lobster63
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Hey folks, I've been looking to buy my first brand new regulator. whats the difference between a piston regulator and a diaphram regulator?
regulator types
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halocline
Contributor
If you try a search for exactly that, you'll get lots of information. The best overall explanation I've seen of how each type of 1st stage operates is in the Vance Harlow book on regulator repair. Another good one, more detailed but also more difficult to understand, is in a book called "reg savvy" by Peter Wolfinger.
Basically, the way any 1st stage works is, there's a valve that is formed by the junction of a seat and orifice, and that valve is held open with a big spring when the reg is not pressurized. When pressurized, air flows through the valve, filling a chamber to a point at which either a piston or a diaphragm is forced in the direction against the spring and closes this valve, shutting off further air flow. So now you have two chambers in the 1st stage, one at tank pressure on the "upstream" side of the valve, and one at this specified pressure (usually around 125-150PSI) which is on the "downstream" side of the valve. This is the intermediate pressure (IP) chamber. There are ports in this chamber leading to the 2nd stage. When you inhale, you lower the pressure in the 2nd stage, which then lowers the pressure in the IP chamber, which then opens the 1st stage valve and allows air to flow from the tank, which quickly fills the IP chamber, forcing the piston or diaphragm to close the valve, etc. This cycle repeats with each breath.
There are two basic piston designs, the flow-by unbalanced piston like the SP MK2, and the flow-through balanced piston design like the MK25. You'll need to see a diagram to get a good idea of the difference. It's important because the two of those are quite different in performance, so if you're trying to compare piston vs diaphragm you really have to specify what kind of piston design you mean.
Diaphragm regs do the same thing, its just that they use a thick rubber or composite diaphragm and a couple of metal parts to accomplish the same thing. Diaphragm 1st stage valves are unbalanced upstream by design, so practically every one made in the last couple of decades uses a balance chamber to "balance" the forces acting on the diaphragm from the tank. This allows more precise IP setting that remains consistent from a full to an empty tank. The flow-by piston design is unbalanced downstream by design, and some manufacturers, like Sherwood, came up with ingenious ways of counteracting those forces to stabilize IP over the whole tank pressure range, and the flow-through piston regs are balanced by design. Consequently, some of the earlier flow-through piston regs, like the MK5, are extremely simple and very high performing.
If you're trying to decide which is a better design for diving, there is no correct answer, there are great and not-so-great regs of each design easily available. Try not to worry too much, even the cheap regs these days are super reliable and deliver lots more air than you'll ever need. New divers often buy high end regs out of sales pressure, hype, extreme concern about their safety, etc...when just a few decades ago professional divers were routinely making extreme dives on regs that many OW students wouldn't take in a pool today. It's too bad, actually. Some old regs are still great performers and fun to dive with.
Basically, the way any 1st stage works is, there's a valve that is formed by the junction of a seat and orifice, and that valve is held open with a big spring when the reg is not pressurized. When pressurized, air flows through the valve, filling a chamber to a point at which either a piston or a diaphragm is forced in the direction against the spring and closes this valve, shutting off further air flow. So now you have two chambers in the 1st stage, one at tank pressure on the "upstream" side of the valve, and one at this specified pressure (usually around 125-150PSI) which is on the "downstream" side of the valve. This is the intermediate pressure (IP) chamber. There are ports in this chamber leading to the 2nd stage. When you inhale, you lower the pressure in the 2nd stage, which then lowers the pressure in the IP chamber, which then opens the 1st stage valve and allows air to flow from the tank, which quickly fills the IP chamber, forcing the piston or diaphragm to close the valve, etc. This cycle repeats with each breath.
There are two basic piston designs, the flow-by unbalanced piston like the SP MK2, and the flow-through balanced piston design like the MK25. You'll need to see a diagram to get a good idea of the difference. It's important because the two of those are quite different in performance, so if you're trying to compare piston vs diaphragm you really have to specify what kind of piston design you mean.
Diaphragm regs do the same thing, its just that they use a thick rubber or composite diaphragm and a couple of metal parts to accomplish the same thing. Diaphragm 1st stage valves are unbalanced upstream by design, so practically every one made in the last couple of decades uses a balance chamber to "balance" the forces acting on the diaphragm from the tank. This allows more precise IP setting that remains consistent from a full to an empty tank. The flow-by piston design is unbalanced downstream by design, and some manufacturers, like Sherwood, came up with ingenious ways of counteracting those forces to stabilize IP over the whole tank pressure range, and the flow-through piston regs are balanced by design. Consequently, some of the earlier flow-through piston regs, like the MK5, are extremely simple and very high performing.
If you're trying to decide which is a better design for diving, there is no correct answer, there are great and not-so-great regs of each design easily available. Try not to worry too much, even the cheap regs these days are super reliable and deliver lots more air than you'll ever need. New divers often buy high end regs out of sales pressure, hype, extreme concern about their safety, etc...when just a few decades ago professional divers were routinely making extreme dives on regs that many OW students wouldn't take in a pool today. It's too bad, actually. Some old regs are still great performers and fun to dive with.
