Question re: diaphragm vs. piston first stage

[concollective]

Ever notice how the same kinds of conventional wisdom get repeated over and over on certain scuba technologies -- so often that you're not sure whether divers are speaking from personal experience, or just repeating what they've heard?

So it goes with the choice between a diaphragm vs. piston first-stage regulator. I have heard/read the following things over and over:

- A diaphragm regulator, because it is typically (interestingly, not invariably) sealed, is therefore better for (a) cold-water diving because a ice doesn't form in the first stage, freezing the valve open, (b) diving in very silty or contaminated water, because no water actually enters the interior of the first stage. However, its design is somewhat more complex and it's a bit more expensive to service.

- A piston regulator may be slightly less expensive to service and is somewhat more reliable than a diaphragm reg.

So here are my questions to people who have substantial experience in diving on both kinds: should any of the conventional wisdom above be taken that seriously? Does the choice matter much? Or are the differences really so marginal in most diving applications that the choice doesn't matter at all? Assume that I don't dive in very cold (say, colder than 40F) water, don't dive in sewage, am not that concerned about say, a $25 differential in the cost of servicing a reg, and don't expect my reg to deliver the volume of air required by an elephant at 300 feet.

Any sage advice would be appreciated!

 

[Thalassamania]

I take it seriously; I think that the choice does matter; most of my regulators are diaphragm. Most repair places change by the stage, they don't care if it is a piston or a diaphragm. Even if you don't dive under harsh conditions, the movement of an o-ring up and down in the barrel of a piston regulator that is hard to clean out almost guarantees wear due to small salt crystals. I'll take a diaphragm every time.

 

[LeadTurn_SD]

All of my regulators have diaphragm 1st stages. I prefer them. I service all of them.... so I'd argue that they are far from "complex" (or I would not be doing my own servicing ).

Having said the above, if someone were to give me a high performance, balanced, flow-through piston 1st stage (say, a Scubapro or Atomic), I certainly wouldn't turn it down

I think you'll be very happy with either piston or diaphragm for warm or temperate water diving, and really should base the decision on where you plan to have your regs serviced, because at the end of the day the skill of the service tech has a greater impact on how well your regulator performs rather than whether it is a piston or diaphragm design...

Also, my feeling is the 2nd stage design and tuning is the primary contributer to breathing performance.... most modern 1st stages will supply plenty of air at a stable pressure to the 2nd stage; but if the 2nd stage is not properly tuned, even a $1,500 reg can breath like a dog....

Have fun with whatever you decide.

 

[concollective]

Thanks to both Thalassamania and LeadTurn SD. If it makes any difference, I'm looking at the Scubapro A700 second stage, and I'm trying to make a choice between the MK17 (diaphragm) and MK25 (piston). I live in New York City, where there's a great dive shop on the West Side that is known for top-notch Scubapro service.

 

[LeadTurn_SD]

Thanks to both Thalassamania and LeadTurn SD. If it makes any difference, I'm looking at the Scubapro A700 second stage, and I'm trying to make a choice between the MK17 (diaphragm) and MK25 (piston). I live in New York City, where there's a great dive shop on the West Side that is known for top-notch Scubapro service.

If I lived where you do, I'd choose the Mk17. While you may have no immediate plans to do cold water diving, you might want to in the future... and the Mk17 is the reg I'd choose for that.

Here in Hawaii, either would be fine.... I'm biased so I'd still choose the diaphragm reg, but the Mk25 is a super reg too....

It is nice to have such good regs to choose from

Best wishes.

 

[Thalassamania]

Both great regs ... I'd take the Mk17. But there are a lot of other brands that I'd buy first, for a number of reasons. I'd take a hard look at HOG, at OCEANIC, and at MARES before I plunked down my hard earned cash.

 

[halocline]

I use piston regs, specifically SP MK 5/10/15. There are two basic designs to piston regs, flow-by, which sometimes are balanced by use of spring washers, but more typically unbalanced, and flow-through, which are always balanced. The two designs are pretty different. 90% of my diving is with the flow-through design. It's an amazingly proven design with decades of reliability and the regs are very simple. The thing is, there are diaphragm 1st stages, like the aqualung titan, that can make basically the same claim.

So it's more or less a "fudd vs duff" scenario, unless you're talking about extremely cold water, in which case sealed diaphragm regs do offer a real advantage. There are sealed piston regs too, though, but they're a little less common. Atomic makes one, Sherwood makes one (SR1, I would avoid for other reasons) and I'm sure some other companies have them.

There's no real-world difference in performance between a high quality piston first stage and a high quality diaphragm 1st stage. Unless, of course, you're planning on sharing air with several of your closest friends by using multiple 2nd stages on the same first stage supplied by a high-flow unlimited air source at extreme depths!

 

[herman]

I pretty much agree with the rest, it's pretty much a wash unless you are doing cold water diving. I tend to prefer diaphram regs due to the ability to adjust the IP without dissambling the reg. That said, IF I could only have one first stage and was stuck in the middle of nowhere, my choice would be a simple flow by piston (Scubapro MK-2 or the like). They are about as bullet proof as a reg comes, simple to repair and parts can be DIYed if necessary.

To add a little to LeadTurns post, I would not go with the MK-25 simply because it is more complex and expensive than is necessary. Contrary to what a lot of (well meaning??) folks will tell you, the first stage has almost nothing to do with the performance of the reg. A basic simple unbalanced piston stage coupled with a well tuned balanced second stage will hold it's own with most any other first/second stage combo and it will still be diving when the fancy reg is long gone.

 

[Scott L]

Both great regs ... I'd take the Mk17. But there are a lot of other brands that I'd buy first, for a number of reasons. I'd take a hard look at HOG, at OCEANIC, and at MARES before I plunked down my hard earned cash.

You got paid for diving. Woe is me...

 

[berk]

I've used MK-25 / G-250's in NYC for the past 9-10 years and they've served me well. I like the hose routing in them. I think that might be one of the main considerations if you believe all else is really equal. I would not argue against environmentally sealer diaphragms being more resistant to freeze up; seems to make sense. There are kits and goo that you can put in pistons to help with cold water.

Some people say turrets are a potential failure point (available on both diaphragm and piston) and that might be more of a consideration than anything. -to some people

Supposedly piston perform better after being flooded so they are the choice for stages. I have it backwards and have diaphragms on my stages and had to switch one over underwater at one point. It did not do that well afterwards (fine after service).

I always find pistons to be a little more crisp when they open and close, almost like a snap to it. Admittedly personal taste and subjective.

For general diving I think the dry-air bleed system in Sherwood regs makes them rough and tough. How could a zillion rental regs be wrong?

-matt