There are a couple of different positions you can put the sensors depending on what you want to monitor.

If you are running an RO unit without a DI resin pod it is usually best to position the 'IN' sensor on the tap water coming in to the unit and the 'OUT' on the RO water line from the membrane. This way you will be able to work out if the membrane is performing properly and can give you an indication of when the unit needs filters replacing.

On units using a DI resin pod after the RO membrane the 'IN' sensor is usually located between the membrane and the DI pod and the 'OUT' on the outlet of the DI pod. This way you can monitor the condition of the membrane and tell when the DI resin needs replacing.