Basic Chemistry Question

[H-12 1/2 Mike]

Yesterday my chemistry check had the following results:

Cl = 7.0
ph = 7.7

TA= 85
CH=210
CYA=50
Temp=81

Today the results are:

Cl=2.5
ph=7.7
TA=85
CH=270

I added 8 lbs of Calcium Chloride only yesterday. Would that cause my Chlorine level to drop 4.5ppm when it has been consistently at 7 - 7.5 ppm. ?? Just curious. I do check my chlorine and ph each day.

 

[Texas Splash]

Hello! To try and get you the most accurate reply, can you confirm a couple things first please:
1 - What test set did you use for these readings?
2 - You listed your chlorine simply as CL. Can you list your FC and CC instead please?

Thanks very much.

 

[H-12 1/2 Mike]

FC and CC are the same: 2.5 and 7.0. The water has been crystal clear for months. I use a Taylor K-2006 test kit.

 

[Texas Splash]

Other threads on this topic show that typically Calcium Chloride should have no effect on FC unless possibly there were any contaminants or organic matter in the batch. Does that answers your original question?

 

[Richard320]

FC and CC are the same: 2.5 and 7.0. The water has been crystal clear for months. I use a Taylor K-2006 test kit.

Let's make sure we're using the same acronyms.
FC = Free Chlorine
CC = Combined Chlorine
TC = Total Chlorine
CH - Calcium Hardness

The colormatching test block reads TC, that's FC+ CC
The powder and drop chlorine test first tests FC. Then you add the R-003 and if it turns pink you have CC. Then you add drops to make that disappear and you know CC.

FC good, CC bad. If your CC is really at 7, you'll got something growing in the water you don't want. The only possible exception is if you used some MPS non-chlorine "shock" which messes up that test.

If you did the test properly and you have 7 CC, you need to SLAM. You have algae starting. If you lost 4.5 FC in a day, you might have something starting and an overnight loss test will tell you one way or the other. Both the SLAM and OCLT tests are in Pool School - How To

 

[H-12 1/2 Mike]

Sorry for the brain f..... the free Chlorine is 7.0 and the Total Chlorine is 7.0.

 

[Texas Splash]

FC 7 should indicate 0 CC, so that's a good thing. We know from other threads that typically Calcium Chloride should have no effect on FC unless possibly there were any contaminants or organic matter in the batch. So now I guess we're back to your original question right? "Why is your FC dropping"? Also note that we've also learned that a CC of zero is not the only factor to consider to ensure water is healthy. So I would pass this back to you for consideration:
- Try an OLCT to make sure you're not losing more than 1ppm FC over night
- If that passes fine, consider bumping the CYA up slightly to save on FC.

Some pool owners do just fine at a CYA of 40 or 50, some go up slightly more with the understanding that if they ever had to SLAM, they would need more bleach. But it saves them bleach in the long-run. That's my thoughts at this point. Perhaps Richard or some other veterans will chime-in as well with something else.