Mystery: 1st phenol red pH reading too low

[Ronald U.]

Greetings, pool chemistry buffs. Anyone have any ideas? My first pH reading is often way too low. The second reading makes more sense. Same rinsing with tap water 3x, then with pool water 3x, same pool water sample. Thank you for your consideration. Cheers.

 

[HermanTX]

Greetings, pool chemistry buffs. Anyone have any ideas? My first pH reading is often way too low. The second reading makes more sense. Same rinsing with tap water 3x, then with pool water 3x, same pool water sample. Thank you for your consideration. Cheers.

I never rinse with tap water - always pool water. Also, I dry it with a paper towel after use and rinsing with the pool water. I then do a quick rinse with pool water sample prior to my next test (to remove any dust, etc.)
Are the 2 readings you obtain significantly different, i.e. on opposite sides of scale or just off by 1 measurement?

I also use a pH meter and my readings of pH using the chemical solution tend to be always lower then my pH meter but not significant (i.e. 0.2 off - i.e. 7.6 by chemical test and 7.8 on pH meter. My pH meter has accuracy of +/-0.1

 

[JoyfulNoise]

Not sure I understand what you are seeing.

Are you saying that you scoop up some pool water and get a low pH reading. Then you toss that out, rinse the tube, get more pool water and your pH reading is higher?

Can you show some pictures??

 

[Ronald U.]

Like today for example. First pH reading 7.8. Second 8.0 which is more in line with recent readings. Some times the difference is more like 7.4 to 8.0.