CYA disappears

[alben]

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

 

[dmanb2b]

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

 

[alben]

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.

 

[chem geek]

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).

 

[anonapersona]

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

 

[alben]

This is a followup to an old post that I started back in 2011. I still have significant CYA loss over the winter. Not that it really matters as I simply use trichlor and dichlor until the CYA is up, then I switch to liquid chlorine. I am just curious as to what is happening. This spring 2016 CYA was 0. I use the TF-100 test kit. In Minnesota our season is short. Typically open Memorial weekend and close week after labor day.

Here are my results from 2011 - 2016 when doing an initial water test when opening pool in the spring. Pump is running for about an hour when I take the measurement. I close the pool with a shock FC of about 15-20.

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

2011 ended with CYA 45
Note: Replaced liner. New water fill. Obviously no CYA
2012 06/04
FC 0.0
CC 0.0
TC 0.0
pH 8.2
T/A 160
CH 90
CYA 0
Temp 87F

2012 ended with CYA 50
2013 05/14
FC 0.0
CC 0.0
TC 0.0
pH 7.0
T/A 90
CH 90
CYA 35
Temp 44F

2013 ended with CYA 45
2014 05/23
FC 0.0
CC 0.0
TC 0.0
pH 6.8
T/A 70
CH 70
CYA 35
Temp 51F

2014 ended with CYA 60
2015 06/09
FC 14.5
CC 0.5
TC 15.0
pH 6.9
T/A 70
CH 70
CYA 20
Temp 53F

2015 ended with CYA 40
2016 05/27 21:00
FC 0.0
CC 0.0
TC 0.0
pH 6.8
TA 70
CH 50
CYA 0
Temp 52F

 

[bobodaclown]

I think you're experiencing is due to splash out and such. It's doesn't seem too out of line.

 

[alben]

Zero CYA in the spring? Can't be splash out.

 

[pooldv]

I agree that CYA drops over time, more over winter usually. Mine does too and we don't close.

 

[tphaggerty]

I'm in NY, use a mesh trampoline style cover when closing, so we get a lot of fine silt in the pool during the offseason. Open to a zero CYA pool every year for the last 10 years. Maybe it's the soil bacteria. We also drain down about 18 inches when closing, that gets replaced by snow and rain, so some dilution for sure. Part of my opening is to add CYA directly or sometimes with dichlor until the SWG is set to go. Once I'm at 60 to 80 ppm, I'm generally good for the season.

 

[mikemass]

OP - There's an current thread ongoing where a member posted the same problem. Then one of the senior TFP guys chimed in on how bacteria turning to ammonia can eat up CYA over the winter. I have owned 2 pools over the course of about 27 years now and this is the answer I was finally looking for. I would like to read some more scientific evidence on this. But so far this explanation makes a lot of sense to me. Read about it here:

Opened pool, CYA has gone