pH Discrepancies

Dallas_James

Member
Aug 6, 2020
14
Dallas/TX
Hello
I have a nice digital pH meter by Thermoworks. I recently noticed discrepancies between the digital meter and the Taylor color match test. I’ve calibrated my meter in the buffer solution and then got a reading of 7.68 on my pool water. Seems perfect. When I did the color match test, my eyes see it at about 8. Which result should I be trusting? It seems to me that the color match test is “fool proof” while the digital meter has room for user error.
 

proavia

LifeTime Supporter
TFP Guide
Feb 6, 2015
2,421
Chandler AZ
Pool Size
12300
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Chlorine
Liquid Chlorine
I always believe the drop test results. To better determine the colors, you can hold the vial in front of a white surface. Some use a white paper or plastic plate. This can provide a better indication of the reading. I tend to use 4 drops instead of 5 as it's easier for me to determine the color.
 

jark87

Silver Supporter
Jun 5, 2011
433
Flower Mound, TX
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Salt Water Generator
SWG Type
Jandy Aquapure 1400
I’ve always wondered about the accuracy of the drop tests. Seems like every new bottle I buy has different sized drops that come out. Nothing that would make a significant difference, but probably enough to make the difference between your readings.
 

mgtfp

Bronze Supporter
Mar 5, 2020
1,026
Melbourne, Australia
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66000
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Chlorine
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Astral VX 7T
I’ve always wondered about the accuracy of the drop tests. Seems like every new bottle I buy has different sized drops that come out. Nothing that would make a significant difference, but probably enough to make the difference between your readings.

That shouldn't be too much of a problem with the pH test, it's just an indicator dye. More or less indicator will change the colour intensity slightly, but not the hue. It can actually be helpful to add 1 drop less (or more) of the pH reagent, some say that makes it easier to differentiate the different hues. Different drop size would of course have an impact in a titration based test.
 

mgtfp

Bronze Supporter
Mar 5, 2020
1,026
Melbourne, Australia
Pool Size
66000
Surface
Plaster
Chlorine
Salt Water Generator
SWG Type
Astral VX 7T
Hello
I have a nice digital pH meter by Thermoworks. I recently noticed discrepancies between the digital meter and the Taylor color match test. I’ve calibrated my meter in the buffer solution and then got a reading of 7.68 on my pool water. Seems perfect. When I did the color match test, my eyes see it at about 8. Which result should I be trusting? It seems to me that the color match test is “fool proof” while the digital meter has room for user error.

Do you have a base and acid demand drops in your kit? I found it really helpful to use those drops to go through the whole colour scale and learn to see the different hues. I mostly use my pH-meter, but it took some time to until my tests got reproducible enough, making sure that it's properly calibrated, how long to wait for the reading , making sure that the temperatures of water and probe are not too different, etc.
 
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