SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

Playing the odds is something the various US regional plumbing codes have dispensed with, and they are correct to do so. Those codes basically declare the entire outdoors to be toxic, and require that it has no possible avenue of entry into a potable water supply. Since toxic backflow is prevented only by those devices rated to do so, that disallows a "dual check" device to serve as sprinkler system backflow protection, as it is rated as effective only against non-toxic backflow.

This "toxic backflow" designation might seem extreme to some people, but employing it creates genuine water-supply protection, and does so in a way that does not require constant vigilance, while also making it possible to confirm that the devices operate correctly (the "dual check" is a piece of brass with a label stuck on it, and you have absolutely no idea whether it functions or not - rest assured the manufacturers did not guarantee it would work perfectly forever and ever)

I respect both sides of this argument.
If I am the only one in the city practicing safe water protection, what is the point. This is a system fail.
For good information and pictures of why backflow devices fail.www.backflowinformation.com

I appreciate that you already have done substantial research.

The frustrating part for me is that in one region irrigation backflow can be a low hazard device (double check below grade).
In other regions it must be a high hazard device (PVB or RP).

I am happy to be involved in the water quality field. I began my adventure as a Sprinkler Contractor.

The choice of contractors installing toxic-rated backflow protection is the PVB, located at a height that is at least a foot above anything downstream of it. It would be worth the expense to install, because even a humble single-family home can become the target of a "backflow event", and a PVB at the correct elevation is trustworthy protection. It works on gravity, and gravity is generally considered to be in good working order at all times. I like the Wilkins 720, for its durable construction.