Strange shifts in numbers

[rjward]

Hello all – here is a summary of my current conundrum.

This is a new pool – completed in November 2012.
Up until now my CH numbers have been consistent at 300-330 (using my TFT100), even though the three pool stores that I visit for monthly comparison have showed me at 230-260. Same for my TA, small changes being occasionally made to a tee using my spreadsheet and the pool calculator.

Here were my numbers last weekend:

TC/FC = 4.5 (taken in the morning)
pH = 7.5
TA = 70
CH = 300
CYA = 35
Borates = 50

I added 16oz of baking soda which I would expect to bring up TA by around 4 according to the pool calculator. I also added 19oz of CYA which I expected to raise CYA by about 8, which appears to have occured

Today I test and I have the following numbers (measured twice just to confirm)

TC/FC = 3.5 (taken in the afternoon)
pH = 7.8
TA = 90
CH = 140
CYA = 45
Borates = 50

1) Any thoughts on what would have consumed over ½ of my calcium?
2) Any thoughts on the large jump in TA?

If somehow ½ of my water was to have been replaced than I would think that my chlorine would plummet since I am on a scheduled 1ppm dose each night with my stenner pump. That has not happened.

Thoughts?
Do I dare add calcium since I am below 200?

Thanks for reading!

 

[jblizzle]

Those are some interesting results.

First the TA ... this is only accurate to +/-10ppm and susceptible to the static charge affecting the drop size ... so this does not trouble be too much.

The CH is the odd one. Since nothing else dropped (CYA and borates), then loss of water is not likely and nothing as far as I know consumes the calcium. Is there any chance on the second test you used 10ml of water instead of 25ml?

 

[duraleigh]

Do I dare add calcium since I am below 200?

I certainly wouldn't. CH is one of the parameters that can be changed slowly if need be. I would first look for a testing error since CH never goes away on it's own unless you have an autofill device and an undetected leak....which I seriously doubt.

Try the CH test again on your regular testing cycle...it'll probably be fine. If not, then I would take some steps to correct it.

It sounds to me like your testing procedures are pretty darn good. That said the TA test has a tolerance of plus or minus 10ppm so your TA could be 80 and your results would be normal. Again, not an emergency item. I would simply monitor it.