Tank Diameter - How To Measure and Why It Matters
* Yes, you measure the fiberglass tank that contains the resins ( or other
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 ).
EASY WAY - Take a piece of string, and wrap around the tank.
Then cut or mark the string so you can measure the total Length.
Divide that number by 3.14 to get your Diameter in Inches.
The number will always be higher, since you are measuring the outside of the tank.
And you should "round down" to the nearest whole number
( i.e. 8.34 would be 8 inches tank diameter).
Why do you need this measurement?
* The proper Backwash Flow Rate to be used is determined by the diameter of
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
( our PN AP25F ) that has a small number ( and the words "Backwash" ) on it.
This number normally corresponds to the diameter of the resin tank.
Autotrol Backwash Plug Sizes
If your Backwash Plug Number is LARGER than actual Tank Diameter, this would mean you have a "Turbulator" installed inside your Resin Tank ( instead of a normal one pipe with Bottom Distributor, like our MD1236 ).
NOTE: It is recommended to increase the Control Valve's backwash flow control rate by 30% when using a Turbulator.
So, an 8" tank might have the # 10 Backwash Plug, and a 9 or 10 inch tank might have the # 12 Backwash Plug installed.
*** NOTE: Some Autotrol valves, if "backwash only" of Filter media ( NOT water softener resin application )
will have the Backwash Plug with NO Flow Control Ball ( AP4F, or PN 1030502 ),
to allow full flow ( 5+ GPM when heavy filter media is used --- such as BIRM, Calcite, Greensand, etc )
* Fleck valves use DLFC ( drain line flow control ) buttons. The gpm is
sometimes marked near the drain fitting outlet.
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,
Common Autotrol Injector Sizes / Colors PN AP28F
And for Fleck Valves,
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.
Common Tank Sizes and Amount of Resins Inside
8" x 35" 0.64 cu. ft. ( 20,480 grains )
8" x 44" 0.75 cu. ft. ( 24,000 grains )
9" x 35" 0.75 cu. ft. ( 24,000 grains )
9" x 40" 1.00 cu.ft. ( 32,000 grains )
9" x 48" 1.00 cu. ft. ( 32,000 grains )
10" x 35" 1.00 cu. ft.( 32,000 grains )
10" x 40" 1.00 cu .ft. ( 32,000 grains )
10" x 44" 1.25 cu. ft. ( 40,000 grains )
10" x 54" 1.50 cu. ft. ( 48,000 grains )
12" x 52" 2.00 cu. ft. ( 64,000 grains )
13" x 54" 2.50 cu. ft. ( 80,000 grains )