Many thanks for that Mark, Chart saved for future reference.

What you have said about cartridge having the lowest head loss is what I was expecting. As you say the internal fittings are where the head loss occurs which is why I said the "designers" are lazy etc because it could easily be improved and needs to be given that we are going Eco. I have fitted a 15 PSI gauge to my filter as I needed to see the 2 PSI I run at also. I notice the almost exponential graph as the flow increases, which fits with my empirical data and is where I am investigating the head loss. Is it through the media or bad plumbing? Not an issue in the past as a powerful pump always makes sure the water gets where it's going but reducing the power brings on new challenges.

Thanks for you help on this, it's appreciated.

Regards
John