FC change after chlorine addition

[tobydog]

FC 11.5
CC 0
TA 110
CH 290
CYA 80
PH 7.6

Since I have gotten the pool cleared up and looking good with the TFP method, I have been testing everyday to find out how much chlorine the pool needs every day. Eventually, I want to add an automated chlorine system to the pool. I tested last night after dark and found the FC to be 7.5. In order to get to my target FC of 9, the pool calculator said to add 67 oz of 10%. For the first time, I actually measured down to the oz instead of eyeballing it. When I tested this AM before the sun hit the pool, the FC was up to 11.5. Any ideas why this would happen? I messed with the # of gallons on the pool calculator and there would have to be a huge difference in gallons to make a big difference to the amount of FC change from 67 oz of chlorine.

 

[woodyp]

Had your pool been circulated well before you tested it at 7.5?

 

[tobydog]

Yeah, probably 6 hours worth of filtration. After I added the 67 oz last night, I ran the pump for about an hour on the high speed (2 speed pump). The chlorine was added in the stream of the return jet on the deep end of the pool.

 

[pooldv]

There are only a few options
Testing variability in one of the tests, one of the tests was off
Strength of the chlorine added, they accidentally put 12% in the bottle or it was really, really fresh
Amount of chlorine added, you put in more than you thought
Pool gallons are off, or wrong numbers in Poolmath
Neighbor snuck over and added some as a prank

 

[tobydog]

Looks like the test could have been off....I just retested with the 25ml sample and came up with FC of 8.8- 9.0.