Diaphragm vs. Piston Regulators

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Oldbear

Teaching Neutral Diving
Scuba Instructor
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Location
Melbourne Florida
# of dives
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I know divers have their personal preferences and brand/model loyalty but strictly from a functional stand point what are the advantages and disadvantages between balanced diaphragm 1st/2nd stages and balanced piston 1st/2nd stages?
 
Aside from the environmental sealing stuff (you can environmentally seal a piston, btw), it's very easy to adjust the intermediate pressure on a diaphragm. Piston's will require shims.

Whether or not that's important to you, that's for you to decide.
 
Second stages are second stages so there is no difference there, at least in P vs D debate. They do come in balanced and unbalanced varities but that is a different subject.
While it is true you can environmentally seal a piston reg, its a messy affair, both to install and clean up during service. Diaphragms rule in environmental sealing and adjustability. Even if a diaphragm reg is not "environmentally sealed", by it's basic design, its internal parts are better protected than those of an unsealed piston reg, only the spring is exposed in a diaphragm reg as opposed to those of a piston reg where the piston, spring and to some extent the main sealing orings are exposed to the water. In short, an abused diaphragm reg is much more likely to function than a piston reg (unsealed) . As a general rule, balanced piston regs are capable of flowing a lot more gas but this is more sales BS than anything useful. Any reg made in the last 30 years or so can flow WAY more gas than any 2 divers can breath so its a moot point. There are some piston regs that you can adjust the IP externally but they are the minority. Diaphragm regs have more moving parts but all these parts are internal to the reg and most do not have a sealing function as opposed to a piston reg which has fewer moving parts but relies on several exposed oring for sealing. There is one dynamic oring in a diaphragm reg (balance chamber) which sees little movement and is well protected as opposed to 2 in a piston reg which are both exposed to the environment (unless sealed). Both of these orings see much more movement than the one is a diaphragm reg.
From a service standpoint, pistons are a little easier to service but tend to require more specialized tools to do it properly. (bullet tools and piston oring installation tools) but once you have done a few, both are easy to service IMO. Service kits for diaphragm regs tend be be a little more expensive and many of these parts are not available other than from a dealer. Piston regs have few parts and with the exception of the HP seat, the orings are usually common sizes that can be purchased inexpensively at a number of places.

Both are reliable and work fine, which you choose is up to you . I prefer diaphragm regs but if I were stuck on a remote place where parts were hard to come by, a simple flow by piston reg (SP MK-2 for example) would likely be my choice. A bag of orings and I can make it work for many many years.
 
also worth noting the diaphragm regs tend to offer better balancing in terms of IP swing. Even the MK25 has a 25+psi IP swing from a full tank to empty. Not important for balanced second stages, but for unbalanced regs it is noticeable
 
Aside from the environmental sealing stuff (you can environmentally seal a piston, btw), it's very easy to adjust the intermediate pressure on a diaphragm. Piston's will require shims.

Whether or not that's important to you, that's for you to decide.

Shims?
I need to correct the date stamp on my SB profile... I didn't realize it was still 1980!
 
Yes, shims- thin metal or plastic washers placed at the end of the spring to increase the spring pressure. All piston regs use them unless they are one few that have an external adjustment feature.
 
also worth noting the diaphragm regs tend to offer better balancing in terms of IP swing. Even the MK25 has a 25+psi IP swing from a full tank to empty. Not important for balanced second stages, but for unbalanced regs it is noticeable

Something very wrong here. My Mk5/10s will see an IP swing of 7 to 10 psi as the tank goes from full to 150 psi, ut my Mk20 show little to no change.
 
... it's very easy to adjust the intermediate pressure on a diaphragm. Piston's will require shims.

My Cressi AC2 unbalanced piston reg requires no shims. I only have to turn a ring to get it set dead on.
 
... From a service standpoint, pistons are a little easier to service but tend to require more specialized tools to do it properly. (bullet tools and piston oring installation tools) but once you have done a few, both are easy to service IMO. Service kits for diaphragm regs tend be be a little more expensive and many of these parts are not available other than from a dealer. Piston regs have few parts and with the exception of the HP seat, the orings are usually common sizes that can be purchased inexpensively at a number of places.

I don't need any specialized tools to disassemble or adjust either my Cressi AC2 (1st) or XS2 (2nd) stages. The parts are very inexpensive, they are readily available, the service manuals are online, and it's easy to do. I always have a set of service parts for both on every trip. I can rebuild either reg in a half hour (actually much less) no matter where I am.

Both are reliable and work fine, which you choose is up to you . I prefer diaphragm regs but if I were stuck on a remote place where parts were hard to come by, a simple flow by piston reg (SP MK-2 for example) would likely be my choice. A bag of orings and I can make it work for many many years.

As far as functionality and reliability, last year I dove the Radau in spring on Lake George. My buddies fancy, high priced, environmentally sealed diaphragm regs free flowed at 102 feet and 43°F. We had to abort. My Cressi's - perfect.
 
Any reg made in the last 30 years or so can flow WAY more gas than any 2 divers can breath so its a moot point.

I am definitely not trying to dispute you. This is a serious and sincere question: Is that true for depths beyond recreational limits? Or maybe I should ask, at what depths do the differences between "good" current 1st stages and "less good" ones come into play?
 
https://www.shearwater.com/products/perdix-ai/
http://cavediveflorida.com/Rum_House.htm

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