consistent measure of stating the characteristic.

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