Why would CH go down, and what should be done about it?

[Gooserider]

I've been doing BBB for a month or so now, and am happy as heck with it... Most all of my numbers have been pretty good, and I'm keeping the CL sort of reasonable (It tends to get down to 1-2 FC before I bump it back up to 4, but I'm staying nice and clear thanks to the Borax...)

However, since I started, my CH has been slowly dropping - I understand why the other numbers are changing, to the extent that they are, but I don't see why my CH has been changing... According to Pool School one lowers CH by water replacement, which I haven't been doing on purpose. We have been getting a lot of rain, and my pool has probably gotten 2-3" deeper, but isn't (far as I can tell) at the point where it would be overflowing...

However my CH has been dropping - I've been keeping a log of my numbers every time I do a test. I don't do the CH test that often as it uses a lot of reagents, and I've been told it should have been pretty stable and didn't need testing that often.

However, on 5/25 when I got started on BBB testing, I had a CH of 300. I did do some backwashing getting the opening water into shape, but by 6/09 I was done and in control, with borates and CYA where I wanted them, and a CH of 290

Today I ran the full set of tests outside in order to verify all my numbers (I'd been testing indoors at night and the "One Sheet Instruction" thread suggested that the color change tests are easier to read in daylight...) and found that everything else was what I expected, but my CH was only 260... I have a plaster pool, so I know CH is more of a concern, but that is still w/in the limits (barely) though my CSI is at -0.47, which is getting on the low side.

Any idea what would cause my CH to drop so much? Does it need anything done about it?

Todays numbers...

OTO CL 1 (approx)
pH 7.2 (I added some MA the other day, and it dropped more than I was expecting - I'm planning to just let it drift up naturally as it doesn't seem that far off)
DPD - FC 2 (Added 54 oz of 12% CL to bring it back to 4)
DPD - CC 0
DPD - TC 2
TA - 95 (has been around 100, but I'm letting it slowly drop, as ideal s/b around 80)
CH - 260
CYA - 40 or a hair under
Borates - 50, (I think, though it was very hard to tell whether it was closer to 50 or 80)

Aside from the CH dropping, everything else looks fine, so that's mostly what I'm concerned about...

 

[JasonLion]

CH, CYA, and Borates will all fall slowly through the pool season. The more frequently you clean your filter, and the more splashing there is, the faster they will fall. What you describe is a little faster than normal, but not dramatically so.

 

[Gooserider]

OK, sounds like I won't worry about it overmuch... I will say though that I haven't done much of the things you mentioned as factors... This has been a cold and rainy June, so I've been in the pool TWICE, for about one or two minutes each time, so not much splashing. (I also took the home brew aerator out a week or two ago as not needed w/ current numbers)

I get a fair number of dead mice in the skimmer :x but I doubt that they really do all that much splashing before getting sucked into the skimmer...

I also haven't cleaned the filter in over a month - don't need to, as it is not showing any pressure increase to speak of - w/ clean DE it shows 10psi, 8 if the booster pump is running. It is now around 12 / 10, and I normally don't clean it until it gets over about 15 - 20, as that is when it starts showing an impact on the water flow through the system.

In the past, I'd need to clean it about once a month to six weeks, but in retrospect that was probably with lots of borderline algae blooms - BBB is keeping the water so nice I may be able to go all season w/o cleaning it again...

Gooserider