Can CYA ever drop on its own? Or is my new CYA reagent different somehow?

[FailedChemistry]

Hey everyone!

Still humming along in my first year using the TroubleFree method. Thanks to you all.
i have a question about my CYA level dropping 10 points.

i had to drain the pool several times at the beginning of the season to get my CYA down to 50. (from 100!)
Then, satisfied with 50, i've been operating with it at that level for most of the season.
Plus i hadn't even been testing the CYA the last 2 months or so, because i ran out of the R-0013 CYA reagent, and am no longer using chlorine pucks.

i recently bought some more reagent.
So, this morning (after a couple months or so) i tested the CYA and got a 40.

Now this puzzles me because i haven't been draining the pool since i tested at 50.
i did drain the filter twice this year so i could clean it out, but i wouldn't have thought that'd be enough to affect the 11,813 gallon pool.
We've also gotten a bit of rain this year, but as i said, i haven't been draining it.

i thought once CYA is in your water, it's there to stay.
Could my new refill R-0013 reagent be reading differently?
i mean granted, that test is kind of funky, but i'm... sort of confident i did it right.
At the very least, i know i did it the same way as before.

Latest test if that matters
FC - 6
CC - 0
TA - 90
CH - 250
CYA - 40
pH - 7.5

Any ideas?
i guess i'm looking a gift horse in the mouth. Maybe i should just be happy it's down!

 

[pwrstrk]

Your fine. You will lose a couple of ppm per month along with backwashing the filter, splash out and the rain filling it back up will change things.
You should test it once a month to see where it is. That's what I do and I just had to add some as mine was down to 30. I keep mine at 40. ?

 

[Suley]

Remember that the CYA test is accurate +/- 15 anyways...

 

[tim5055]

I'm going to say that a 10 point drop in a couple of months is not unusual with backwashes, splash out and rain filling the pool.

 

[FailedChemistry]

Ok cool. i will assume it actually is at 40 then.

You guys keep mentioning backwashing.
I think with my setup i can't do that. The pump only flows one way.
What i do is drain the filter tank, open it up, pull out the cartridges, and hose 'em off.

Does that change your answers?

 

[jblizzle]

You are right. You do not backwash cartridge filters ... which is what your signature says

 

[DogsHouse_MyPool]

You can always pick up a bottle of R-7065 CYA Standard Solution at tftestkits. This will allow you to test your reagent and your testing skills against a standard 50ppm CYA solution. I would do that before I assume my 30 ppm test results were really 40 ppm.