To control the flow on my open loop system, I use a ball valve.  To read the flow I use a variable area flow meter.   I currently have installed the flow meter inline with the supply coming into the unit.  

Since then I've seen pictures of a few installations where the flow meter is piped in parallel arrangment with the supply line, with isolation valves that allow the water to go through the flow meter when ballancing, but around it when in normal operation. 

I'm thinking the parallel arrangment would be an improvement as the readout on my meter is beginning to get a little cloudy/discolored from constant exposure to hardwater over the past year or so.  

Can anyone else comment on series vs. parallel ? 

With the parallel arrangement, how do you ensure that both flow paths have equal pressure drop and thus what you measure/see through the meter when ballancing is the same as what you are getting through your unit once the valves are thrown over ??