CYA drop

[skydude]

I test my CYA around twice a month. On 7/7 and 8/6 my CYA was at 60. I tested it today and it was at 40.

When I test my CYA I look at the cylinder, in sunlight at waist level and I have my wife do the same. This gives the test a little more validity in my opinion.

I live in Phoenix and we have not had that much rain recently. What would explain the drop of 20 points in just over two weeks?

I decided to throw a few chlorine tabs in my floater as well to elevate my CYA back to ~60.

 

[pooldv]

There are a few things that can/will reduce cya.

You normally lose 2-3 ppm per month, up to 5 ppm in 90°+ water
High fc, such as SLAMing can lower CYA a bit faster
Rain overflow
Splash out
Draining and refilling
FC dropping below minimum can allow bacteria to grow and consume it and convert it to ammonia or nitrates
The test is typically +/-10 ppm

 

[skydude]

That's exactly what I thought pooldv. Our FC never dropped below 6.0. We did splash a bit of water out to lower our TA. And with a margin of error of +/-10 I suppose we are still in range