My CYA all of the sudden dropped from 80 to 50 ???

[ashmoussa]

Hi TFP,
First, my current numbers are:

FC = 25.5 (had my SWG at 60-80% while it was in the 90's which kept it in the teens, now its cloudy and in the 70's-80's. Need to decrease SWG %)
CC = 0
Calcium = 275-300
PH = 7.4 (but rises quickly quite often)
CYA = tested 4 times different times and different samples all when sunny out at 50ppm (background: was at 70ppm about 2 weeks ago and I decided to increase it to 80ppm by adding a little CYA. Somehow it dropped to 50!!??!)
TA = 70

My CYA has been in the 70-80 range since June. I test weekly.
Why would CYA drop like that in a short amount of time especially after adding more CYA 2 weeks ago to raise it?? Doesn't make sense, unless it is due to high FC.
Help please. Thx.

 

[Silver_2000]

rain ?
Leak and autofill ?

 

[ashmoussa]

I don't have any leaks that I know of or an auto fill. However I have topped off the water due to evaporation.

 

[JoyfulNoise]

Did you do the test outdoors in the proper lighting? Have you been consistent in how you test?

Just by standing under my side door awning I can make the apparent CYA level in my view tube 20ppm lower. I always try to do the test at high noon (give or take an hour) with sun to my back and the view tube held waist high in my shadow. I make sure my fingers do not touch the tube in anyway (you can do that with a Taylor 2000 series comparator block but not with the CYA tube that comes with the TF-100). If it's even the slightest bit cloudy or too late in the day, I wait until the next day to do the test. As I said, all I have to do with the same exact solution is stand under my awning but otherwise do nothing else different and the results drop by 20ppm.

 

[Divin Dave]

I agree with JFN, even slight changes in light conditions can make a noticeable different in test result. And also, hold the test tube with just your forefinger and thumb. Using several fingers to hold it will block the light from going through the solution like it should.

You might re-test, and then pour the solution back in the bottle and then squirt it back in the tube again. Repeat 3 or 4 times and take an average of the test results and use that for your CYA level.

 

[Marlahoutex]

I know this is a dumb question, but could you have accidentally used the sample reagent? The one that shows you what a CYA of 50 is? If you have a fairly new test kit, I think it's included.

 

[Divin Dave]

That's a good point Marla, thanks for pointing that out.

 

[ashmoussa]

Thanks for all the input. I am aware of the factors you all have mentioned. I usually do multiple tests, with back to sun, at waist, very sunny, and use 2 fingers to hold tube. However I have never seen the sample reagent. I have the TF100 that I got last year and this year I bought the full refill kit.
I'll try the test again tomorrow.
Thx.

 

[SilverSD]

I have the exact same question.

Tested on 9/14 - CYA 80, Tested 9/21 CYA 50! (same measurement conditions - retested to confirm)

Only 2 changes during that time: a few days of *light* rain here in southern CA, and adding a lot of acid to drop the pH/reduce TA. Not sure if either of those would have affected the CYA.

 

[borjis]

Do you get any tree debris in your pool?

I had a TON of red fir tree pollen nodules fall in over the may-june period and my cya dropped from 50 to 20.

I did nothing but top up evaporation as you did and I test cya the proper way.

My theory is somehow those nodules had bacteria that ate the cya...no other logical reason it would go down.

They stopped falling in and my cya remains at 50 (I brought it back up in july)

 

[Kathe]

My CYA dropped from 48 to 19 in a month's time. We're in MI and had some rain in that time period. I haven't had to top off the pool because of the rain. No leaks that I know of. What are some of the reasons other than losing water, that cya might lower? The pool is clean, numbers are good, no debris, using solar cover whenever not in pool.

 

[ashmoussa]

I don't have much debris at all. Maybe a leaf or two. I'm thinking it has to do with the rain lately. Even though it had rained hard at the beginning of the season and it really didn't effect anything. For some reason the rain towards the end of this season has been doing something to throw off my CYA and possibly my PH. Sounds a bit weird.