Accuracy in Testing

crokett

Well-known member
Jul 17, 2007
554
Hillsborough, NC
I have one of the liquid-drop type kits that tests for chlorine, pH alkalinity and acid demand. I tested Monday night and got:

7.6 pH
140 PPM alkalinity

I tested last night and got:
7.6 pH
130 PPM alkalinity

I took a sample to the pool store today and got
8.0 pH
53 alkalinity

Chlorine being low I understand. I need stabilizer and I am fighting some algae.

Now it rained overnight so that could explain why the pH drifted up, but alkalinity is way off. I realize the test is subjective regarding color changes but that is a huge difference. Could the rain make that much of a difference?
 

CaOCl2

LifeTime Supporter
May 23, 2007
326
Montreal Canada
If it's a Taylor test kit the error margin for the alkalinity test is 1 drop, so if you used a 25 ml sample this is 10 ppm, so you're within tolerance. The endpoint of the alkalinity titration is very a sharp and vibrant red leaving very little room for interpretation. I wouldn't trust the 53 alkalinity the LPS measured.
 

crokett

Well-known member
Jul 17, 2007
554
Hillsborough, NC
The test kit is a generic Ace but the sample size would be about 25 ml. I'd understand a few points difference withe LPS, but 80?? I will retest tonight before I treat for alkalinity. Again a little subjective but that is a huge difference. I tested last night and pulled a water sample this morning but did not test again. The rain overnight may have screwed things up.
 

JohnT

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Mod Squad
TFP Expert
Apr 4, 2007
9,614
SW Indiana
Your alkalinity is fine unless you are struggling to keep your pH down. My fill water is over 400ppm TA, so I know high TA. This is the first year ever that my TA has been less than 180ppm.
 

JasonLion

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May 7, 2007
37,880
Silver Spring, MD
Some pool store tests can be wildly off. The amount of training the staff has and care with which they do the tests varies quite a bit, even at the same store at different times. I would trust your results.
 

duraleigh

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Apr 1, 2007
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Sebring, Florida
Crokett,

You indicated an algae issue but haven't said anything about adding chlorine....you are, right?
 

crokett

Well-known member
Jul 17, 2007
554
Hillsborough, NC
Duraleigh. yes I am. Tonight I will vac out as much as I can, add the stabilizer and shock. By the way, since I am in your area, the place I went is Poolside. They were recommended.

This latest with the test kit is just one more thing that gives me more questions than answers.
 

duraleigh

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Apr 1, 2007
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This latest with the test kit is just one more thing that gives me more questions than answers.
As you learn more and more from this forum, you will see just how inaccurate pool store testing can be. Measure and test accurately and trust your own results.
 

crokett

Well-known member
Jul 17, 2007
554
Hillsborough, NC
Well actually I think this was my screwup this time. I tested again twice this evening (it was getting dark last night) from different parts of the pool and came out with 70PPM and 60 PPM. Neither of those is that far off from 53. I still say my pH is fine. They measured it at 8, I came out with 7.6 both times.