There are three common problems that might be encountered when calibrating a pH sensor. These are:
- Difficulty in Achieving a Zero Point Calibration
- No Success in Obtaining a Slope Calibration
- A Very Long Response Time (longer than 3 minutes)
There could be various reasons for the above mentioned problems. The most frequent causes are listed below:
- The buffer solutions used for pH calibration are either contaminated or expired. It also could be that the buffer solutions used are incorrect — therefore never store buffer solutions in unmarked bottles or open in a beaker.
- The reference electrode electrolyte is contaminated and/or the diaphragm is blocked. This may be apparent through discoloration of the ceramic junction or a color change of the KCl electrolyte.
- An older sensor is used which no longer generates a proper mV signal.
- The pH sensor has dried out or has not been hydrated long enough (after dry storage or after cleaning with an acid solution).
- Hairline cracks are present in the glass membrane of the measurement electrode.
- An electrostatic charge has developed due to rubbing the electrode shaft with cloth instead of gently dabbing it with tissue paper.
- A temperature difference between pH sensor and the liquid buffer solution of more than 10°C during calibration.
- The zero point of the pH electrode assembly and the pH meter differ. (Note: This is seldom the case as normally both sensor and electronics use pH 7 as their zero potential. In exceptional cases a poorly manufactured pH sensor could have a zero point which differs from pH 7).
- The electrical connection between the pH sensor and measurement transmitter could also cause problems. This is normally recognized as either open-circuit in the cable or short-circuit in cable and/or connection plug (usually due to moisture ingress).
Note: Hamilton Arc Intelligent sensors allow for diagnostics checks of many of the error messages described above. These sensors convert electrical measurements into either digital protocol (Modbus RTU) or Bluetooth wireless communication. The resulting diagnostic error messages can be displayed on a PC, mobile phone or tablet using ArcAir Software. Consult Hamilton Support for more information.