Why do piston regs cost more than diaphragm regs?

[bryankwatts]

The problem with that 'test' is that the tank valve is by far the weakest link in the supply chain. For a 1st stage to supply 100 2nd stages with air, you need a much higher flow supply than a single scuba tank, and in fact that's exactly what they use to measure flow rates. The MK25 has something like a maximum flow rate of over 200SCFM, which means theoretically emptying a AL80 in about 20 seconds. Obviously the tank itself can't keep up with that.

The only 'real world' application for flow rates is something tangential, which is IP recovery. A high quality 1st stage will allow IP to dip a minimal amount under demand, and recover very quickly. This is a little tricky to measure at home because IP gauges are not designed to measure pressure with air flowing. There's a venturi effect in the hoses and/or 1st stage whiich makes it appear that IP is dropping more while air is flowing. You'd need an actual flow meter to accurately measure the pressure drop.

Getting away from this boring and lengthy monologue, the short answer is that the MK19-17 don't have anywhere near the theoretical top flow rate of the MK25 but it has absolutely no real-world impact on how they perform diving.

A great explanation. Thank you.

 

[eelpout]

I really think that basic physics explains air consumption going up as depth increases.....

 

[HaloBlo]

We all have different priorities. While I prioritize consumers getting parts directly from the manufacturer (not working through a connection at a shop) and also get training for servicing is a huge plus for me. Others couldn't care less about either of those. From @rsingler's experiment, there is a difference in second stage design where some second stages perform better in certain conditions and worse in others. He actually did things I didn't even think about (such as while scootering).

Do you have a link to the experiment you mention?

 

[HaloBlo]

How does an unbalanced reg make you use more gas?
You use what you use.
Besides that, if someone is diving deep enough when the tank gets low enough to begin feeling resistance (also using an unbalanced second stage) then they need to rethink their dive plan and gas supply choices. If you are breathing a tank down that low then you should be about finishing your dive and at your safety stop. For some people an unbalanced reg is a great idea; the ones that have trouble remembering to look at their air gauge.
Beyond that, all you have to do is pit a high quality pneumatically balanced second stage on an unbalanced 1st stage and it will breath all the way down to zero with little resistance.

If you dive a high performance adjustable second stage on an unbalanced piston, you will find that performance drops significantly over the entire pressure range.

In magnehelic terms it goes from being a 1.1" reg to a 2.3" reg ( with an IP that ranges from 145 to around 125) unless you unload the spring tension with the adjuster. And it happens throughout the tank pressure change.

As someone who has spent a lot of time doing this, with a couple of Atomic Second stages on MK2 first stages, on all kinds of dives on all kinds of depths, (because that is what is on my sidemout tanks) you are specifically incorrect about this part. It is brutal switching to a non-adjustable balanced second stage from one that is properly adjusted to match the current IP of the unbalanced second stage, and immediately obvious.

"Beyond that, all you have to do is pit a high quality pneumatically balanced second stage on an unbalanced 1st stage and it will breath all the way down to zero with little resistance."

No, the breathing resistance (unless you unload the spring with the adjustment knob) will continuously increase to a fairly badly performing reg. Because the IP falls continuously throughout the dive and thus the cracking effort continuously increases.

Most people just get used to the slowly increasing cracking effort throughout the typical dive.

If you have three or four first stage second stage combinations like I often have on extended sidemount dives, so you do not just get used to gradually worse and worse performance, the effort on an unbalanced piston first stage becomes really obvious, and only bearable with an appropriately setup adjustable second stage

Appropriately setup in this case means breathes low effort at low IPs, and leaks on a full tank unless the adjuster knob is all the way in to give enough spring tension to stop the leak from the higher IP.

 

[L13]

If you dive a high performance adjustable second stage on an unbalanced piston, you will find that performance drops significantly over the entire pressure range.

In magnehelic terms it goes from being a 1.1" reg to a 2.3" reg ( with an IP that ranges from 145 to around 125) unless you unload the spring tension with the adjuster. And it happens throughout the tank pressure change.
...
<other stuff that also seem wrong>

This seemed wrong. The entire purpose of a balanced second stage is to compensate for changes in IP pressure. So I decided to do an experiment.

I took my ScubaPro G260 and measured it's cracking pressure at 135 psi, I like it tuned a little hot, and it was 0.8". I adjusted the IP pressure from 125 psi to 145 psi and the cracking pressure barely moved +- 0.1" (I did not touch the adjustment screw).