Quote Originally Posted by spica View Post
I then vacuumed the DE from the bottom surface. It's difficult to see, but it looks like it still goes right through the filter.
I have to admit that the DE is not from a pool store. It's a brewery grade, the datasheet states that 96% of the particles are smaller than 105um.
I've always suspected that this would happen -- but we haven't had confirmed cases of people using non-pool DE. Now, we have. Until you test with pool DE, your test tells us nothing.

Does the test really work? Depends on what you mean. If you mean, does using pool DE to check filter function work, the answer is "Yes, generally". If you mean, does it work to use DE to enhance sand filtration, no, not really (sorry mods!). A tiny sand filter coated with DE is converted into a microscopic DE filter.

Here's what I mean: a minimally sized sand filter, for a 25 GPM flow is going to be 18" in diameter. Except for the shallow sand bed, your filter (at 16") is OK at 18 GPM. But, a minimally sized DE filter for 18 GPM is going to to have 9 sft of surface . . . and yours has only 1.36 sft. So you take a marginally acceptable sand filter, and turn it into a DE filter that is 1/6 of a marginally acceptable size.