ScubaPro MK10 IP Question

[jgttrey]

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?

 

[Angelo Farina]

I find it normal in the MK10. It has a smaller piston head compared to MK5, so it is slower reacting to pressure drop caused by pushing the purge button or inhaling.
The MK10 is a great reg, but it is designed for being matched with a balanced second, such as a 109 modified to BA, a 156 or a G250.
These regs do not suffer of the pressure drop of the MK10.
If using an unbalanced second, you probably will get more constant performances with a MK5.

 

[Open Ocean Diver]

I find it normal in the MK10. It has a smaller piston head compared to MK5, so it is slower reacting to pressure drop caused by pushing the purge button or inhaling.
The MK10 is a great reg, but it is designed for being matched with a balanced second, such as a 109 modified to BA, a 156 or a G250.
These regs do not suffer of the pressure drop of the MK10.
If using an unbalanced second, you probably will get more constant performances with a MK5.

What would be the understanding. smaller piston, larger stroke to move same amount of air?

 

[rhwestfall]

What lube and how much did you use?

 

[Angelo Farina]

What would be the understanding. smaller piston, larger stroke to move same amount of air?

The point is not "moving air", this is not a pump! The point is being sensitive to pressure drop. When the pressure reduces as the valve on second stage opens, the piston must move from the closure position, opening a gap through which the high pressure air gets laminated, until the increase of pressure shuts the piston back again the seat.
A piston with a large stem (allowing for a large flow) but a small head is slower reacting to pressure reduction. The MK5 was faster, thanks to the stiffer spring acting against a piston with larger head.
In both regs the piston travels for the same stroke.
A piston reg is not a piston engine...

 

[Eric Sedletzky]

In any scenario with any of those SP piston regs, they will deliver way more air than would be needed in any scenario with two divers huffing away on the same reg at any depth. So the whole pressure drop point is somewhat moot anyway.
Pistons excel at airflow because there really is no limit to how far that well lubed piston can slide back to keep up with demand, except for spring pressure.
If you want to see what miserable performance is with two second stages fully at work at depth on a single 1st stage, try and old diaphragm reg that had a small diameter thick diaphragm with hardly any flex and a small volcano orifice. Those were truelly dismal breathers!

 

[scrane]

My Mk5 which was rebuilt not too long ago has an IP of 130. It will drop to 115 and sustain that pressure throughout a long breath, never dropping below that. It recovers quickly.
A Mk5 that has a pressure drop from 135 to 100 under load right after a rebuild doesn't seem right to me.
Does the IP spring back to 135 quickly after the breath/ purge? Like almost instantaneously?

 

[Angelo Farina]

My Mk5 which was rebuilt not too long ago has an IP of 130. It will drop to 115 and sustain that pressure throughout a long breath, never dropping below that. It recovers quickly.
A Mk5 that has a pressure drop from 135 to 100 under load right after a rebuild doesn't seem right to me.
Does the IP spring back to 135 quickly after the breath/ purge? Like almost instantaneously?

You are correct. In a MK5 the max drop during air flow is around 15 PSI.
But here we are talking of a MK10.
During a massive air flow, it is normal that a MK10 drops 30 PSI. But the max air flow is also almost twice that of a MK5...
That's the price to pay for such a massive air flow.
And this pressure drop does not cause significant problems to a balanced second stage.

 

[scrane]

OOPS! Thanks for your gentle correction.

 

[jgttrey]

Thanks all. Much appreciated.

@rhwestfall, I used Tribolube 71.