pH Discrepancies

[Dallas_James]

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]

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.

 

[Donldson]

Just to rule it out: your FC is less than 10 ppm, correct?

 

[Dallas_James]

Just to rule it out: your FC is less than 10 ppm, correct?

 

[jark87]

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]

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]

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.