How to do flow monitoring on main return pump

[smithpl]

Hi,
I found Flow-sensor for profilux.
I would like to use this for monitor main return pump in tank but... this pump has flow 4500l/h with pipe 25mm
So I have question how to do this.
Maybe should I do bypass with thinner pipe for Flow-sensor ?

[Wirus]

How would it be to use the flow sensor for up to 9000l/h with DN25 (http://www.ghl-store.com/sensoren/durchfluss-sensoren/flow-sensor-9000-l_h.html)?

[smithpl]

good idea, thanks :-)