ScubaPro MK10 IP Question

[rhwestfall]

Lube - One issue can be how much. Too little is problematic as well as too much. Much the reason the "wiper" was added to the MK-10+ (and I believe the 20/25?).

 

[Open Ocean Diver]

Deleted, corrected myself.

 

[axxel57]

So I've got two MK10s that I just revived from the bin at the shop and serviced.

One had an old red seat, the other had a -II gray seat. Even though everything was in good shape, because these were basically bags of parts, I did not have the opportunity to test them before service, so I don't know what the IP would have been.

After service, IP on both was low (120-ish) when I used new -II seats but lockup was solid and the dip in IP when a breathed on a reg seemed normal. It would have been very satisfactory but for the fact that the IP was low.

Since these were already pretty much shimmed up, I switched to the -I seats on both to bump up the IP. IP is now on the money (135). So far so good.

Here's the question: Now when I breath on the regs I get a much larger swing in IP. The first stages do not feel "wide open" until IP dips to about 100psi. They crack open just fine and deliver plenty of air, but as I breath deeper, the dip in IP increases (i..e, IP drops) until it hits about 100 and then things seem wide open and no amount of extra breathing demand gets the IP to drop further. IP rebounds immediately to 135 with no creep or drift as soon as I stop inhaling.

Is this normal behavior?

What you experienced after service sounds a bit strange to me.

If your IPs in both MK10s had been in the low 120s psi using -II seats, it‘s not clear for me how you ended up with 135psi on both regs using just -I seats. Normally they increase or decrease the static IP by only 3-5psi.

The seats also shouldn’t be responsible for a higher dynamic IP swing, at least not to a higher extent.

A swing of 35psi dynamic IP is pretty high, but not uncommon with these old regs.

Responsible for the high IP swing is mostly the main spring which tends to get stiffer over the years and use, so it produces a higher IP and a higher IP swing ( because the stiffer spring prevents the correct seat-piston separation).

If you invest ( what I suggest) in a new MK10 main spring, you will realize that the IP will be significantly lower, so will be the dynamic IP swing.

A new main spring will produce an IP swing of about 15-20psi

If the dyn. IP drops to less than 100psi, I usually change the main spring for a new one, I don’t feel comfortable with such a low IP, especially as a Pro responsible for students and other divers.

Things can happen, and when you end up having a distressed diver on your octo, it will lower the dyn. IP even a couple of psi more, and when your first delivers in the end 40 or 45psi less than the specs ask for, you might have at least a quite uncomfortable situation with two 2nds that have now a higher cracking effort, even if the 1st delivers enough air.

As mentioned before, I still don’t understand how the change of the HP seats could produce such a significant higher dyn. IP swing.

 

[Angelo Farina]

What you experienced after service sounds a bit strange to me.

If your IPs in both MK10s had been in the low 120s psi using -II seats, it‘s not clear for me how you ended up with 135psi on both regs using just -I seats. Normally they increase or decrease the static IP by only 3-5psi.

The seats also shouldn’t be responsible for a higher dynamic IP swing, at least not to a higher extent.

A swing of 35psi dynamic IP is pretty high, but not uncommon with these old regs.

Responsible for the high IP swing is mostly the main spring which tends to get stiffer over the years and use, so it produces a higher IP and a higher IP swing ( because the stiffer spring prevents the correct seat-piston separation).

If you invest ( what I suggest) in a new MK10 main spring, you will realize that the IP will be significantly lower, so will be the dynamic IP swing.

A new main spring will produce an IP swing of about 15-20psi

If the dyn. IP drops to less than 100psi, I usually change the main spring for a new one, I don’t feel comfortable with such a low IP, especially as a Pro responsible for students and other divers.

Things can happen, and when you end up having a distressed diver on your octo, it will lower the dyn. IP even a couple of psi more, and when your first delivers in the end 40 or 45psi less than the specs ask for, you might have at least a quite uncomfortable situation with two 2nds that have now a higher cracking effort, even if the 1st delivers enough air.

