My CYA gradually disappears. My location in central Minnesota. Pool temp is kept between 83F and 87F. Whenever we are not in the pool, the safety cover is on, so the pool gets relatively no sunlight. To counter act the CYA disappearance, I periodically add dichlor or trichlor instead of liquid bleach. Any idea what is happening here? This past week the chlorine suddenly went to zero and the water got slightly cloudy, so I added a lot of liquid bleach. Today (Aug 5), I noticed that the CYA is suddenly lower. Hmmm....

Here is a complete history of this year's chem numbers and the chems added:

City Water tested on 2011 05/28
FC 1
pH 7.8
T/A 120
CH 130
Temp 48F

2011 05/17 Open pool and start pump
FC 0
CC 0
TC 0
pH 7.5
T/A 130
CH 60
CYA 0
Temp 48F
Borates 15ppm

2011 05/18
FC 3
CC 25
TC 28
pH 7.2
T/A 130

2011 05/19
FC 2
CC 19
TC 21
pH 7.2
T/A 130
CH 70
CYA 30
Temp 49F

2011 05/21
FC 1
CC 12
TC 13
pH 7.0

2011 05/22
FC 1
CC 7
TC 8
pH 7.0

2011 05/23
FC 7
CC 1
TC 8
pH 7.0
Temp 54F

2011 05/24
FC 8
CC 1
TC 9
pH 7.2
Temp 54F

2011 05/25
pH 7.3

2011 05/26 1
FC 12
CC 1
TC 13
pH 7.4
Temp 54F

2011 05/26 2
FC 16
CC 1
TC 17
pH 7.4
T/A 120
CH 70
CYA 35
Temp 56F

2011 06/02
FC 5.5
CC 2
TC 7.5
pH 7.7
T/A 100
CH 70
CYA 30
Temp 81F

2011 06/05
FC 2.5
CC 0.5
TC 3
pH 7.5
Temp 89F

2011 06/07
FC 6
CC 1
TC 7
pH 7.6
Temp 87F

2011 06/09
TC 2
pH 7.8
Temp 82F

2011 06/11
FC 7
CC 0.5
TC 7.5
pH 7.8
Temp 78F

2011 06/18
FC 7.5
CC 0.5
TC 8
pH 7.8
Temp 87F

2011 06/25
FC 2
CC 0.5
TC 2.5
pH 7.8
T/A 100
CH 70
CYA 33
Temp 87F

2011 07/02
FC 1.5
CC 0.5
TC 2
pH 7.7
Temp 88F

2011 07/03
FC 6
CC 0.5
TC 6.5
pH 7.6
Temp 88F

2011 07/05
FC 0
pH 7.8
Temp 84F

2011 07/06
FC 23
CC 0.5
TC 23.5
pH 8.2
Temp 83F

2011 07/07
FC 19.5
CC 0.5
TC 20
pH 8.2
Temp 83F

2011 07/08
FC 14
CC 0.5
TC 14.5
pH 7.4
T/A 90
CH 70
CYA 30
Temp 82F

2011 07/09
FC 9
CC 0.5
TC 9.5
pH 7.4
T/A 90
CH 80
CYA 25
Temp 86F

2011 07/10
FC 7
CC 0.5
TC 7.5
pH 7.4
Temp 85F

2011 07/13
FC 12
CC 0.5
TC 12.5
pH 7.4
Temp 82F

2011 07/16
FC 10
CC 0.5
TC 10.5
pH 7.5
Temp 78F

2011 07/22
FC 12
CC 0
TC 12
pH 7.4
T/A 90
CH 80
CYA 20
Temp 88F

2011 07/25
FC 8
CC 0.5
TC 8.5
pH 7.5
Temp 86F

2011 07/29
FC 8
CC 0.5
TC 8.5
pH 7.5
T/A 80
CH 70
CYA 30
Temp 87F

2011 07/31
FC 6.5
CC 0.5
TC 7.0
pH 7.4
T/A 80
CH 70
CYA 30
Temp 86F

2011 08/02 1
FC 0
pH 7.2
Temp 83F

2011 08/02 2
FC 13.5
CC 1.0
TC 14.5
pH 7.3
Temp 84F

2011 08/03
FC 17.5
CC 1.0
TC 18.5
pH 7.4
Temp 83F

2011 08/04
FC 13.0
CC 0.5
TC 13.5
pH 7.5
Temp 83F

2011 08/05
FC 8.0
CC 0.5
TC 8.5
pH 7.5
T/A 100
CH 70
CYA 10
Temp 82F
Borates 20ppm

