Usually used with a return pump, this flow sensor can be placed in line with the flow on any pipe, connected to the FMM, and communicate to your Apex the flow rate of water through that pipe. The FS-100 will work with flow rates from 150-1500 gph.

Please note that union adaptors are sold separately  - 1inch threaded adaptor (female)
https://charterhouse-aquatics.com/shop/aquatics/fittings/female-adaptor-sockets/adaptor-socket-female-32mm-x-1