Mueller 108-904RP Backflow Preventer Vacuum Breaker - Brass Anti-Siphon Valve for Irrigation Systems, Lawn Sprinklers & Outdoor Water Faucets - Prevent Contaminated Water Backflow - Meets ASSE 1019 Standards

This really works. I got a cheaper plastic one first that kept leaking. Had no trouble with this one. I didn't even use the white plumbers tape. It makes a soft sound and does spray water when it activates, but at least I know its working. I have a soaker hose system setup for my raised garden beds with a timer. So the hose is on all the time under pressure. This goes off anytime the water is used in the house (sinks, toilet, shower, washing machine) so I know it really works. Just don't set it up somewhere you don't want to get wet.