uloset
I have the same Nautilus Filter (NS 36) with a Pentair 36 Sq Ft Grid Assembly 59023500 .
I've been using the following D.E. charging amounts for several years:
With a new or cleaned (removed & cleaned) Grid Assembly, I charge with 3.6 Lbs (same as recommended by the Nautilus NS 36 manual and the template attached to the actual filter tank).
I use a "Leslie's Pool" light blue D.E. scoop which = approx .5 Lbs D.E. It's actually = 5 Sq Ft of D.E. coverage but that's close enough to consider the scoop = .5 Lbs.
So for a new (or removed/cleaned) Grid Assembly, I use 7 D.E. scoops.
When I backwash, I charge with 6 scoops which is = 80% of a full charge. That's recommended since a backwash won't remove all of the old D.E. from the filter.
That's = to 6 D.E. scoops.
Using the above charging methods, here's my filter backwash history for the past couple of years, since switching to TFP methods (Bleach/CYA ratio dosing)
05/15/14 Backwashed. This was the last Backwash done before switching to TFP methods. Prior to this, I usually had to backwash every 3-5 weeks, sometimes sooner.
11/07/14 Backwashed. 6 D.E. scoops Duration from last BW 25 weeks ~6 mo
02/17/15 Backwashed. 6 D.E. scoops Duration from last BW 15 weeks ~3.5 mo
06/01/15 Backwashed. 6 D.E. scoops Duration from last BW 15 weeks ~3.5 mo
11/23/15 Backwashed. 6 D.E. scoops Duration from last BW 25 weeks ~6 mo
04/18/16 Backwashed. 6 D.E. scoops Duration from last BW 21 weeks ~5 mo
I last replaced (or pulled the assy for complete cleaning) on 09/18/13
I backwash using 10 BW/Rinse cycles with my Multiport Valve. I think this is the reason I haven't had to pull the Grids sooner since I'm removing almost all of the D.E. in the Grids with multiple BW/Rinse cycles.
My "starting/best case" PSI (right after BW and before charging with D.E.) is about the same as with a completely new Grid Assembly.