Chemicals added and the expected result based on TFP calculator iPhone app:
Pool startup
2011 05/17
84oz dichlor FC 20.2
CYA 18 pH down .77
4 jugs 12.5% chlorine FC 28.8
2 boxes Borax pH up .35

2011 05/18
32oz dichlor FC 7.7
CYA 7 pH down .29
2 jugs 12.5% chlorine FC 14.4
2 boxes Borax pH up .35

2011 05/19
4 jugs 12.5% chlorine FC 28.8

2011 05/20
2 jugs 12.5% chlorine FC 14.4

2011 05/21
2 jugs 12.5% chlorine FC 14.4

2011 05/22
4 jugs 12.5% chlorine FC 28.8

2011 05/23
1 jugs 12.5% chlorine FC 7.2

2011 05/24
1 jugs 12.5% chlorine FC 7.2

2011 05/26
20oz dichlor FC 4.8
CYA 4 pH down .18

2011 06/01
1 jugs 12.5% chlorine FC 7.2

2011 06/02
32oz dichlor FC 7.7
CYA 7 pH down .29

2011 06/05
1 jugs 12.5% chlorine FC 7.2

2011 06/08
16oz dichlor FC 3.8
CYA 3 pH down .15

2011 06/15
1 jugs 12.5% chlorine FC 7.2

2011 06/25
1 tablet 7oz trichlor FC 2.8
CYA 2 pH down .15

2011 06/27
1 tablet 7oz trichlor FC 2.8
CYA 2 pH down .15

2011 07/02
1 jugs 12.5% chlorine FC 7.2

2011 07/05
3 jugs 12.5% chlorine FC 21.6

2011 07/07
2 quarts 31.45% 20 baume muriatic acid pH 0.7 down

2011 07/10
3 lbs baking soda pH up 0.4
3 tablets 7oz trichlor FC 8.3
CYA 5 pH down .45

2011 07/16
2 tablets 7oz trichlor FC 5.6
CYA 3 pH down .30

2011 07/24
1 tablet 7oz trichlor FC 2.8
CYA 2 pH down .15

2011 07/28
1 tablet 7oz trichlor FC 2.8
CYA 2 pH down .15

2011 07/31
3 lbs baking soda pH up 0.4
1 jugs 12.5% chlorine FC 7.2

2011 08/02
3 jugs 12.5% chlorine FC 21.6
1 jugs 12.5% chlorine FC 7.2

Hi, sorry to here about your troubles. What test kit are you using to measure CYA down to 2ppm? The K2006 or TF100 at best can only measure down to 20ppm, anything lower should be considered zero. That said, any chance you have an autofill device that may be masking a leak? How long does the pool stay covered for?

We have heard of cases where CYA converts to ammonia over the winter, but odd to see that happen during the swim season

I am using the TF-100 test kit.
I think you read my post wrong. The 1st half of the post is my actual measurements. The 2nd half of the post is the chemicals I added and their expected change to my pool chemistry based on the TFP iPhone Pool calculator app.

No autofill device. Actually we have had so much rain this summer, that I seldom have had to add water. Since the pool opening and initial top off, I probably have only added 6 inches of city water total this summer.

The pool probably gets used every 3rd day, where the cover will be off for 3 hours. So say, maybe it gets exposed to the sun 8-10 hours a week. The rest of the time it is covered with an extreme safety cover that lets very little light through it.

High chlorine levels can slowly oxidize CYA. Usually though, normal chlorine levels would lower CYA at a rate of around 2-3 ppm per month. In a hot spa we typically see around 5 ppm CYA per month drop. It's possible something in the water is catalyzing the reaction to go faster or there could be some water leak/refill going on (you can do a bucket Leak Detection test to verify that).

Has the rain been enough to make the pool overflow? or to make you have to lower the water level on purpose?