Tank Diameter - How to Measure and Why It Matters

Do you just measure the outer resin tank diameter?

* Yes, you measure the fiberglass tank that contains the resins ( or other
media ).
Do not be "fooled" by plastic decorative "jackets" sometimes placed over the
fiberglass ( or polyglass ) tank.
The diameter is the measurement ACROSS the WIDTH of the tank, or the
circumference ( around ) the tank divided by pi ( 3.14 ).
And you should "round down" to the whole number ( i.e. 8.34 would be 8
inches ).

Why do you need this measurement?

* The proper Backwash Flow Rate to be used is determined by the diameter of
the tank.
The larger the diameter, the higher the flow rate required to properly
expand and clean the resin bed.
If your current valve body is an "Autotrol" type, it has a "Backwash" Plug
( PN 25F ) that has a small
number ( and the words "Backwash" ) on it. This number normally corresponds
to
the diameter of the resin tank.
See:
Autotrol Backwash Plug Sizes

* Fleck valves use DLFC ( drain line flow control ) buttons. The gpm is
sometimes marked near the drain fitting outlet.
See:
Common 2 gpm DLFC Button PN 12087

* Also, the injector size used in the Valve is determined by the diameter
of the resin
tank.
Autotrol valve examples,
See:
Common Autotrol Injector Sizes / Colors PN AP28F

And for Fleck Valves,
See:
Common Fleck Injector Sizes / Colors
All these are DIRECTLY based in the Tank Diameter, because the wider the tank, the higher the flow rates need to be for proper washing and rinsing of the media in the tank.