PH reading differencies

Hi,

I have an alarming situation with my PH readings through Profilux and ProfiluxControl -program. ProfiluxControl is showing PH as 10,5 (when connected with usb cable to pc) and profilux is showing 5,15 (when not connected to pc). I have tested the PH-probe in RO/DI water and the reading was around 7,4 (I saw the figure change, when connected to pc). The PH-probe has been calibrated many times and still the same problem continues. Also, when displaying the measurement data, it shows strange staright line from the days I have not connected the profilux to my pc... Something must be off and not right. What to do? :confused:

ProfiluxControl 5.1.7.2
Profilux firmware 5.17

Attachment 6581

Attachment 6582

there are 2 possibilities:

a) the probe is bad

b) there is a problem with a ground loop (another device in the tank causes interferences, and they are transmitted this path: water-probe-ProfiLux-USB-cable-laptop
-> find device
or maybe a grounded power supply for ProfiLux (with earth pole) will help

did it ever work?
if so - what caused the change?

Hi Matthias,

Thank you for your reply.

I changed the Profilux wall plug to a different socket (I had all my tank electronics connected to the same socket) and that changed the PH figure to 8,2 in Profilux (when not connected to the pc). Now that is the correct figure, i am after. However, again, when I connected the Profilux to my pc with the usb cable, I am getting a different reading (9,2). So there must be something causing troubles with the connection to pc? Can it be the cable or the software? Or is my pc causing the interference? This is very odd...

Hello,

this could be one reason, see Matthias' comment:

"b) there is a problem with a ground loop (another device in the tank causes interferences, and they are transmitted this path: water-probe-ProfiLux-USB-cable-laptop
-> find device
or maybe a grounded power supply for ProfiLux (with earth pole) will help"