Pentair filter - PSI is the same after adding D.E. after a backwash - how?

[just4747]

Just did the first backwash in a few months on my Pentair 24 filter, as the pressure was at about 25 PSI (it took a while to get there over summer). This is in prep to SLAM and close my pool.

After backwashing and rinsing until the sight glass was clear, the new pressure reading was 18psi. I ran the filter and shortly after, added two full orange scoops of DE, but the PSI reading has remained just about the same, maybe 1 higher at 19 PSI since.

I expected it to go up a bit right away, so that was weird. I then also remembered that 18 PSI was the starting pressure AFTER I added fresh D.E. last time I backwashed months ago, which is now confusing me even more.

So two questions:

1. How was I at 18 PSI or lower AFTER adding D.E. after a backwash months ago and now I'm at 18 PSI after backwashing but BEFORE adding D.E. now??

2. Does it make sense that I am starting at 18 PSI after backwash and then staying almost the same (19ish) after adding 2 scoops of D.E., or should the pressure have risen up right after? What could cause this?

Thanks.

 

[ajw22]

I think you are fine.

When do you breakdown your DE filter and hose off the grids?

During the season backwashes only get rid of about 80% of the DE and the rest is caked on the filter. You do not see that much of a pressure change as the caked on DE accumulates.

 

[just4747]

I think you are fine.

When do you breakdown your DE filter and hose off the grids?

During the season backwashes only get rid of about 80% of the DE and the rest is caked on the filter. You do not see that much of a pressure change as the caked on DE accumulates.

I did a full disassemble and cleaning with Magic Jack spray/hose at the beginning of the summer and haven't done that or backwashed since, as the pressure was just recently getting to around 25 PSI.

I realized though that after that cleaning months ago, I had written down a starting pressure (on the freshly cleaned filter after adding 2.5 cups of initial DE) was 15-18 PSI - not sure why I put a range but maybe that explains it and that normally it's around 15 after a full clean and initial 2.5 cups of DE, and the fact that I'm seeing 18 now after a backwash is due to the leftover DE that stayed?

Still wondering though why the pressure didn't change at all after adding the 2 cups of DE today. Shouldn't it rise noticeably still, or not if the above is true?

Thanks.

 

[ajw22]

I don't know what a cup size is. You should be adding 2.4 lbs to a clean FNS DE filter and 80% or about 2lbs to a backwashed FNS DE filter.

I use a food scale to measure the amount of DE I am adding.

 

[just4747]

I don't know what a cup size is. You should be adding 2.4 lbs to a clean FNS DE filter and 80% or about 2lbs to a backwashed FNS DE filter.

I use a food scale to measure the amount of DE I am adding.

View attachment 615652

Yes exactly that. Sorry, I was referring to the popular orange scooper that many people have, which is 1lb per scoop. I did 2 of those this time after backwashing and after the full clean earlier in the year I did 2.5 scoops per manual.

 

[ajw22]

You ever measure the weight your orange scooper actually holds?

 

[just4747]

You ever measure the weight your orange scooper actually holds?

No I haven't. I inherited the pool a year ago and have learned so much since, but all I ever read about those scoops is that they hold 1lb worth and that was that lol. Not sure if any small difference would matter but even the original owners had the 2 and 2.5 scoops numbers written down in a document.

 

[ajw22]

I believe in trust but verify.