I purchased a 9000 l/h sensor, thinking that I can plumb this into my return line. My return line is 1-1/2", which splits into two 1" returns at the tank (one on both sides). Since the flow sensor has an internal diameter of about 1", and threaded fittings that are 1-1/4" BSPP, I thought I would be able to plumb the flow sensor directly to my 1" lines. This is not the case: there does not seem to be a way to connect the BSPP threads in that size to 1" PVC in the US. The only solution for making this connection appears to be to use half of a 25MM union, which will step down the 1" internal diameter of the sensor to 3/4" PVC.

I am flowing about 1000-1200 gph (call it 4000 l/h) through this 1" pipe. If I step this down to 3/4", I am concerned that I will be adding a lot more head to my return line. This has me considering a bypass configuration. Alternatively, I could connect the 25mm union to the 3/4" pvc and then step that right up to 1" pvc, but that 3/4" restriction will still be in the line.

Which brings me to my question - in the US, what is the actual max flow for this sensor that people are achieving? Since our only option here is to use 3/4" pipe, and since that size pipe has a max flow rate (at 12 f/s) of something like 1400 gph (5000 l/h), the math would say that this sensor can not achieve 9000 l/h. But, that is the theoretical picture - what do people see in the real world?