Chlorine test question?

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dgibby

Active member
Feb 21, 2025
25
Central Texas
This might be a dumb question, but I have the pro kit for testing.
There is the block thing that you test chlorine and pH daily. Then with the drops and beakers, there’s a free chlorine test and a total chlorine test. What chlorine test is the one in the little block with ph considered? Free chlorine or total chlorine or something totally different.
Thanks
 
Free chlorine. We don't measure TC...we measure FC and CC.

The color block with drops really only tells you if you have chlorine. We recommend that you use the FAS-DPD 0870 powder and 0871 reagent to test your FC and R0003 to test CC.

www.troublefreepool.com

FAS-DPD Chlorine Test

The FAS-DPD chlorine test is used to find the level of Free Chlorine in the … Read more…
www.troublefreepool.com www.troublefreepool.com
 
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dgibby said:
Then with the drops and beakers, there’s a free chlorine test and a total chlorine test.
Click to expand...
This test provides the value for FC and CC (not total chlorine). This is called the FAS/DPD chlorine test. However for clarity Total Chlorine (TC) = FC + CC - but we don’t really care about TC - only track FC and CC.

dgibby said:
What chlorine test is the one in the little block with ph considered?
Click to expand...
The block color comparator that uses the R-006 drop measures TC. It is called the OTO Chlorine test. It really is just detecting the presence of chlorine in your pool, rather than the precise level of chlorine that is required to properly sanitize the pool.
 
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HermanTX said:
This test provides the value for FC and CC (not total chlorine). This is called the FAS/DPD chlorine test. However for clarity Total Chlorine (TC) = FC + CC - but we don’t really care about TC - only track FC and CC.


The block color comparator that uses the R-006 drop measures TC. It is called the OTO Chlorine test. It really is just detecting the presence of chlorine in your pool, rather than the precise level of chlorine that is required to properly sanitize the pool.
Click to expand...
So why use it to test daily as recommended and do you even use the number in pool
math?
 
clambert1273 said:
I assume you're referring to the OTO block, I have NEVER used it nor will I. I only record FC from the powder.
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Yes, from the block. The pro kit came with a recommended testing sheet that says use the block to test chlorine and pH daily and only check with the powder, the free chlorine, once a week. But then I see people are putting chlorine in their pool every day or every other day, so are they doing the free chlorine test that often or weekly as the instructions recommended?
 
dgibby said:
So why use it to test daily as recommended
Click to expand...
Its for more advanced users. You may decide to use it after many comparisons, or not.
dgibby said:
do you even use the number in pool
math?
Click to expand...
It does not give you a #, only a vague idea. Nobody's eyes see the same thing and you and I may record different values.
 
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I use the comparator block most often so that I can make a decision to add chlorine and acid or not (don't have the new cell up yet.) I save the powder for when I do more than those tests. But I know my pool and how it behaves so it's easy for me to do this. I am one with my pool. We've been best buds for 20+ years.
 
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dgibby said:
So why use it to test daily as recommended and do you even use the number in pool
math?
Click to expand...
You do not use the number in pool math when testing chlorine with the comparator block. That is a total chlorine (TC) number and does not tell you how much active chlorine (FC) you have in your pool.

When I first started testing using the TF Test kit, I tested daily for about 2 weeks using the FAS/DPD test for FC and CC. I began to understand how my pool acted with some sunny days then with rainy days. I then backed off to testing every 2nd or 3rd day and did that for another serveral weeks. I rarely used the color comparator for chlorine other just see any change after a rain storm.

Some members stretch out to testing to 5 or more days for FC and CC. They may use the color camparator for chlorine in between those tests to confirm they still have chlorine.

Also many of us keep chlorine at the up edge of the range for a given CYA value. So if you do that, when you use the color comparator, the color turns orange and if you see that then you know you are OKAY on chlorine.
 
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HermanTX said:
You do not use the number in pool math when testing chlorine with the comparator block. That is a total chlorine (TC) number and does not tell you how much active chlorine (FC) you have in your pool.

When I first started testing using the TF Test kit, I tested daily for about 2 weeks using the FAS/DPD test for FC and CC. I began to understand how my pool acted with some sunny days then with rainy days. I then backed off to testing every 2nd or 3rd day and did that for another serveral weeks. I rarely used the color comparator for chlorine other just see any change after a rain storm.

Some members stretch out to testing to 5 or more days for FC and CC. They may use the color camparator for chlorine in between those tests to confirm they still have chlorine.

Also many of us keep chlorine at the up edge of the range for a given CYA value. So if you do that, when you use the color comparator, the color turns orange and if you see that then you know you are OKAY on chlorine.
Click to expand...
Good to know. Thanks.
I assumed that everyone was using the block since the instructions in the test kit said use it daily. I see many post about people adding chlorine every other day or so and assume that was the test they were using. I’ll start using the other one and mainly use the block to check my pH
 
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dgibby said:
So why use it to test daily as recommended and do you even use the number in pool
math?
Click to expand...

This has been nicely covered but I will add that there is a bit of history to it and it depends a bit on how a pool is chlorinated.

The history is related to the evolution of test equipment in the pool industry. The shades of yellow OTO total chlorine test was the first kit to be readily available and it was a combination pH/OTO comparator. Next came the shades of purpleDPD test which was eventually modified into the FAS/DPD test. The DPD test is done in the pH/DPD combination comparator. But the point is that a stand alone pH comparator is not available, they are all combinations. At some point a decision had to be made, which combination to use when selecting a pH comparator? And then do they include the reagents and instructions for it?

My Taylor kit uses a pH/DPD/CyA combo and simply leaves out the DPD reagents and instructions. I default to the FAS/DPD test for FC. But the OTO test can be a very useful tool. Particularly if you’re manually dosing with liquid chlorine, LC. I guess the instructions are for the best practice and cover most situations but it can be adapted to suit the situation. For example, if I were manually dosing I would be adding X amount of LC daily and use the OTO test every second day looking for a bright orange yellow to ensure it’s good. Then use the FAS/DPD test every alternate day to give me a more precise number and adjust my daily dose amount.
 
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