CYA testing...and the black dot magic trick

Docpatel

Active member
Oct 25, 2020
28
Denton, TX
Folks, I've read through some good articles on TFP but I can't find an answer to a question I have on how I should be doing the test...

When I'm testing the pool water (room temperature)....I do the 7ml pool water, and then fill to 14ml mark with R-0013. Then I shake for 30-35 seconds.

I hold the tube waist level, back against the sun. When I start pouring it in the tube....I pour it until the black dot disappears. Great right?

Here's where I'm confused: So the black dot has disappeared...BUT THEN if I wait about 15-20 seconds, the dot's BACK! I can definitely see it at about 30-45 seconds.

QUESTION: Should I continue repeating adding more of the liquid in until it disappears for good? OR should I use the reading after the first time it disappears?

Thanks in advance!!
 

ajw22

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Glance at the dot. When you don’t see the dot you are done.


As much as possible you should do the CYA test in the same consistent light conditions:

  • Outside in full sun or use the brightest indirect indoor light that you can if you have to test at night
  • Back to sun
  • Don't stare at the dot as you pour
  • Fill to a line, hold the tube waist high, and glance at the dot
  • If you see the dot fill to the next line and glance at the dot
  • When the dot disappears you round up to the previous line. If you saw the dot at 40 and not at 30 then you call it 40
 

Newdude

Gold Supporter
TFP Guide
Jun 16, 2019
6,156
NY
Here's where I'm confused: So the black dot has disappeared...BUT THEN if I wait about 15-20 seconds, the dot's BACK! I can definitely see it at about 30-45 seconds
your eyes adjust to being cross eyed and rest assured It gets us ALL. Do the first glance and call it a day.

*also, officially submitting request to rename it what it IS. The googely eye test.
 

geh

Gold Supporter
Nov 28, 2020
90
New Orleans
When you do the CYA test, try this next time.

Once you have your solution ready, back to the sun, etc. Fill the vial to a line, say 80, lower the vial to your waist level and glance for the dot, you see it, add solution to the 70 line, glance, see it, repeat until you no longer see it with a glance. Then use the CYA value one step above the line you read. So if you stopped at 50, use 60 ppm CYA.

The vial is in logarithmic scale. So it is not viable to interpolate between the lines. Just use the whole numbers, such as 50, 40, 30, ....

Funny I just asked the same question this morning in another thread I started. The very helpful mknauss gave me this reply above and that worked well for me.
 
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Docpatel

Active member
Oct 25, 2020
28
Denton, TX
Thanks everyone. I just tested using advice above on simply glancing at it. I think I was staring in the past so I kept seeing it....or maybe I THOUGHT I was seeing it.
It just disappeared at 70 so I rounded it up to 80.
 

AUSpool

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Thanks everyone. I just tested using advice above on simply glancing at it. I think I was staring in the past so I kept seeing it....or maybe I THOUGHT I was seeing it.
It just disappeared at 70 so I rounded it up to 80.

If your using trichlor or dichlor, pucks etc, now is the time to stop and switch to bleach. You don’t want that CYA to get any higher.
 

woodyp

Mod Squad
TFP Expert
LifeTime Supporter
Apr 17, 2010
11,124
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You can always pout that same sample back and forth 2 or 3 times and take the average.
If you see THAT much difference between them....it ain't the test......it's YOU!
 

Dtkokay

Well-known member
Dec 31, 2019
379
Houston, Texas
You can buy a calibrated sample of water with 50 CYA so that you can learn how much the dot should disappear. It's from the same site where you buy the TF-100.
 

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Mandy

Silver Supporter
Jul 5, 2017
43
Dubbo Australia
Glance at the dot. When you don’t see the dot you are done.


As much as possible you should do the CYA test in the same consistent light conditions:

  • Outside in full sun or use the brightest indirect indoor light that you can if you have to test at night
  • Back to sun
  • Don't stare at the dot as you pour
  • Fill to a line, hold the tube waist high, and glance at the dot
  • If you see the dot fill to the next line and glance at the dot
  • When the dot disappears you round up to the previous line. If you saw the dot at 40 and not at 30 then you call it 40
I have a question regarding your last bullet point. I thought if the dot disappeared between 30 and 40 (eg at half way point 35) you would round up to 40 and if it disappeared bang on 30 then it would be read as 30. Have I been doing this incorrectly?
 

ajw22

Gold Supporter
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Jul 21, 2013
23,586
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I have a question regarding your last bullet point. I thought if the dot disappeared between 30 and 40 (eg at half way point 35) you would round up to 40 and if it disappeared bang on 30 then it would be read as 30. Have I been doing this incorrectly?

If you are good enough to say the dot disappeared bang on 30 then so it is.

The fact is getting the CYA test +/- 10 ppm is good enough.

Giving yourself a higher CYA value gives you a bit more chlorine which is better. The only reason you care about your CYA is to set your FC levels following the FC/CYA Chart. You win no prize for being bang on any CYA value.
 
Last edited:

wireform

Silver Supporter
In The Industry
Aug 15, 2017
1,950
Spring Valley, NY
Better to round up for the extra insurance. Rounding down can put the FC borderline in the event of a heavier pool load gone unnoticed.
 

needsajet

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Then I shake for 30-35 seconds.
Joyful Matt, Chem Guru made a good point about this once. The idea is to just invert the mixture 3 or 4 times to mix poolwater with reagent, and then set it to rest for 30 seconds. Excessive shaking will introduce little tiny bubbles that stay suspended for a while. Could be what you're experiencing, but also heaps of good advice above.
 

Bradenwb

Member
Jul 13, 2020
10
Minnesota
I do it 3x and average. I am usually within 5 or so each time.
This is a great point. I did not even think to re-use the sample until someone pointed it out last year on here.

Not to threadjack but a question I would like to clarify, where is the best place to take the sample? I thought I read in pool math that is was ~18ish inches down where the pool starts to decline. The PB says I should take it near the jet in the deep end to make sure I am getting well circulated water between where the water comes in and where it drains on bottom. That actually makes sense to me. Wondering all your thoughts on this?
 
Last edited:

Winger 03

Well-known member
Sep 7, 2009
312
Frederick County, MD
This is a great point. I did not even think to re-use the sample until someone pointed it out last year on here.

Not to threadjack but a question I would like to clarify, where is the best place to take the sample? I thought I read in pool math that is was ~18ish inches down where the pool starts to decline. The PB says I should take it near the jet in the deep end to make sure I am getting well circulated water between where the water comes in and where it drains on bottom. That actually makes sense to me. Wondering all your thoughts on this?
My pump runs 24/7 so I figure all the water is the same concentration. I just go 18" by the transition.
 

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