How to properly use electronic PH Tester Pen?

[Obriy]

I would like to share my problem with PH tester pens that I purchased on Amazon. When I test them directly in the aquarium tank, both testers show PH levels around 0.3-0.4 less then when I get a cup of water from the tank and place the testers in the cup. Just now, the tester pens show average PH level of 7.73 (there is some small discrepancy between them, so I use averages) when measured directly in tank and 8.10 when tested the tank water in a cup.
Does anyone know what is the right way to do measurements using these testers?

 

[FUNGI]

Same......so I used VivoSun's directions and now just do it in a beaker....

 

[Obriy]

Same......so I used VivoSun's directions and now just do it in a beaker....

Can you explain, please?

 

[Randy Holmes-Farley Verified Community Expert]

That sort of difference is not normal, but if you calibrate in a cup and see such a difference that is not a temperature difference then measure in a cup.

There are some complications to pH measurement, such as electrical interference from nearby devices, and ground loop issues.

 

[Obriy]

That sort of difference is not normal, but if you calibrate in a cup and see such a difference that is not a temperature difference then measure in a cup.

There are some complications to pH measurement, such as electrical interference from nearby devices, and ground loop issues.

Thank you Randy.
I have two follow-up questions:
1 - When I calibrate the testers, should I heat the temperature of calibrating mix to the tank temperature? My tank temperature is 79 degrees and my home (and, therefore, RODI) is 76 degrees.
2 - If in fact there is some electrical interference, or ground loop issues, how big of a problem can it be for the tank and how to find it?

 

[scotty333]

Following this
I took my pen to my lfs to check against their tank and he took a beaker of water from the tank but never questioned it

 

[Randy Holmes-Farley Verified Community Expert]

Thank you Randy.
I have two follow-up questions:
1 - When I calibrate the testers, should I heat the temperature of calibrating mix to the tank temperature? My tank temperature is 79 degrees and my home (and, therefore, RODI) is 76 degrees.
2 - If in fact there is some electrical interference, or ground loop issues, how big of a problem can it be for the tank and how to find it?

Warming to tank temp can be a good idea, and I believe that ground loops are only a pH measurement problem, not a tank problem.