Wild swings in Free Chlorine?!?

[gsfergy]

This is my first post and I’m truly baffled.

This year when I opened my SW AGP, I did a test strip and free chlorine seemed very low. I put my Pentair Intellichlor SWG to 100% for a 3-4 hours and checked again and I was getting a normal range result. I turned SWG down to 60% and thought problem solved. After an afternoon of swimming the next day, I thought for fun I’d check since this a new issue for me - and it tested at almost zero FC :S

In response, I let the SWG run overnight and I was back at a normal range by morning. I left late morning and went to the pool store - my water tested 3 for free chlorine, and pH, alkalinity, and salt content in normal range. I came home - took cover off - turned SWG to 60% thinking it was a good opportunity to see if it would recur and it did. After my two kids spent about 1.5 hrs in pool and cover was off for less than 3 hours — a test strip shows almost no FC again.

What is happening?!

[Teapot1]

What was the CYA (chlorine stabiliser ) level? The most important chemical in your pool, without it chlorine will be burnt off by the sun in around 2 hours.