https://terrylove.com/forums/index.php?threads/flow-inducer-sleeve.95740/No Dole valve or CSV in current use, can you point me to the right thread for the flow sleeve? I searched as you say but you get a lot of hits. I expressed my probably unwarranted concern on the sleeve in my response to Valveman above.
How about the tank diameter? 10 inch? This table is for Katalox Light, one of the lighter effective medias.I'll have to see what I can dig up for info on the filter, I'm sure there's a label on it somewhere. The filter itself is a completely enclosed fiberglass shell and stands about 4.5' tall, you don't change the media so I doubt I'll be able to provide anything there.
I think you would want between 30% and 40% bed expansion, tho 25% might be enough. More expansion is usually better.
Here is the deal... some media, such as Filox, etc, are very dense, and need a lot of gpm/sqin. And yes, it is the cross sectional area that matters for getting the backwash rate right. Some media are lighter.
If you have a jacket over the tank, that makes measuring the diameter of the tank harder, so compensate. You would not be able to knock on the tank to learn about the bed expansion, and you certainly could not shine a light through.
Is the fiberglass natural tan color? If so, you can shine a light through, in the dark, and see if there is bed expansion during backwash. If painted, some can detect the different sound of the fluffed-up/expanded media when knocking onto the side of the tank during backwash.
Would you say your iron filter is effective (not broke)?