MK2 overhaul question

[halocline]

There's no question that in the flow-by piston design, changing the ratio between the area of the piston head and the size of the orifice will change the amount of IP drop over the supply range. It's just math. How that translates into our personal ability to measure IP drop and those areas is anyone's guess, I suppose.

I have always been under the impression that this is the limiting factor in orifice size for the MK2 and other flow by designs. If SP wanted to increase the orifice size to increase flow, it would need to enlarge the piston head area as well or tolerate a larger drop in IP over the supply range.

Sherwood got around this by putting the orifice on a spring washer (actually a few of them, I think). When the tank is full, supply pressure pushes the orifice enough to compress the spring washers, reducing the amount the main spring has to compress for the piston to contact the orifice. As supply pressure drops, the orifice is pushed away from the piston by the spring washers, and the whole arrangement keeps IP fairly steady throughout the supply range. It's their method of balancing flow by 1st stages, although it doesn't involve diverting air into a balance chamber.

One would think that this would allow sherwood to significantly increase the orifice size and thus flow rates, but I have never heard that the sherwood 1sts have higher flow rates than the MK2. I'm pretty confident that to the real world it doesn't matter much. The MK2 has good fast IP recovery under demand and flows close to 100SCFM at full tank pressures. That's the equivalent of emptying an AL80 in under a minute. Flow rates in 1st stages are pretty much useless numbers to an actual diver.

 

[axxel57]

To measure the IP drop in general is pretty easy, but I use either my GMC Flow Module or my Scubatools A.I.R. Flow Analyzer.

It helps me to correlate the exact amount of air flow to the dynamic IP and the cracking effort at a certain tank pressure.

But I would not even really need that to get a good picture of the performance of my 1st concerning the dynamic IP and the IP drop at different tank pressures.

Everybody who has an IP meter can check his 1sts dynamic IP and its main spring condition by putting the reg on a tank and the IP meter on the inflator hose or 1st stage, adjusting the venturi on minimum (if possible) and start to press slowly the purge button watching the IP meter .

In the moment you press the purge button the static IP on your meter gets dynamic and drops.

If you keep on pressing harder the dynamic IP will drop further until to a certain point near where the 2nd stage reaches its limits, because you cannot press deeper anymore.

That would be the maximum flow this 2nd can flow and the dynamic IP would be at its lowest point.

You can repeat these tests with different supply pressures and static IPs adjusted to different levels.

Obviously are Flow modules more comfortable to operate, more precise and in designs like the R190 (ca. 12 SCFM at max press on the purge button) I could force the 2nd to higher flow rates than I could do by pressing the purge button, but the dynamic IP has usually stopped dropping already earlier than max, so with this method you would watch the same max drop as I do with my machine.

You could not correlate the cracking effort and the venturi with the dropping dynamic IP, but that was not the issue anyway right now.

The Sherwood design with the disc springs (5) balances remarkably well, one could look at it as an extremely well balanced unbalanced design because the design is an inline piston 1st design and maybe one could consider it as a hybrid.

But for me also the Sherwood 1st has the same limitations and correlations concerning the sealing-spring-supply pressure forces as the MK2, the disc shims and their balancing effect change nothing to that.

As I wrote before, the flow rates of a 1st stage could be only interesting for someone who wants to put a couple of divers on one 1st stage or dive extremely deep.

Halo is right here, as long as the 2nd stage valves are the limiting factor for max flow, boasting with idiotic high flow rates of a 1st stages is just BS.

 

[halocline]

I don't think measuring IP drop during demand with a regular IP gauge on the LP inflator hose is accurate. There are venturi forces at work that I believe cause pressure drops in the turret. The only way to measure it accurately would be with an inline flow meter right at the 2nd stage. And that's just a guess.

 

[axxel57]

Before I wrote the post yesterday, I checked the thing with a MK2 and a R190 first on the flow bench and then with my handheld IP meter (ANSI Grade A) to be sure.
The IP at full tank was 147psi and dropped at full purge button press (about 12 SCFM flow) to 139-140psi, which is a pretty good value.
At 600psi the IP was 129psi and dropped to 118 -119psi.
I was testing the same on my Flow Bench, testing it to 15 SCFM, but the IP was not dropping more than the values I had in my purge button test and this drop stopped at around 9,5 SCFM and stayed stable.
How accurate in absolute the measurements are measured through the inflator hose, I can't tell, but I'm sure in proportion they are correct anyway you measure it.
The Venturi doesn't play much of a roll I think if you have switched it off, but if you should be worried about that, just use the valve without the housing (if possible,it's tricky, but with some practice anybody can do that also on high flow rates).

 

[halocline]

I'm not talking about venturi in the 2nd stage, and you can't really "switch it off" anyhow. Whenever there is air flowing in a linear manner there will be a pressure drop around the edges of the flowing air. The only thing that the venturi adjustment switch does is direct a small vane to break up the linear flow near the mouthpiece.

I'm talking about venturi forces in the 1st stage, as when air flows through one LP port, there's a pressure drop at other LP ports. This explains why a MK15 could show a bigger IP drop than a MK2 under demand, when connected to the same 2nd stage and IP drop measured through the LP inflator hose. I talked with Wolfinger about this a number of years ago, shortly before he sold peterbuilt.

 

[axxel57]

I'm not talking about venturi in the 2nd stage, and you can't really "switch it off" anyhow. Whenever there is air flowing in a linear manner there will be a pressure drop around the edges of the flowing air. The only thing that the venturi adjustment switch does is direct a small vane to break up the linear flow near the mouthpiece.

