Here's a little more data, and it looks good for the D420.
First, the ground rules: any regulator with a cracking effort less than 1 1/2", inserted diaphragm down, will open its valve and flow air. That's the definition of "the sink test", yes? Whether or not it stops depends on two factors: 1) whether the Venturi kicks the reg into true freeflow, and 2) whether water can seep into the case, fill it up, and neutralize the negative pressure created by the escaping air.
The attached 5-sec video shows a D420 and a G260, both on the same Mk17 first stage, being dunked in the water. Both were tuned to 1.1" cracking effort.
Two previous tests (not shown) are not surprising:
1. With the Pre-dive switch in the "Dive" position, both regs dunked "diaphragm down" went into full freeflow and sprayed water all over the bathroom!
2. With the Pre-dive switch in the "Pre-dive" position, if I inserted either reg in the water with the diaphragm down and then kept the mouthpiece facing perfectly up, the regs didn't go into massive freeflow, but the valves never closed, either. The escaping air kept all water out of the case, and air continued to flow, because the distance from the diaphragm to the top of the mouthpiece in inches was greater than cracking effort.
So, on to the test. I attempted to simulate a water entry in disadvantageous conditions. The regs were held nearly diaphragm down, and clearly in a position that would cause the valves to open. Both were tuned to the same cracking effort, and the D420 was further challenged by removing the mouthpiece restriction. For visibility in the video, the pink mouthpiece was cut to just leave the restriction on the G260, with no bite blocks. I don't think I'll regret sacrificing my pink mouthpiece, do you?
As you can see, the D420, even without a mouthpiece restriction, displayed no inclination to freeflow. Despite equal or better air delivery than a G260, and an absent mouthpiece to facilitate possible freeflow, the D420 stopped leaking faster than its sister.
I think she'll do just fine as an octopus.
Of course, ya gotta remember to put your octo in Pre-dive!
