CYA is lower than expected

johig

Gold Supporter
Since my pool was filled (December 2019) I have been gradually raising the CYA. The pool guy started out with chlorine pucks when it was too cold for the SWG to run. After startup was complete, I began adding solid CYA a couple of pounds at a time using the sock method. I am up to a total of 10.5 lbs of CYA added to approximately 17,000 gallons over about 6 months, but I'm reading my CYA test at around 50 ppm maximum. Some days I swear I can still see the black dot at closer to 40 ppm. Based on pool math, I was expecting the reading to be higher by now. Is there anything I'm missing here? I really don't want to add too much CYA. I'm topping up the pool regularly due to evaporation, but I am not backwashing or doing anything that causes a lot of water to be lost from the pool. Any suggestions on what is happening here? I suppose I could take a sample of my water to the local pool store and see what they come back with on the CYA test. Thanks in advance for any insight you can provide!
 

tim5055

Mod Squad
TFP Expert
LifeTime Supporter
May 11, 2014
11,018
Franklin, NC
Well, if your pool is 17,000 gallons then 10.5 pounds of stabilizer should raise the CYA to 73. Each puck used would have been additional 2 CYA

We generally see a natural decay of maybe 2 CYA a month, so that may be 10 or so less

If the pool is a littel larger than 17,000 and with the margin of error in the test you are getting close, but still 10-20 off.
 

johig

Gold Supporter
Well, if your pool is 17,000 gallons then 10.5 pounds of stabilizer should raise the CYA to 73. Each puck used would have been additional 2 CYA

We generally see a natural decay of maybe 2 CYA a month, so that may be 10 or so less

If the pool is a littel larger than 17,000 and with the margin of error in the test you are getting close, but still 10-20 off.
Thanks! This is reassuring. My volume might be a bit larger than 17,000. I try to keep it conservative in Pool Math to make sure I don't add excess chemicals.
 

Richard320

TFP Expert
LifeTime Supporter
Jan 6, 2010
23,930
San Dimas, CA (LA County)
Did you ever get enough rain at any one time to make you empty some water? I know we had a couple storms a bit north of you where I had to empty about 4" out before the next storm, and another couple after that. 6" is 10% of my water.
 

johig

Gold Supporter
Did you ever get enough rain at any one time to make you empty some water? I know we had a couple storms a bit north of you where I had to empty about 4" out before the next storm, and another couple after that. 6" is 10% of my water.
This could be it, too. We did have some decent rain storms and that would have sent some water out of the overflow.