How to test for Combined Chlorine when Free Chlorine is high

volkyl

New member
Nov 21, 2021
3
San Francisco
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!
 

Mdragger88

Bronze Supporter
TFP Guide
Jun 1, 2018
4,288
Hernando, Ms
Pool Size
26000
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Intex Krystal Clear
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.

Well-known member
Apr 12, 2016
551
Kershaw, South Carolina
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
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.
 

Mdragger88

Bronze Supporter
TFP Guide
Jun 1, 2018
4,288
Hernando, Ms
Pool Size
26000
Surface
Vinyl
Chlorine
Salt Water Generator
SWG Type
Intex Krystal Clear
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.
 
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