Scubapro Metal 109 vs 156: A Short Report

[Zung]

As an experiment, I set up the following during my last dive trip:

• SP MK20, IP 9 bar / 131 PSI, no creep
• SP 109, duro poppet, cracking pressure 22 mm / .9 inch
• SP 156, s-wing poppet, cracking pressure 22 mm / .9 inch

Both seconds are freshly serviced with all new soft parts.

I did a side by side comparison and it was REALLY hard to tell the difference. Maybe the 109 has a bit more of an on/off effect, and the 156 is a tiny bit smoother, but I couldn’t consistently tell which is which without looking at the cheater. Half way into the dive, I gave up because the difference is too small to make the experiment worthwhile.

And yes, at that setting, both seconds free flow in a face down position. Interestingly, a G250 at a similar state of tune does not.

The morale of the story is: while it’s totally cool to be able to balance the 109, the effectiveness in terms of breathing is close to zero, and the cost of a balanced seat is maybe a hundred times more than an awap seat.

 

[herman]

Nice to know, thanks for the eval.
I have wondered if it was worth converting the 109 and had come to the conclusion it was not, this just verifies it.

 

[rhwestfall]

Very good news. Seeing that I have a number of 109's (okay, 8, just don't tell my wife ), and a single 156 (I know one has been upgraded as I had it done years ago, including the logo face plate), so its great to know I can get the desired performance without the substantial investment.....

 

[redacted]

With identical cracking pressures and the very stable IP of a Mk20/25, it should take a machine to detect the difference. With less stable IP the BA should have an advantage but may still be like the princess and the pea. I usually use a balanced primary and a 109 for my secondary. And I see some benefits in a pair of 109s in terms of simplicity and parts availability.

 

[Six2Life]

As a suggestion for a possible future experiment...

Make up two sets of regs; one MK2 with a 109 and 156 tuned identically, and one MK5 or MK10 with the same 2nds. It would be interesting to see if on two successive dives using those sets if any differences would be noticed between the various combinations. I'd be most interested in seeing how a MK2/156 would compare to a MK5/109. I wonder if having just one stage balanced would make up for the other being unbalanced.

 

[herman]

It will, balancing 2 is redundant.
The whole purpose of balancing is to keep cracking pressure constant as tank pressure changes. A balanced first does it by keeping the IP constant while a balanced second does it by balancing the forces across the spring of the second stage, both have the same effect. The one diffence I can see, is the balanced second will likely work down to a lower tank pressure since the first stage balancing will stop once the tank reaches IP pressure.
Then again, a little "manual balancing" with a tweek of the adjustment screw will do the same thing so maybe no balancing at all is needed.

 

[Zung]

Check these NAVY charts out:

• Difference between the Adjustable and the Balanced Adjustable is slight
• Difference between the BA and the G250 is huge

Note that the MK9 is a MK10 less the turret.



The HP (108) doesn't fare as well.



Moral of the story: there're some additional advantages with the barrel poppet design vs the classic downstream.

 

[Rusty Shackleford]

Question: When upgraded to the S-wing poppet, what would the difference be between the 109, the 156 and the classic G-250?

Wouldn't the 109 and the 156 Bal Adj essentially be the same reg after the upgrade?

And wouldn't the air barrel be the same as the old school G-250's the only real advantage for the 350 being the venturi effect from the VIVA?

I just purchased a 109 and a Bal Adj both without the S-Wing poppet upgrade. I am planning to upgrade both. What am I gaining and what might I be losing by the conversion?

Would it not make them essentially compatible with the old style G-250 when it comes to rebuild kits from that point forward?

 

[halocline]

1. The 109 and 156 are the same except for the poppet and spring. So after a conversion, yes they're the same.
2. The G250 feels different to me, I'm guessing the geometry of the case and differing venturi assist is what causes that.
3. It is the same rebuild kit, there's just some leftover o-rings. The 109 only has two o-rings; the orifice and the adjusting knob. The G250 also has o-rings at either end of the air barrel.

 

[Zung]

What halo says.

In addition:

3. I prefer the G250 because it has a better geometry, and so it can be tuned to crack at lower effort

4. I would upgrade both the 109 and the Bal Adj because it would break my heart to downgrade the Bal Adj to a 109. But then again, you can "awap" the seats for the 109 for literally a peanut a piece.

5. If you already have some G250's, it a very good excuse to upgrade so you only need to stock up 1 single type of parts. That's my case.