Today is the big day! I finally found a way to mount my water sensors on the water circuit. After a lot of unsuccessful and/or wet tries, I came to the idea of using the same Gardena elements as my sprinklers. They provides a 25mm PVC pipe, Tee fittings with 3/4 connectors and barbs to connect to flexible hoses.
As I had some pipes left from my sprinkler installation, I only had to buy a set of Tees in order to assemble my sensors bypass. From now on, I only have to start the secondary pump in order to get some fresh water in the pipe to take my water quality measures!