,Are you spending a lot of money on pH sensors? Like most process instruments, proper installation and maintenance play a significant role in determining the effective life of your sensor. In this writeup, we address the top four things you can do to extend the electrode’s life through proper pH sensor maintenance.

1. Keep the Sensing Element and Junction Hydrated

Whether in storage, the lab, or in your process, it’s critical that you keep your pH sensor hydrated. Allowing a glass or ISFET pH sensor to dry out will, at a minimum, cause inaccuracies and slow response time when reintroduced to the measured medium.

We recommend the following steps that will protect your sensors:

• While in storage, sensor should have a wetting cap. The cap should contain 3 mol/l KCl solution or pH 7 buffer. DO NOT STORE IN DEIONIZED WATER, as it will leach the electrolyte from the reference system.
• When not in use in the lab, soak the sensor in 3 mol/l KCl solution or pH 7 buffer
• If you are operating a batch process or your plant is in shut down, you should either:
  • Remove the sensor from the process and soak in 3 mol/l KCl solution or pH 7 buffer
  • Retract the sensor into a retractable holder, introduce either 3 mol/l KCl solution or pH 7 buffer through a cleaning port, and then plug the port and drain

BONUS TIP: If left dry for even a short period of time some sensors may be re-hydrated by immersing in a 3 mol/l KCl solution for 24-hrs; calibrate and/or diagnose further to determine if sensor remains useful.

2. Calibration best practices

It is an absolute truth that bad calibration practices will directly impact pH sensor performance and cause additional maintenance. In other words, the better the calibration the greater the accuracy and reliability.

When doing calibrations in the field, operators often prioritize analog sensors based on need and the analyzer’s location. This is often no matter the situational or environmental conditions. This can cause calibration accuracy issues because of bad weather conditions, physical installation difficulties, and electrical noise.

The ideal calibration is best performed in a controlled environment at a time when most convenient for the user. With the advent of digital sensor technology, the option to perform offline calibrations is now available. Calibrations can now be proactive rather than reactive. Offline calibrations eliminate some of the main contributors to poor sensor performance.

Listed below are a few helpful hints to ensure a reliable calibration is performed.

• If possible, perform the calibration in the instrument/analyzer shop or lab (digital sensors)
• Use fresh, clean calibration standards (Buffers) and rinsing solution
• Make sure to use a pH sensor with fast response to both pH and temperature change – Be patient with your calibration process. Wait for the sensor output to stabilize in the buffer prior to performing each calibration point

For a more detailed overview of how to develop a comprehensive calibration procedure please review our post “How accurate is my pH sensor? You should be asking, how accurate is my calibration procedure?”.

BONUS TIP: Buffers of pH 9 and greater are volatile and degrade upon exposure to the atmosphere. Date any newly opened containers of these buffers and dispose of after 30-days unless using from a single dose or isolated dosing dispensers.

3. Clean the sensing element and junction

The buildup of any kind on the surface of the pH sensor or junction can shorten sensor life. As a result it will cause measurement inaccuracies. Proper cleaning of the sensor, done on a regular basis, is necessary for those applications where suspended solids or adhering media are present. Chemical cleaning of the sensor is preferred to mechanical cleaning as it reduces the possibility of damage to the sensor and the sensitive pH glass.

Here are a few helpful hints to help you maintain measurement accuracy and extend sensor life:

• For inorganic coating or buildup, you should soak the sensor for several minutes (max. 1 hour) in a diluted Pepsin/HCl or NaOH (0.1 mol/l) at room temperature. Pepsin/HCl is also recommended for use in applications where proteins or sulfides buildup on the sensor.
• For organic coating or buildup, you should rinse the sensor with an organic solvent such as ethanol or acetone.
• When wiping off the sensor, be sure to use a soft, damp cloth to prevent polarization of the sensing glass and/or damage the pH glass.
• If buildup occurs quickly and the cleaning requirement becomes excessive, we recommend implementing an automated cleaning system.

BONUS TIP: If returning the sensor to storage after cleaning, use deionized water to rinse off any residual cleaning chemical, then dab the sensor dry before applying a wetting cap to prevent cross-contamination of the cleaning and storage solutions.

4. Monitor sensor diagnostics

Technicains should utilize data, extracted from the pH sensor, for a better understaning of maintenance requirements which can extend sensor life.

Analog pH sensor diagnostics only evaluate sensor calibration (zero, slope, response time) and glass impedance. In order to obtain this information, the user must extract it from the device that was used for calibration. For most process applications, this means going out to the field transmitter at or near the point of measurement. Once able to view the data on the transmitter, the user must then understand what the information means and what actions to take.

Digital sensors offer the user a wide array of diagnostic data available via the field transmitter, portable meter, and in the control room (through the use of HART or other BUS protocols). Information now available from within the sensor:

• Calibration validation (Zero, Slope, Response Time)
• Wear
• Operation Time
• Lifetime Calculator
• Fault icons and codes
• Cleaning and Sterilization cycle counts

Technicians can configure digital pH loops provide an indication that service needed prior to measurement drift or failure. If the sensor does fail, they now have the ability to understand why. They can then perform the proper maintenance technique and see if the sensor is repairable and able to be reused in the process.

Properly configuring a digital sensor and transmitter and then monitoring the diagnostic data provided will reduce pH sensor maintenance and downtime.