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Last night we put the DE into the skimmer, but we can't see the returns at the bottom because of the cloudy water. Nothing came to the top - does that mean the filters are OK? We had to backwash within 1 hour of adding the DE. The pool does not look any clearer today. Does the DE go away with backwashing? Should we keep doing this again?
If you can't see the water stream from the returns . . . it doesn't tell you anything. The DE is either on top of the sand in the filter, and will be backwashed out OR it's sitting on the bottom of the pool along with the dead algae.
*If* the DE is on the filter, you should see filter pressures rise much more quickly than normal. If this is not happening, I'd recommend opening one of the filters and checking sand levels.
Something just occurred to me -- if someone has backwashed the filters one at a time, instead of doing both simultaneously, odds are you've blown much of the sand out of the filter. I'm guessing the flow curve below shows your pump on the "F" curve. If so, that pump will deliver WAY too much water, during backwash, for a single filter. Maximum backwash flow on a TR-140 is about 110 GPM. That pump can deliver 2x that rate of flow.
If the standard practice at your pool has not been to backwash both pumps simultaneously, you'll need to (a) replace the missing sand and (b) develop a process that will ALWAYS be followed during backwash, in order to prevent excessive backwash rates.
Sorry it took so long to write back. We used to backwash both at the same time and then someone told us this was wrong and we backwashed one filter at a time. Tried to get into the sand filters and broke the handles of the tool.
Our pool is 34 years old - maybe the plastic tool got brittle. Got a new handle and broke it again. Last night we finally managed to open the filter tops and a lot of sand is missing. We added the sand and it starts to look better already. Thanks a lot for all your help. I hope it is looking clear tomorrow!
Glad you worked it out!