IP drop in Piston 1st stages vs Diaphragm

[reefrat]

The Piston Vs Diaphragm debate regarding IP drop during inhalation/purging has being going around for a while. I have made the same observations as you using Scubapro and Atomic pistons vs Mares and Aqualung/USD diaphragms, basically the piston firsts drop about twice as much as the diaphragms.
Halocline has stated that this is most likely due to a venturi effect causing a lower pressure in other hoses due to the high flow rate of the pistons- which sounds really plausible but I am not 100% convinced. I would expect a “venturi” effect to be most pronounced when the first stage is purged hard because there would be a high flow rate at that time. However I have repeatedly seen that when the second stage is inhaled (sipped) on very slowly and gently the same larger IP drop occurs in piston firsts and there is generally a rather slow return to the set IP- I would suggest that any Venturi effect would be minimal during this cycle. The diaphragms, as usual, showed a much lesser IP drop and faster recovery.
Conversely, a fast stab on the purge button usually resulted in the same larger IP drop but a much faster recovery on the piston first stages.
My take on this is that the piston first stages are just a little more sluggish to respond to an IP drop due to the mass of the piston and the friction between the piston O ring and the internal wall of the ambient/IP chamber. Apparently the piston first stages are capable of ultimately higher gas flow rates but I am not sure how relevant this is for typical dive conditions.
So it seems to me that:
Pistons- especially flow by designs have less parts and greater inherent reliability provided they are appropriately maintained and serviced. Also they may be better suited to pnuematicall balanced second stages where any breathing cycle IP drop is minimized (maybe why Scubapro, who only produced pistons at that time invented the balanced adjustable second stage in the first place).
Diaphragms- more complexity and parts along with an “upstream” HP seat would suggest a higher potential to fail but having most components sealed from the environment may negate this provided the design and quality of manufacture is good. Diaphragm designs may have less inertia to overcome when the IP is lowered and therefore respond more quickly resulting in less IP drop and quicker recovery during a normal breathing cycle, if this is true then they would probably be a better choice for an unbalanced second stage.

Sound like your MK10 might benefit from a service?!

 

[halocline]

There is no question in my mind that you are not getting accurate readings about IP drop under inhalation with a standard IP gauge. So I don't think you have any idea what the relative IP drop of pistons vs diaphragms actually is. Get a flow meter and infer the pressure via the flow measurement.

Keep in mind that flow and pressure are proportional in identical spaces, like a LP hose. So if, say a MK25 really has twice the IP drop that a mares diaphragm 1st stage has under demand, then it MUST mean that the mares is flowing more air. (Assuming both 1st stages have the same static IP). Clearly, that is not the case, as the MK25 flows 300 scfm.

There are two radically different piston designs, flow by (MK2) and flow through (MK10). There is no question that the flow through design flows more air; that's what it was designed for. Yet, if you compare the apparent IP drop in those two 1st stages (MK2/MK10) by observing an IP gauge at the end of the LP inflator hose, you'll see that the MK10 needle drops quite a bit more. It's definitely not flowing less air. So the measurement means must be inaccurate. That is the only plausible explanation at least that I can come with.

And it's not just me, several years ago I was curious about this so I called Peter Wolfinger; he used to own scubatools, he wrote regulator savvy, and was an engineer at scubapro for a long tine. He agreed, it's the only explanation.

 

[Eric Sedletzky]

I used to notice a bigger IP drop on pistons too compared to diaphragms. I thought what the fk is this? Pistons are supposed to flow more?
So I did a test.
I hooked up two different regs up to a full tank, a diaphragm model (Aeris) and a MK20 piston, both had two second stages on them. I pressed one purge button full blast, then pressed the other one full blast holding them both wide open. The Aeris diaphragm regs lost more than half their flow (maybe more) when I opened both seconds up. I did the same test with the piston reg and there was almost no change in flow from one being wide open to both being wide open.
After doing this test I proved to myself that indeed it’s true that pistons flow more, way more. So didn’t worry about it anymore.

I also found that using pneumatic tools off a scuba tank on dry land, diapragm 1st stages didn’t flow enough air to make some tools work like mini grinders or spray guns, but pistons flowed way more air and the tools would work. However, I also noticed that with flowing that much air out, if there was any moisture in the air the piston reg would ice up on the outside and get freezing cold.