I'm talking about venturi forces in the 1st stage, as when air flows through one LP port, there's a pressure drop at other LP ports. This explains why a MK15 could show a bigger IP drop than a MK2 under demand, when connected to the same 2nd stage and IP drop measured through the LP inflator hose. I talked with Wolfinger about this a number of years ago, shortly before he sold peterbuilt.

Okay, so you don’t talk about the2nd stage venturi, but of course there are lots of 2nds which don’t show any venturi assist even at 12,5 SCFM when adjusted on venturi minimum.

Where else than in the mouth piece shaft you think you could produce ‘edges of flowing air’?

Maybe between the aspirator and the mouth piece shaft?

I don’t believe that, the air entering the 2nd stage housing through the aspirator will be turbulent until it is channelized in the mouth piece shaft where it is measured.

The Flow restrictor simply produces in those 2nds there enough turbulent air flow to prevent a laminar airflow strong enough to trigger the venturi, but we had that discussion before.

Concerning your 1st stage venturi, I think you’re mixing something up.

Of course will the IP drop in the 1st stage when air is flowing through one LP port, also at the other ports.

That’s called dynamic IP and is always lower than the static IP (a drop).

Has nothing to do with venturi, especially not in the MK2.

I think you are referring to something well known for decades, that the SP MK 10-20 models produce a certain venturi in the swivel, but that works only for the port on top of the swivel.

This is one of the reasons why I and many other pros use this port since the eighties for their primary 2nds like what is nowadays known as a ‘Preferred Port’.

In practice I admit I think hardly anybody can feel the slightly higher dynamic IP produced by the venturi boost and their effect on the cracking or inhaling effort, this is why SP is boasting with high flow rates instead.

The association: higher flow rate – more safety works better.

 

[redacted]

In practice I admit I think hardly anybody can feel the slightly higher dynamic IP produced by the venturi boost and their effect on the cracking or inhaling effort, this is why SP is boasting with high flow rates instead.

About that higher flow rate claim, it looks to me like the sale folks came up with that rather than the engineers.

SCUBAPRO - Regulator feature comparison

Physics 101: increase supply pressure 15% and you see an increase in flow rate of 15%. But it will probably sell regulators.

I also found a video on the Mk2 EVO and it looks to me like the differences are quite extensive and incompatible with the current Mk2. If the EVO catches on, I predict we will see Scubapro withdraw support for the Mk2 that has been around for about 40 year.

 

[halocline]

Concerning your 1st stage venturi, I think you’re mixing something up.

Of course will the IP drop in the 1st stage when air is flowing through one LP port, also at the other ports.

That’s called dynamic IP and is always lower than the static IP (a drop).

Has nothing to do with venturi, especially not in the MK2.

I'm not talking about dynamic IP. I am saying that an IP gauge on the LP inflator hose is not capable of accurately measuring IP while air is flowing because the air flowing into the 2nd stage hose causes a pressure drop in the LP inflator hose. IE, the pressure in the 2nd stage hose at the 2nd stage valve is higher than in the LP inflator hose. I'm confident that this is the case, but have no accurate means of testing it and I don't think you do either, unless you have some very sophisticated measurement tools.

Regarding the end port on the BP 1sts, that port is considered higher flow by SP, but I bet this is measured by opening all the LP ports at once and simultaneously measuring flow through them. Since the source of air is flowing linearly up the piston shaft directly into the end port, this creates lower pressure on the sides of the turret, (venturi forces) hence lower flow out the those ports. But I'm pretty confident that this is the case ONLY when both end and side ports are flowing at the same time.

 

[axxel57]

I'm not talking about dynamic IP. I am saying that an IP gauge on the LP inflator hose is not capable of accurately measuring IP while air is flowing because the air flowing into the 2nd stage hose causes a pressure drop in the LP inflator hose. IE, the pressure in the 2nd stage hose at the 2nd stage valve is higher than in the LP inflator hose. I'm confident that this is the case, but have no accurate means of testing it and I don't think you do either, unless you have some very sophisticated measurement tools.

Regarding the end port on the BP 1sts, that port is considered higher flow by SP, but I bet this is measured by opening all the LP ports at once and simultaneously measuring flow through them. Since the source of air is flowing linearly up the piston shaft directly into the end port, this creates lower pressure on the sides of the turret, (venturi forces) hence lower flow out the those ports. But I'm pretty confident that this is the case ONLY when both end and side ports are flowing at the same time.

I know you think you are not talking about dynamic IP, but you are describing exact its working.

I’m with you that measuring, and that has always been said,the IP in the inflator hose might be for the reasons you state less accurate than measuring directly on the 1st stage, but that is completely irrelevant for the procedure I suggested to check parts of the performance of your reg (dynamic IP).

The difference must be very small, otherwise it would not be industry standard to measure the IP on an inflator hose.

The proportion in dynamic IP drop stays the same, so If I see for example my dynamic IP dropping to 90-psi while pressing the purge button, I would consider changing the main spring of my 1st.

And I’m pretty sure you are wrong about the high flow port.

While you are probably right that the total max flow rate of a 1st is measured with all LP ports open, I would like to know how you imagine a turret with all ports open to develop a venturi.

Obviously the venturi can only be triggered when the others but the top port are closed and the air at the side of the air flow path can be converted into a vacuum.

The air which is streaming out of the turret also through the 90’ ports, how do want to get that air backwards to create a vacuum?

High Flow Port means pretty sure only that the venturi boost pushes more air into the inline port producing a lower dynamic IP.

It’s the same principle used quite successful by Mares in their Abyss 1sts for decades.