Golan68
Registered
am I the only one who think that all the above regulators are using the same piston? is that mean that they all produce the same performance? any thoughts?
Scubapro mk15 mk20 and mk25
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The Mk 15 used an all stainless steel knife edged piston that is not interchangeable with the Mk 20/25.
The Mk 20 used 3 separate pistons over the course of it's production. The original was an all stainless knife edged piston very similar to the Mk 15. The major difference between the two was the redesign of the bushing arrangement in the Mk 20 as many techs found the Mk 15 hard to service as it used a C clip to retain the inner bushing, piston stem o-ring, and middle bushing. The outer bushing was not replaceable - yet some tech tried to do exactly that.
The middle production Mk 20 used a brass tipped rounded edged piston that was supposed to improve flow rate - like the Mk 10 Plus. But in practice that piston demonstrated a very soft and creepy lock up and had often had a fairly short service life. Scubapro replaced them for free during annual service if the reg was in warranty and was suffering from IP creep, so there are not many of them still out there.
The late production Mk 20 used a composite piston - identical to the piston used in the early Mk 25 and nearly identical to the current piston used in the current Mk 25. The difference in these pistons is a higher degree of polish on the stem to help retard ice development on the stem as part of the TIS kit. In both cases the composite piston uses a seating surface that is a lot closer to a knife edge than a rounded edge and you'll want to use a piston bullet to install it.
In fact, the last Mk 20 was exactly like the first Mk 25 with the exception of the Mk 25s adjustable seat retainer. It was that change, one of many changes in a long series of changes in the evolution of the Mk 20/25 that served as the excuse to call the Mk 20 a Mk 25. The DIN and Yoke retainers were recalled on the Mk 20 to be replaced with Mk 25 retainers, however the new "Mk 25" retainer was we were already being used on late production Mk 20s. The remainder of the changes to the Mk 25 over the years have been cosmetic in nature that there is no performance difference between the Mk 20 and 25.
I'll also argue there is no noticeable difference in performance between the Mk 15 and the Mk 20/25.
If there is a noticeable difference it is in terms of the SPEC kit used on the Mk 15 and very early Mk 20) which is far more effective in cold water than the TIS kit used on the Mk 20 and 25.
The caution about the Mk 15 is that Scubapro no longer makes hard parts for them so if you damage the piston you are basically screwed unless it's still under warranty.
The Mk 20 used 3 separate pistons over the course of it's production. The original was an all stainless knife edged piston very similar to the Mk 15. The major difference between the two was the redesign of the bushing arrangement in the Mk 20 as many techs found the Mk 15 hard to service as it used a C clip to retain the inner bushing, piston stem o-ring, and middle bushing. The outer bushing was not replaceable - yet some tech tried to do exactly that.
The middle production Mk 20 used a brass tipped rounded edged piston that was supposed to improve flow rate - like the Mk 10 Plus. But in practice that piston demonstrated a very soft and creepy lock up and had often had a fairly short service life. Scubapro replaced them for free during annual service if the reg was in warranty and was suffering from IP creep, so there are not many of them still out there.
The late production Mk 20 used a composite piston - identical to the piston used in the early Mk 25 and nearly identical to the current piston used in the current Mk 25. The difference in these pistons is a higher degree of polish on the stem to help retard ice development on the stem as part of the TIS kit. In both cases the composite piston uses a seating surface that is a lot closer to a knife edge than a rounded edge and you'll want to use a piston bullet to install it.
In fact, the last Mk 20 was exactly like the first Mk 25 with the exception of the Mk 25s adjustable seat retainer. It was that change, one of many changes in a long series of changes in the evolution of the Mk 20/25 that served as the excuse to call the Mk 20 a Mk 25. The DIN and Yoke retainers were recalled on the Mk 20 to be replaced with Mk 25 retainers, however the new "Mk 25" retainer was we were already being used on late production Mk 20s. The remainder of the changes to the Mk 25 over the years have been cosmetic in nature that there is no performance difference between the Mk 20 and 25.
I'll also argue there is no noticeable difference in performance between the Mk 15 and the Mk 20/25.
If there is a noticeable difference it is in terms of the SPEC kit used on the Mk 15 and very early Mk 20) which is far more effective in cold water than the TIS kit used on the Mk 20 and 25.
The caution about the Mk 15 is that Scubapro no longer makes hard parts for them so if you damage the piston you are basically screwed unless it's still under warranty.
halocline
Contributor
Ask a question and you get an excellent answer! Thank you DA for continuing to share your extensive knowledge.
The MK15 seat is certainly different than any of the MK20 seats I've ever seen. I have always assumed that none of the MK20 pistons were knife edged in the way the MK15 is. Was there an early MK20 seat that I've never seen?
I believe that the MK20 with the newer piston likely has a higher flow rating than the MK15. IP drop during inhalation on a MK20 with the composite piston is impressively low, and lock up impressively immediate and stable. However, I'm sure that any increase in rated flow is superfluous as the MK15 flows A LOT of air and the tank valve and 2nd stage would be a limiting factor.
