Pressure drop of first stage

[DA Aquamaster]

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.

I agree an IP gauge on the end of an inflator hose has possible limitations when it comes to accurately measuring the magnitude of IP swing. The geenral trend and the final stablized IP are fine, but just not the more dynamic aspescts of it.

An IP gauge on an inline adjuster makes a lot more sense and it pretty much ensures that you are measuring at the second stage. But then again, if the adjuster itself reduces the flow, you can't really see the full impact of maximum flow rates. Still the one Scubatools sells has worked very well for me and does not seem to impede the flow rate much more than the orifice itself.

There is also I think an effect on IP caused by response time. I think the composite Mk 25 piston demonstrates less IP swing as it is much lighter and allows the valve to repond faster to IP drops. In that regard the lubricant used can have an effect as can dirt, salt, etc - or for that matter water rather than air in the ambient chamber as the water is denser with more mass and takes more energy to move and displace as the piston moves back up and down of the seat.

Response time is I think also impacted by piston head diameter in a balanced piston reg. The larger piston head requires less pressure drop to develop the same amount of force to open the valve, and in general a larger piston can maintain a slightly larger distance off the seat at a given IP drop than a smaller piston if the IP drop is identical. So a larger piston diameter equates to greater working range and greater flow, all other things being equal, at an identical IP drop. It is not coincidence that SP went back to the larger piston head size with the Mk 15, and with the Mk 200 and Mk 2 Plus, leaving the Mk 10 and MK 3 as fairly short small head piston experiments.

In an unbalanced piston design a smaller piston head means a larger change in IP as the tank pressure drops, so larger is better on two levels in that situaion.

I think you are probably most accurately displaying the differences at high flow rates where you observed the IP at high sustain flow rates was smaller for the Mk 15 compared to the Mk 5 and Mk 10. Doing the same thing at low rates won't show the differences as the capabilities of the lesser performing regs are not really challenged.

 

[hmoffatt]

Thanks all, ordered the GT3 / CDX5 DVT combo and saved my $200 Aussie for other toys

 

[Greg Barlow]

The intermediate pressure is monitored on an ANSTI simulator. The computer tracks the drop in pressure during the full breathing cycle. It is fairly common for many regs to drop 10-14psi during an inhalation of "serious" nature. That being 2.5 liters per breath at a rate of 25 breaths per minute at a depth of around 50 meters. Typically, large, balanced piston designs will demonstrate the least amount of drop. That said, a well designed diaphragm design can easily meet the needs of any diver and scenario.

Greg Barlow

 

[halocline]

Greg, what's the supply source for these tests? High flow, constant 3000 psi? My point about regulator flow rates is that the tank valve is almost certainly capable of flowing less air than any medium-high performance reg.

An interesting experiment would be to take all the LP port plugs out of a reg, attach it to a full tank, place it next to another full tank with no reg, open both valves and see what happens. I'd bet anything they empty at the same rate.

 

[Greg Barlow]

Greg, what's the supply source for these tests? High flow, constant 3000 psi? My point about regulator flow rates is that the tank valve is almost certainly capable of flowing less air than any medium-high performance reg.

An interesting experiment would be to take all the LP port plugs out of a reg, attach it to a full tank, place it next to another full tank with no reg, open both valves and see what happens. I'd bet anything they empty at the same rate.

The standard supply pressure is 1,500psi (+- 100psi). Most facilities run that off of a cascade system with a outgoing pressure regulator. When I worked for Rodale's, they would often reduce the pressure to 750-800 to determine what impact it had on the performance. When the depths approached 200fsw, and the demand was high it had a pretty significant impact on the overall WOB. Amazingly, though, many "medium peformance" regs could maintain an overall WOB of under 2.0j/l with the lower air supply. A reg delivering a score of 1.4j/l with a demand of 62.5lpm and at a depth of 198fw earns the coveted USN Class A award.

It is pretty darn tough to sustain a work load of 62.5lpm for very long. That is the equivalent of swimming at around 70-80% of your maximum kicking speed. That can be compared to running 800m as fast as you can.

I found it really neat to find that one of my restored Voit/Swimaster MR-12 regs delivered a score of 1.68j/l at 132ffw, with the 62.5lpm setting. Not bad for a reg designed in the late 60's. A late 70's USD Calypso VI with a balanced piston first stage had a score of 1.46j/l. The IP on the old regs fell off a bit more than modern designs. The MR-12 dropped 25psi at the full inhalation at 132ffw, and the USD Calypso VI was in that ballpark. You need to remember that alternate air sources were very uncommon when that particular reg was built in 1971. The first stage could probably only flow 110-115cfm or so, and I bet that the second stage would be lucky to flow 40-50cfm. Given those figures, the reg would be really tough to over breathe with but one second stage.

As far as the "overblown" flow rates for a first stage. Why does the MK25 need to have a flow rate of 300cfm? I bet that the S600 can only flow around 70-75cfm or so. Of course, a really high flow rate means that the IP drops very little.

I've used vintage double hose single stage regs at 135ffw, and they still breath well at average swimming paces.

Greg

 

[halocline]

It's not so much the supply pressure, it's the flow. I'm sure the test machines deliver air to the regulators at a much higher flow capacity than tank valves.

I agree that some test data is way overblown. Just a few decades ago, divers routinely went to extreme depths and conditions with regs that most divers to day wouldn't take in a pool. I'm also a double hose diver, and while the WOB is definitely higher due to the position sensitivity, it's still really fun to dive with and I would not have any hesitation taking one to my depth limits.

 

[captain]

I've used vintage double hose single stage regs at 135ffw, and they still breath well at average swimming paces.

Greg

Now that's a real pressure drop, tank psi to ambient.

 

[Greg Barlow]

Now that's a real pressure drop, tank psi to ambient.

A few years ago, I had a "reg tech" explain to me how it was impossible for a reg not to have an intermediate pressure setting. I earnestly tried to explain to the young man as to how a single stage mechanism operates. After a couple of minutes, I realized that the "tech" was already an expert and that my explanation defied the laws of physics.

It probably would have been better had I told him that my 50 Fathom reg had an intermediate pressure of 14.7psi while at the surface.

As my Dad used to say: "There are automobile mechanics and there are parts changers. Always search out a mechanic."

Greg