I am a licensed backflow tester by the ASSE and have been since 2000. I need to get several backflow test kits recalibrated, as I'm pretty sure they are supposed to be every year. I guess I don't know of a requirement that they be recalibrated at an independent facility, or if I could simply recalibrate them myself. I'm really not even asking what the standard is so much as I am asking if anyone has ever tried to or seen somebody build a calibration station using water column(s) to recalibrate their backflow tester. I guess I'm just kind of curious if it can be done in the workshop rather than send it away and wait for it to come back for a couple of weeks and spend $100+ plus per instrument each time, thats all. And no, I would never 'calibrate' it to any different 'standard' than what is proper, simply to sell backflow rebuilds. I hate rebuilding backflow preventers, they're a pain in the a$$ usually, and customers ABHOR it when you tell them their BF failed (probably because many testers 'pass' them regardeless of test results. I would much rather the device simply pass, I collect the measly amount of money we charge them, and I be on my way to more profitable ventures.