Ryan_Flowers
Lifetime Supporting Member
Hello,
I am replacing a pH probe that was determined to be bad in a waste water treatment system. I am replacing it with a 2-wire Aqua Metrix 4-20mA probe; Manual Here. The original probe was also a 2-wire but the part number has long been washed away with no way of completely identifying it. This probe is wired into a 1769-IF8 analog input card Manual Here on my Compact Logix controller bank. The input channel it is using is set to 4-20mA and is also set to read Raw/Proportional with a 60 Hz filter.
The logic already has an SCP instruction taking that analog signal and scaling to a min value of 4 and a max value of 10. (Attached Picture).
My questions are; What should my resolution be for this device? Why wouldn't the pH min/max scale already be set to 0-14 instead of 4-10? According to the 1769-IF8 literature my input min should be -32767 and my input max should be +32767. When I change each scale to what I think they should be I get 8+ pH output with the sensor submerged in a beaker filled with city water. Now the city water could very well be +8 pH, but I don't have any buffer solution to test the other pH values to say yes or no on it. The overall problem is we are out of compliance with our readings sent back to the city and I don't currently have an answer for them.
Any help here would be greatly appreciated since analog resolutions always throw me for a loop...
Thanks
I am replacing a pH probe that was determined to be bad in a waste water treatment system. I am replacing it with a 2-wire Aqua Metrix 4-20mA probe; Manual Here. The original probe was also a 2-wire but the part number has long been washed away with no way of completely identifying it. This probe is wired into a 1769-IF8 analog input card Manual Here on my Compact Logix controller bank. The input channel it is using is set to 4-20mA and is also set to read Raw/Proportional with a 60 Hz filter.
The logic already has an SCP instruction taking that analog signal and scaling to a min value of 4 and a max value of 10. (Attached Picture).
My questions are; What should my resolution be for this device? Why wouldn't the pH min/max scale already be set to 0-14 instead of 4-10? According to the 1769-IF8 literature my input min should be -32767 and my input max should be +32767. When I change each scale to what I think they should be I get 8+ pH output with the sensor submerged in a beaker filled with city water. Now the city water could very well be +8 pH, but I don't have any buffer solution to test the other pH values to say yes or no on it. The overall problem is we are out of compliance with our readings sent back to the city and I don't currently have an answer for them.
Any help here would be greatly appreciated since analog resolutions always throw me for a loop...
Thanks