The calibration interval is highly dependent on the measurement conditions. The time interval between calibrations depends on a number of factors, and no generally valid interval can be given. The frequency at which process sensors should be calibrated can be determined by using pH transmitters with sensor diagnostics or by experience.
To help you gain experience about your process and your chosen pH electrode you can use different ways. Two useful methods are to monitor the scattering of the measured values and to regular check your electrode.
The frequency of calibration really depends on whether you think there
is any need for adjustments. An adjustment is probably unnecessary if
the measured pH value is within the specification limits.
An online pH measurement will, in most cases, show scattered measured values, even the process media are supposed to be stable. Scattered measured values alone are not always an indication of a fault. It can be caused by normal variations in the measured media. However, the scattering of the measured values will increase with the age of the pH electrode.
A plot on the measured pH value will probably vary according to the picture to the right. As long the measurement value is within the approved tolerance value, your pH sensor is still in calibration and no need for re-calibration. A process pH loop is in many cases connected to a computer or control system and will make it easy to produce a plot on the scattered measured values.
On a regular base check your sensor performance by measuring an
appropriate pH buffer. If the measured value is within the required
specification, do not recalibrate the sensor. Return it to the process.
After you have done these checking a couple of times, it will become apparent how long your pH electrode holds calibration. Based on this knowledge the calibration frequency can then be determined.
How often should you check your pH sensor has to be estimated for each individual application. For a clean process, a couple of weeks can be an appropriate interval, for dirty and aggressive media a couple of days is probably more likely.