ScubaPro MK10 IP Question

[Angelo Farina]

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.

You are right, the conical seat first appeared on the 4-ports version with large holes in 1979. I did remember wrongly, I have just recovered my reg from storage and here is it:

 

[axxel57]

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.

Yes, the conical filter was introduced before the Cave Cone HP Seat, in 1983, the year the MK10 came on the market.....

 

[halocline]

The problem with some of the discussion on this thread is that it is impossible to accurately measure dynamic IP drop with a standard IP gauge. I really experimented with this a number of years ago and even had a lengthy discussion with Peter Wolfinger about it. I found my MK15 had an apparently greater IP drop under purge than my MK2; which is very counterintuitive considering the MK15 has double the flow capacity.

Here's the reason, when you use a regular hobby IP gauge, especially one that fits on the LP inflator hose, it's not going to be accurate while air is flowing, due to venturi forces in the air paths. The only way you can measure pressure with air flowing would be with a dedicated flowmeter or some other device that is designed to measure pressure within a dynamic setting. And of course, the faster the air is flowing, the more pronounced the venturi is.

I don't understand how the smaller piston would have any effect on this other than the total force on the piston head is lower. For someone that doesn't understand that, imagine that the MK10 piston is 1 sq in in area and the MK5 is 2 sq in. At an IP of 135 PSI, the total force pushing the piston into the seat in the MK10 would be135 lbs, while in the MK 5 it would be 270. (Those figures are not accurate, I'm just trying to explain the situation) Why this matters is because there are other forces at work, most notably friction in the HP o-ring around the piston shaft. That friction will have a proportionally greater effect on the MK10 because the overall force is lower. Does that make sense?

A smaller piston by design is not going to be 'slower' reacting; in fact that piston was designed to flow more air, be lighter, and faster acting than the MK5. That was the whole point of the MK10. But the ancillary forces of friction will have a greater effect. That's why static IP in the MK10 at higher supply pressure goes up more than in the MK5. At least that's what I think.

Getting back to the seat change and the big IP change from -II to -I seats, I agree with Axxel that simply changing that seat is not going result in 15PSI increase in IP. There has to be something else going on, and I would suspect the HP piston o-ring, maybe lubrication at that point, and I would also suspect greater seating force due to tiny imperfections in the new seat. Those things are not perfect. Also, it's not clear that the OP measured IP with the same supply pressure in both cases. So he should try pressurizing the reg for an hour or so, cycle the purge a few dozen times, and see if the seat breaks in a little. I'll bet IP comes down some. I've found about 7PSI difference between each seat height. I'd also make sure the HP o-ring is well lubed, the right duro hardness (85-90).

It's fun to talk about this stuff, I'm not sure it much affect on diving, but still fun. Obviously, what's important is that your regulator holds IP consistently, doesn't creep, and lasts a long time between rebuilds.