First, I found some of these techniques useful.

As an engineer I’m very disturbed by others calling this engineering. This is very prime demonstration of technician hands on know how as is most DIY. One of the comments is correct, engineering is not about perfection, it’s about making trade-offs and coming up with solutions to problems. However it is also firstly about ethical designs and secondly about solving the problem. An effective way to take out the trash is to dump it on the neighbor’s lawn and make them pick it up, it is not ethical.

As mentioned by some with something to lose and some without, this style back flow prevention does not work as shown and you will risk poisoning yourself, your family, and your neighbors. Go buy a back flow prevention device that meets code specifications. In my city they follow all standard plumbing recommendations from International Association of Plumbing and Mechanical Officials with the exception that they require very specific standards for back flow prevention devices. I’m quite sure my city isn’t the only one.

Get your install inspected and follow the local code for sprinkler systems! there are very good public safety reasons for the code.

-Signed a concerned engineer with interest in DIY sprinkler systems with no connection to professional installers or back flow prevention manufacturers.