PH Sensor Calibration Rosemount 3900

[hse112]

Hi guys,
Hope you are doing fine, i am having an issue that i want to discuss it with you if it happened to anyone of you.
I am having a rosemount PH\ORP Sensor of model 3900 series connected to a rosemount 1056 analyzer screen.

First , i want to ask about the offset standard value beacause as i know that the standard for the slope is nearly 59 mv according to ernest's equation to calculate PH and for low reading between 42-47 mv the electrode indicates it should be replaced or cleaned.
So offset too is an indication but i dont know the standard reading of it as anew sensor compared to an old one.

Second , i want to know when calibrating an old sensor as auto or manual it gives me offset error and no calibration occurs, why?
i tried to override and change the offset manually but when i retried to calibrate the old sensor nothing happened.
What is really an offset in the process of analyzing ph values, and how it affects the reading cause changing it changes the whole reading.

Thanks in advance.

 

[rlundsrud]

The slope equation is 59.1mV/pH. This means that each pH value above 7 adds 59.1mV. At 7 the value would be 0mV, at 8 it would be 59.1mV, at 9 it would be 118.2mV etc. This is the slope value that is referred to by the Nernst equation. If the slope value drops below 47mV/pH then the tip should be replaced.

When you did the 2 point calibration, what values did you get for 4 and 7 pH? Take a close look at 7 as this is the offset value, this offset should match the pH4 value (displayed value +177.3mV). Also, you need to make sure the settings in the 1056 are correct as this will cause you problems.