How to test for Combined Chlorine when Free Chlorine is high

[volkyl]

My CC tested 0.6 on a FAS-DPD test using a 25mL sample. If I understand correctly, the recommended approach to reducing CC when greater than 0.5 is to shock to 12ppm FC.

I'm not sure if this is technically SLAM, but assuming I need to maintain 12ppm FC (or whatever the CYA/FC table specifies) until my CC comes down to acceptable levels, how to I measure fractional ppm levels of CC when my FC is 12 without blowing through a ton of R-0871 reagent?

I'm assuming that I need to be doing a 25mL sample test to get .2ppm/drop granularity, which would require 60 drops (12ppm * 5 drops/ppm) of reagent to eliminate the FC prior to being able to test the CC.

Is the answer that I just need to test a 10mL sample and be happy when either:

1. The sample is clear after adding my 5 drops of R-0003 (indicating 0 CC), or
2. After adding the R-0003, 1 drop of R-0871 turns the sample clear (indicating 0 <= CC <= 0.5, an acceptable level)

Even with a 10mL sample, I'm still wasting 24 drops (12ppm * 2 drops/ppm) of R-0871 to eliminate the FC prior to being able to test the CC.

I'm a long time lurker, first time poster. Thanks for all of the great advice and knowledge that you all provide here. TFP is an excellent resource and great community!

 

[YippeeSkippy]

I only read your first sentence and am responding. You only need to dwell on CC if it is >0.5ppm. Some pools rarely get to zero but 0.5ppm is acceptable.

Maddie

 

[A.O.]

You can also leave the top off on a sunny day , UV will eat some of the CC too. At .6 CC I would not be too concerned.

 

[Mdragger88]

To answer your other question- yes, use the 10ml sample it is plenty accurate & enjoy your tub!
Be sure after chlorinating/using the tub to leave the cover off for around 30 minutes to allow cc’s created by sanitizing to escape.

 

[A.O.]

To answer your other question- yes, use the 10ml sample it is plenty accurate & enjoy your tub!
Be sure after chlorinating/using the tub to leave the cover off for around 30 minutes to allow cc’s created by sanitizing to escape.

Hmm, never even thought of that, good call.
I do have an ozonater which takes care of some of that.

 

[volkyl]

Thank you all for your replies! Sounds like I need to leave my cover off a bit more.

I guess if I really want to preserve my R-0871 I could just shock up to 12ppm FC and then wait awhile until I expect it to be low enough to test again.

 

[Mdragger88]

Hmm, never even thought of that, good call.
I do have an ozonater which takes care of some of that.

This practice is supposed to also help with the life/longevity of your cover as it’s materials are not quite as resistant as the shell itself.

 

[A.O.]

This practice is supposed to also help with the life/longevity of your cover as it’s materials are not quite as resistant as the shell itself.

By letting it dry out a bit?

 

[Mdragger88]

By letting it dry out a bit?

Allowing the chloromines to escape instead of eating the cover