As mentioned before, I still don’t understand how the change of the HP seats could produce such a significant higher dyn. IP swing.

Probably he is swapping a worn -II seat with a new -I seat.
In my understanding the large IP drop is quite typical of the MK10 due to the smaller size of piston head, which reduces the force applied to the spring compared to a MK10.
Also, in my experience, old springs do not change the elastic modulus of the steel, what changes is their lenght. They become shorter.
kìAnd you fix it with shims, no need to change the spring...
The trick for reducing the IP swing is reducing friction between piston and reg body.
This is obtained polishing the surface where the large O-ring of the piston head moves, and polishing the piston stem in the area where the smaller O-ring works.
And of course proper lubrification with a good, not-sticky grease.

 

[axxel57]

Probably he is swapping a worn -II seat with a new -I seat.
In my understanding the large IP drop is quite typical of the MK10 due to the smaller size of piston head, which reduces the force applied to the spring compared to a MK10.
Also, in my experience, old springs do not change the elastic modulus of the steel, what changes is their lenght. They become shorter.
kìAnd you fix it with shims, no need to change the spring...
The trick for reducing the IP swing is reducing friction between piston and reg body.
This is obtained polishing the surface where the large O-ring of the piston head moves, and polishing the piston stem in the area where the smaller O-ring works.
And of course proper lubrification with a good, not-sticky grease.

Yes, correct, the smaller piston head doesn't respond as good to pressure as models with bigger piston heads, but 15-20psi IP swing is pretty large in comparison to the MK20/25, which usually have less than 10psi swing.
Yes, some springs get shorter and so stiffer, but I measured a couple of MK10 springs and compared them with with a brand new one, couldn't find any significant difference.
I don't know how they get harder with time, but new ones seem to be 'softer' and they 'behave' so.
This is why you can only remove shims, but when there are none anymore, you can only change the spring.
Of course friction can play a role, but I just assume that the OP polished those areas as one should do.
Since Tribolube 71 was used, the lube cannot be the culprit.
The Mk10 was designed to correct a flaw of the MK5 series, which were flowing only less than 1400l/min when at low tank pressure ( ca. 2500l/min at 2000psi tank pressure).
It flew a bit less air at 2000psi (2300l/min), but still almost 2100l/min at low tank.
In the end SP came back to the bigger piston design with a lighter Composite Piston, flowing almost four times as much air than the Mk10 or Mk5 design.

 

[jgttrey]

Probably he is swapping a worn -II seat with a new -I seat.
In my understanding the large IP drop is quite typical of the MK10 due to the smaller size of piston head, which reduces the force applied to the spring compared to a MK10.
Also, in my experience, old springs do not change the elastic modulus of the steel, what changes is their lenght. They become shorter.
kìAnd you fix it with shims, no need to change the spring...
The trick for reducing the IP swing is reducing friction between piston and reg body.
This is obtained polishing the surface where the large O-ring of the piston head moves, and polishing the piston stem in the area where the smaller O-ring works.
And of course proper lubrification with a good, not-sticky grease.

Actually, these were two new -II seats and switching to -I seats bumped the IP by about 12 psi in both regulators. Nothing else was changed. I simply swapped the seats after I saw that the IP was too low and the regs were already fully shimmed (well, one had only two shims, but I didn't have another laying around).

My only theory as to why the seat change, made more of a difference than 3-5 psi is based on something I saw in one of Couv's older posts where he had measured the heights of several examples of the +, -I and -II seats. IIRC, there was a bigger difference between the -I and -II seats than there was between the + and the -I. From his measurements, I inferred that the difference between -I and -II was the equivalent of 2 or 3 shims, which is about what I experienced, while the difference between the + and -I was smaller, and probably only worth about a single shim.

That's just a theory. I'm no expert on this regulator. I can only report that both regs responded identically to going from a new -II seat to a new -I seat, with nothing else changing. Anyway, @Angelo Farina' s explanation about the smaller piston head makes perfect sense to me and explains my biggest question, which was the large dynamic IP drop. Understanding that, the regs are working fine now and I'm a happy camper. Appreciate all the feedback. A great education.

