# DI tank 20x2.5 vs 20x4.5

I need to add a new DI tank to one of my vehicles and it needs to be smaller than a 1/2 cubic foot for space reasons. This will be used after an RO membrane. I know some will recommend I just buy something ready to go but first I want to compare two sizes of DIY tanks…a 20x2.5 inch and a 20x4.5 inch (big blue), both with refillable cartridge inserts. I know the obvious advantages and disadvantages (mainly initial cost, size, weight, and frequency of changing resin differ between the two). I have two specific questions…

1. Is there going to be a flow rate difference?
2. Does anyone know the exact amount of resin that fits in each respective tank? I’ve seen a range of 24 to 32 oz for the smaller tank and 6.5 to 7 lbs for the larger, but would like verification.

I have both.
question 1- no difference in flow
question 2- since resin is sold by volume, not weight, just do the math to calculate total cubic inches each chamber holds

Yes, just take your height of 20" times Pi times R(square) where Pi is 3.14 and R is the radius of your cylinder. This will give you the volume in cubic inches. To get cubic foot, divide this by 12 three times.

Not always. A lot of times smaller quantities are sold by weight, like 5 lb bags. I’m interested to know what the approximate density of DI resin is.

@PressurePoint- if you’ve got the room for it, I would go with the larger vessel. You’ll just save time in not needing to change resin as often.

I bought 1 cubic foot of mixed DI resin from WCR before and it weighs 46 lbs.

that means there is roughly 736 oz in a cubic foot of resin.
there are 1728 cubic inches in a cubic foot.
that means a cubic inch of resin weights .43 oz.
a 4’x20" cartridge will hold 251 cubic inches, or roughly 108 oz. of resin.
a 2.5"x20" cartridge will hold 98 cubic inches, or roughly 42 oz of resin.

^ is my math right? Everything seems right except the last calculation. I have Pi(3.14)*square(1.25)*20=98 cubic inches, not 192 cubic inches. Or roughly 42 oz of resin.

It seems like maybe you used R=1.75 but R should be 2.5/2=1.25 instead.

^correct! entered the wrong value for R on the 2.5" chamber. thanks.