Whew, nice matt. The only thing I can add to that is you should really focus your attention on the second stage rather than the first. Regardless of the hype and pure BS some will hand you, there is very little difference in first stage performance as far as the diver is concerned. Sure some are balanced and some will flow a huge amounts of gas but almost any modern first stage will flow more gas than an average tank valve can flow and certainly more than any 2 divers can breath. The key thing to understand is 99% of the performance of a reg is determined by the second stage. As long as the first stage delivers sufficient amounts of gas at something close to it's design output pressure (know as IP- intermediate pressure) the second stage will perform well. Even with the slight variations of IP across the range of tank pressures resulting from the first stage not being balanced the second stage will perform very acceptably to most divers. If the second stage is balanced, it will perform equally well across the range of tank pressures even when connected to an unbalanced first stage. Balancing both stages is redundant. It does no harm but it also does not buy you anything.
What all this means is you should concentrate on the second stage, not the first. A bottom end first stage connected to a mid to high performance second stage will work great. On the other hand, a top of the line first stage connected to a low end second stage will not perform very well. A low to mid range first stage, piston or diaphragm does not matter, connected to a mid to upper performance second stage is the best value. I will also add that there is little difference in the actual performance of most mid and high cost second stages within a given brand/style. The extra cost is in "pretty" and not function.
What all this means is you should concentrate on the second stage, not the first. A bottom end first stage connected to a mid to high performance second stage will work great. On the other hand, a top of the line first stage connected to a low end second stage will not perform very well. A low to mid range first stage, piston or diaphragm does not matter, connected to a mid to upper performance second stage is the best value. I will also add that there is little difference in the actual performance of most mid and high cost second stages within a given brand/style. The extra cost is in "pretty" and not function.
LeadTurn_SD
Contributor
Just want to give this thread a "bump" because the replies by mattboy and herman are great.
For a new diver (and many of us "oldsters") the array of choices when it comes to picking a new regulator can be staggering. But the really good news: As mentioned above, it has become really hard to make a bad purchase. Most of the regs on the market today will make the average diver very happy.... so pick the one you like, and go have fun!
Best wishes.
For a new diver (and many of us "oldsters") the array of choices when it comes to picking a new regulator can be staggering. But the really good news: As mentioned above, it has become really hard to make a bad purchase. Most of the regs on the market today will make the average diver very happy.... so pick the one you like, and go have fun!
Best wishes.
After you have decided which type, you may also consider features such as swivel for easy hose routing, light weight and compactness if you travel a lot and make sure you can find local dealers to service your regs and to get parts easily. I like budget regulators and then save money for more diving 
theduckguru
Contributor
You could go to your local LDS and ask the repair tech there to show you examples.
pretty much all regs on the market today are very good to great machines, there are minor differences that can make some great and others only good.
a couple recommendations,
1) find out what regs your LDS services commonly. as the familiarity with a design can greatly influence how good the final tune will be. For example- my LDS will not service ScubaPro, so owning it would be of little value as I would have to ship it out to get it serviced (closest SP repair 3hrs away), lucky me, I don't own any SP.
2) try as many regs as you can before you buy, purely to find designs that are uncomfortable, looking for jaw fatigue or pinch points. for example- many regs have too small a air openning for me forcing me to purse my lips more to seal
3) unless you plan to dive in cold water (<45F), you don't need an environmental seal on your reg. for many (if not all) regs, the environmental kit makes them breathe ever so slightly harder, but this kit can often add $30-$100 to the reg. some (Atomic/SP) even add significant cost to the annual rebuild as well.
4) I recommend a pneumatically balanced second stage. they can be tuned right to the edge and will stay there, thus in general they breathe easier.
a couple recommendations,
1) find out what regs your LDS services commonly. as the familiarity with a design can greatly influence how good the final tune will be. For example- my LDS will not service ScubaPro, so owning it would be of little value as I would have to ship it out to get it serviced (closest SP repair 3hrs away), lucky me, I don't own any SP.
2) try as many regs as you can before you buy, purely to find designs that are uncomfortable, looking for jaw fatigue or pinch points. for example- many regs have too small a air openning for me forcing me to purse my lips more to seal
3) unless you plan to dive in cold water (<45F), you don't need an environmental seal on your reg. for many (if not all) regs, the environmental kit makes them breathe ever so slightly harder, but this kit can often add $30-$100 to the reg. some (Atomic/SP) even add significant cost to the annual rebuild as well.
4) I recommend a pneumatically balanced second stage. they can be tuned right to the edge and will stay there, thus in general they breathe easier.
How stuff works how a pretty good flash presentation, HowStuffWorks "Regulators and Scuba Breathing Apparatus".
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