Calibration and storage. pH sensors can only read accurately when correctly calibrated. Proper calibration demands known concentrations of buffers and stable temperature and humidity conditions — ideally, conducted in a lab environment by a trained technician. As already noted, don’t store the sensors in DI water. Instead, use 3 molar KCl (saturated KCl solution) at room temperature.
Hypoionic processes. In water purification processes such as reverse osmosis (demineralization) or distillation, water becomes extremely low in ionic strength. Measuring pH on such processes becomes vary challenging due to low electrical conductance. To overcome this situation requires applying external sources of electrolyte to the sensor — reference electrolyte is supplied from an external reservoir into the reference cell.
Troubleshooting pH sensors
The main components of a typical pH loop are the process, sensor, cable and transmitter. These, individually or in combination, can produce problems with measurements. A five-step approach can help pinpoint the source:
1. Identify the problem. Recognizing there may be a problem with a measurement signal is the start of troubleshooting. Process knowledge, measurement history and data trending are key factors in determining if the pH reading is correct.
2. Isolate the problem. In some cases, the application may be the problem. Process conditions play an important role in successful pH measurement. For example, low conductivity water will require a special reference. If the process is associated with sterilization cycles, PTFE may not be a suitable reference junction. Frequent replacement can be a tip-off to use of the wrong sensor. Always document process conditions.
3. Find the culprit component. With one complete working pH loop, interchange components in the order of sensor, cable and transmitter to isolate the problem part of the pH loop.
4. Characterize the problem. If you’ve identified the sensor as the issue, check the area around the reference junction. Inspect the measuring glass. See if there’s any discoloration of the reference fluid. Occasionally a pH sensor can pass quality control testing at the factory and the performance test at the customer’s site but ultimately suffer a short life.
5. Check for galvanic isolation — ground loop. Use the procedure mentioned earlier. A possible fix to the ground loop may be as simple as removing the output wires from the transmitter (for example, the 4–20 mA output) and checking to see if the reading returns to normal. In some cases however, a sensor that digitizes the signal may provide only real solution.
Take some basic steps
Selecting the appropriate pH sensor technology requires understanding the particular process chemistry and conditions, and their impact on the sensor. This will allow you to choose a sensor that will save chemicals and time. The correct sensor will improve production — avoiding off-spec products and waste — and lead to a more energy efficient and profitable operation.
1Sørensen, S.P.L., “Electrometric method of determining hydronium ion concentration,” Carlsberg Laboratories, Copenhagen, Denmark (1924). (Sørensen’s original paper.)
Bhupen R. Patel is business manager, analytical products — Midwest, for Endress+Hauser, Inc., Greenwood, Ind. Fred Kohlmann is Sussex, Wis., based business manager — upper Midwest for Endress+Hauser. E-mail them at firstname.lastname@example.org and email@example.com.