I frequently see NOS MK15 pistons, springs, and seat retainers for sale, so I wouldn't be too concerned about not being able to service one. It does take a set of skinny snap ring pliers. I like my MK15 and use it on HP tanks. If I were buying again, though, I'd look for an early MK20 and simply install the composite piston. A friend's MK20 equipped with that piston that I work on is now going on it's 4th year with perfect lock up and zero creep.
The MK15 seat is certainly different than any of the MK20 seats I've ever seen. I have always assumed that none of the MK20 pistons were knife edged in the way the MK15 is. Was there an early MK20 seat that I've never seen?
I believe that the MK20 with the newer piston likely has a higher flow rating than the MK15. IP drop during inhalation on a MK20 with the composite piston is impressively low, and lock up impressively immediate and stable. However, I'm sure that any increase in rated flow is superfluous as the MK15 flows A LOT of air and the tank valve and 2nd stage would be a limiting factor.
I frequently see NOS MK15 pistons, springs, and seat retainers for sale, so I wouldn't be too concerned about not being able to service one. It does take a set of skinny snap ring pliers. I like my MK15 and use it on HP tanks. If I were buying again, though, I'd look for an early MK20 and simply install the composite piston. A friend's MK20 equipped with that piston that I work on is now going on it's 4th year with perfect lock up and zero creep.
OP
Golan68
Registered
Thank you both for the compressive answer and for sharing the knowledge .
I think that since the internal diameter of the piston is the same then the max flow rate must be at least very close.
I noticed the difference of the edge hence the size seems to be exact. I also noticed a difference at the angle at the bottom and due to the very high pressure and air density , it might make big difference .
The c clip is indeed a pain.
I was told that mk15 was designed in the USA and Mk20 about the same time at Europe . I suspect that this is another example of giving up a good design due to techs (D400, Pilot, Air 1)
Thank you both for taking the time to answer.
---------- Post added December 10th, 2012 at 10:47 PM ----------
Maybe the edge plays a major rule of the aero dynamics of the air entering the piston (whistling is indication for resistance , isn't it?).
I think that since the internal diameter of the piston is the same then the max flow rate must be at least very close.
I noticed the difference of the edge hence the size seems to be exact. I also noticed a difference at the angle at the bottom and due to the very high pressure and air density , it might make big difference .
The c clip is indeed a pain.
I was told that mk15 was designed in the USA and Mk20 about the same time at Europe . I suspect that this is another example of giving up a good design due to techs (D400, Pilot, Air 1)
Thank you both for taking the time to answer.
---------- Post added December 10th, 2012 at 10:47 PM ----------
Maybe the edge plays a major rule of the aero dynamics of the air entering the piston (whistling is indication for resistance , isn't it?).
The Mk 15, like the Mk 5, Mk 10, etc, used a straight stemmed piston. In comparison the Mk 20 and Mk 25 have a piston where the end with the seating edge is slightly larger in diameter than the section of the stem that passes through the piston stem o-ring. That very slightly belled profile on the piston stem provides some additional area for the seating edge and ensure the area exposed to high pressure gas is the same area as the piston stem where it passes through the piston stem o-ring. The result is near prefect balance.
With the earlier Mk 5, 10, 15 straight piston stem, the area of the seating edge itself as not actually balanced. it was not a huge amount of area but it was enough to change the IP about 4 psi between 3000 psi and 300 psi.
----
The early Mk 15s had significant issues with seat life, and Scubapro went through various seat materials before they found one that worked long term with the concave seat designed used in the Mk 15. The Mk 10 plus had seat life and lockup issues trying to get the concave seat and rounded tip pistons to work well together, and that is I suspect why the initial Mk 20s had a concave seat but a knife edged seating surface on the piston.
The idea behind both was to improve air flow. Rounding the seating edge smoothed the airflow, and making the seat concave provided a bit more room for the gas to turn the corner and flow through the piston, again improving the flow rate.
At this point in time the Mk 10 uses a concave seat and knife edged piston, similar to the overall concept for the Mk 15, and the Mk 10 Plus, Mk 20 and Mk 25 all use the same seat with a compromise seating edge that is a closer to knife edged than rounded.
With the earlier Mk 5, 10, 15 straight piston stem, the area of the seating edge itself as not actually balanced. it was not a huge amount of area but it was enough to change the IP about 4 psi between 3000 psi and 300 psi.
----
The early Mk 15s had significant issues with seat life, and Scubapro went through various seat materials before they found one that worked long term with the concave seat designed used in the Mk 15. The Mk 10 plus had seat life and lockup issues trying to get the concave seat and rounded tip pistons to work well together, and that is I suspect why the initial Mk 20s had a concave seat but a knife edged seating surface on the piston.
The idea behind both was to improve air flow. Rounding the seating edge smoothed the airflow, and making the seat concave provided a bit more room for the gas to turn the corner and flow through the piston, again improving the flow rate.
At this point in time the Mk 10 uses a concave seat and knife edged piston, similar to the overall concept for the Mk 15, and the Mk 10 Plus, Mk 20 and Mk 25 all use the same seat with a compromise seating edge that is a closer to knife edged than rounded.
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