 

[Nemrod]

FWIW, replacing the flat seat in the Mk V with the concave "cave" seats I see less IP drop across the tank pressures from high to low.

Where are we getting the data that a Mk X can flow twice the volume of a Mk V? Just curious, I am not finding that right off. Was this with the "cave" seats?

Of my first stages including a Mk 20 (upgraded) a couple of 10s and a bunch of 5s and now a new Mk 2 Evo, the Mk 2 Evo seems to have the least drop under load (excluding the Mk 20s) and fastest response but also of course IP drops from full to empty tank being unbalanced, I have just started fooling with it so maybe my first impressions will change. At 3200 psi it shows about 145 IP and at 1200 psi it is still at 134 psi. I have seen more drop over that pressure range on a few supposedly balanced first stages. The diaphragm AL Legend stays pretty darn flat but shows a large IP drop under load. A Mk 20 upgraded barely shows a small blip under load and stays pretty tight across varying tank pressures. All of these can flow more than the valve can deliver. If a flow rate of ridiculous amounts is the need, can it be disputed that the Mk 20/25 is standing on the high podium?

 

[axxel57]

FWIW, replacing the flat seat in the Mk V with the concave "cave" seats I see less IP drop across the tank pressures from high to low.

Where are we getting the data that a Mk X can flow twice the volume of a Mk V? Just curious, I am not finding that right off. Was this with the "cave" seats?

Of my first stages including a Mk 20 (upgraded) a couple of 10s and a bunch of 5s and now a new Mk 2 Evo, the Mk 2 Evo seems to have the least drop under load (excluding the Mk 20s) and fastest response but also of course IP drops from full to empty tank being unbalanced, I have just started fooling with it so maybe my first impressions will change. At 3200 psi it shows about 145 IP and at 1200 psi it is still at 134 psi. I have seen more drop over that pressure range on a few supposedly balanced first stages. The diaphragm AL Legend stays pretty darn flat but shows a large IP drop under load. A Mk 20 upgraded barely shows a small blip under load and stays pretty tight across varying tank pressures. All of these can flow more than the valve can deliver. If a flow rate of ridiculous amounts is the need, can it be disputed that the Mk 20/25 is standing on the high podium?

The Cave Cone Seat had been implemented by SP in '87 to reduce the IP Creep and the whistling problems in their 'Flow Through' Firsts at that time.
The Mk10 cannot flow per se the double amount of air, just at low tank pressure. At full tank the MK5 is flowing more than the MK10, and it has nothing to do with the cave cone seat, which came four years later on the market than the MK10.
The OP had questions about the behavior of his reg concerning the dynamic IP, the question was not about how good or bad the first was balancing between full or near empty tank or how much the first is flowing.
I personally have no problem that my MK25 is flowing ridiculous amounts of air, I like Deep Air Diving.
But the point is, what is a first stage in the first line for?
For me it has to provide a stable IP over the tank pressure range, at high flow ( dynamic IP in greater depths), and it should deliver lots of air absolute also at high and low tank pressure.
So for me the MK20/25 is doing a good job.

 

[Angelo Farina]

If I remember correctly the conical filter appeared on the MK5 together with the 4-ports turret and the small holes on the equilibrium chamber.
It was 1979 or perhaps 1980.
Definitely before the MK10 was launched...
It was possible to retrofit it to older models by drilling a larger hole in the joke connector.
It provided less pressure drop, larger air flow and it was more difficult to be clogged by debris or rust coming from the tank.

 

[Nemrod]

If I remember correctly the conical filter appeared on the MK5 together with the 4-ports turret and the small holes on the equilibrium chamber.

My one Scubapro Mk V and over a dozen Mk V clones have four LP ports, conical filters and some of the later ones have cave seats. The flat seats can gently whistle, usually at near full tanks. These all date from the 80s. Flat seat and conical filter. Large ambient holes.