# Thread: CYA Precision vs. CYA/Chlorine Chart

1. ## CYA Precision vs. CYA/Chlorine Chart

I remember reading in the Extended Test Kit directions thread that the CYA measurement has a +/- of 15 when below 90. Given this fact does the CYA/Chlorine chart take this into account already? Is the minimum Chlorine suggested OK if the real CYA level is 15ppm lower than the measured value?

There is no problem that I am trying to solve BTW - just curious mainly.

2. ## Re: CYA Precision vs. CYA/Chlorine Chart

You are safe to swim all the way to shock level.

3. ## Re: CYA Precision vs. CYA/Chlorine Chart

Originally Posted by aasbury
You are safe to swim all the way to shock level.
I'm lost as to what this was in reference to. I agree with what you say above - and I guess you are saying that if it worried someone they could always maintain a higher FC level - so again I agree. What I'm asking about is a more academic question which is does the minimum FC level in the chart already account for the potential precision issues with the CYA test?

4. ## Re: CYA Precision vs. CYA/Chlorine Chart

Let me quantify this a bit more:

Let's say you measure a CYA of 50 and the test has a precision of +/- 15. That means the actual CYA could be anywhere from 35 to 65.

Measured CYA = 50
MIN CYA (based on test precision): 35
MAX CYA (based on test precision): 65

@CYA 35:
Min FC ~ 2.5
Target FC ~ 4.5
Shock FC ~ 14

@CYA 50
Min FC = 4
Target FC = 6
Shock FC = 20

@CYA 65
Min FC ~ 5
Target FC ~ 7.5
Shock FC ~ 26

If you have actually only 35ppm of CYA and maintain per the recommendations for 50ppm your probably OK under normal use never too high (under shock) etc. Presumably because mustard shock levels are still safe you can even shock at 20ppm and not be in any real trouble.

The reverse is more of a concern to me. If I have 50ppm and think I have 35ppm due to the test precision - what is the impact of maintaining a minimum FC level of 2.5 vs. 4 with a target of 4.5 vs. 6.

I think a lot of time when I see advice here the CYA is assumed accurate to 5ppm - I think normally this is not an issue but I wonder about fringe cases.

5. ## Re: CYA Precision vs. CYA/Chlorine Chart

What I meant was that I always keep my chlorine a little higher to account for this. Using your above example If I got a reading of 50 for CYA. I would go with 6-7. That way you would be covered for the minimum FC of 5 for a CYA of 65 and well below shock level for a CYA of 35.

6. ## Re: CYA Precision vs. CYA/Chlorine Chart

The CYA/Chlorine chart already takes the imprecision of the test into account. Get the best reading you can, look that up in the chart, and use the range it recommends.

7. ## Re: CYA Precision vs. CYA/Chlorine Chart

Originally Posted by JasonLion
The CYA/Chlorine chart already takes the imprecision of the test into account. Get the best reading you can, look that up in the chart, and use the range it recommends.
Thanks! That's the answer I was hoping for - it removes any need to fiddle around with things.
Simple is better than complex.

8. ## Re: CYA Precision vs. CYA/Chlorine Chart

Originally Posted by aasbury
What I meant was that I always keep my chlorine a little higher to account for this. Using your above example If I got a reading of 50 for CYA. I would go with 6-7. That way you would be covered for the minimum FC of 5 for a CYA of 65 and well below shock level for a CYA of 35.
OK - that's simple enough. I tend to try to keep my FC about 1-2ppm over the minimum not because of imprecision but to ensure that it does not drop below minimum.

9. ## Re: CYA Precision vs. CYA/Chlorine Chart

It would be nice if we lived in a perfectly quantitative world where the minimum FC/CYA ratio to prevent algae growth were absolutely precise, but it doesn't work that way. There are many species of even green algae and many levels of algae nutrients and varying degrees of circulation, etc. The table is a rough guide that even with test errors works for nearly all pools.

There are no TFP police to come by your pool and make sure you are following the chart. If you wanted to do something different +/- that's perfectly fine. It's your pool and whatever works best for you is appropriate. The charts and recommendations are just that -- guides for you to use as a starting point, especially as a beginner. We get questions such as whether one can have a lower FC/CYA if one is using borates or an algaecide and our answer is usually that we don't have enough data to be able to answer that with any confidence. There isn't any dogma here -- just information based on the most reliable observations and correlations with science that we can do as pool owner lay people.

10. ## Re: CYA Precision vs. CYA/Chlorine Chart

Originally Posted by chem geek
It would be nice if we lived in a perfectly quantitative world where the minimum FC/CYA ratio to prevent algae growth were absolutely precise, but it doesn't work that way. There are many species of even green algae and many levels of algae nutrients and varying degrees of circulation, etc. The table is a rough guide that even with test errors works for nearly all pools.

There are no TFP police to come by your pool and make sure you are following the chart. If you wanted to do something different +/- that's perfectly fine. It's your pool and whatever works best for you is appropriate. The charts and recommendations are just that -- guides for you to use as a starting point, especially as a beginner. We get questions such as whether one can have a lower FC/CYA if one is using borates or an algaecide and our answer is usually that we don't have enough data to be able to answer that with any confidence. There isn't any dogma here -- just information based on the most reliable observations and correlations with science that we can do as pool owner lay people.
Your sage advice is always appreciated... you're the E.F. Hutton of TFP - at least to me.

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