Pilot (Air 1 converted) /MK9 project

[Redbeard04]

I just acquired an Air1 converted Scubapro Pilot that came with a MK9 first stage off of ebay. Today I hooked them both up and tested them, both were in need of a rebuild with the 1st stage IP at 125 @ 500 psi and over 180 at 3000 psi and the pilot free flowing on a good 1st stage set to 138 psi.

I cleaned and rebuilt both and after some tuning with shims and seats I was able to get the MK9 to stay within 135-145 @ 500 and 3000 psi. So far so good. After cleaning and reassembling the pilot I tuned it with the set screw just off of a free flow and the spring cap screwed most of the way down where it seemed happy to seal up and not free flow. Test breathing felt very light on inhalation and exhalation and the magnahelic confirmed it is cracking at about .7 inches. Checking WOB on the flow meter shows it very quickly goes from less than .5 inches resistance to -1, -2, then off the scale past -5 inches above 15 scfm.

I'm all for a low WOB, but would like to avoid excessive positive pressure forcing me air at depth. The pilot tuning instructions reference tuning the aspirator gate much like you would a D400, however the example I have doesn't have one. I wasn't able to find an Air 1 repair guide so I don't know if it follows the same procedure of not. The local SP rep said that the air 1 didn't have a means of tuning the venturi effect and is just always wide open.

So I guess what I'm asking is does the Air 1 and/or converted pilot have a means of tuning the venturi effect? Is my example missing an aspirator gate, and if so what is the best way of sourcing one? This is a regulator I'd like to dive, but I'd rather see it's air delivery a bit more civilized before taking it to depth.

 

[Kupu]

A photo showing conversion kit parts is attached along with Air1 manual & schematic for reference.
The converted pilot will use AIR1 adjustment procedures. If you are missing parts, I have spares available.

The aspirator position is fixed in the converted pilot, not adjustable as it is on original pilot.
I find properly tuned Air1s and converted pilots to be well behaved and a pleasure to dive, so hopefully the tuning procedures in the manual will help sort it out.

 

[axxel57]

Hi Redbeard
First congrats for your nice find!
I love the Pilots, whether they are AIR 1 converted or original, and I like the MK9 optically.
Concerning your tests on the Flow Meter, I must say, the way you describe it, to me that seems quite normal.
If you start the test with a Cracking Effort (CE) of 0,7 inch/h2o, I would just expect the numbers to go into negative territory, and it's not unusual that it goes deeper into minus the higher the flow.
Of course it would be nicer, if the the CE would just go slightly into negative and stays there until 15csfm (or more) flow, and with the D-Series one can adjust so in many cases, but with other models that is often not the case.
Going into negative values doesn't mean automatically that the 2nd will feed force you with air, that's at least my experience.
Point is, when you still can control the flow when conducting the test, going into negative is mostly not a problem.
A problem is, when while doing the test (increasing the flow), the cross over into negative happens within a second and very violently, so the Venturi opens the valve instantly completely and you have a full blown free flow.
Notorious for this are sometimes R190 2nds, which shoot from 1,00 inch to out of scale negative values and 15 or more scfm flow within one second ( and you can only close the tank valve to avoid massive air loss).
I cannot be sure, but I think that was not what you where describing concerning your tests.
With what you were describing, I would not hesitate to take your Pilot for even a deeper dive.
I'm quite sure practically your 2nd will not feed force you with air.

 

[rsingler]

One of the bigger factors in Venturi boost is gas expansion at the valve. At the surface, gas goes from 9 bar to 1 bar, and expands 9-fold. This in itself generates molecule movement which augments Venturi effects.

In contrast, at 100 feet, absolute IP has risen to match the increase in ambient pressure, so absolute IP is now 12 bar. But since ambient pressure is now 4 bar, gas expansion is only 12/4, or 3‐fold. Of course, there are more molecules flowing per breath, but this has less of an effect than the decrease in gas expansion.

In short, what you have seen on your test is less of an issue at depth than you might think.