Maintaining and Calibrating pH Sensors
Because proper pH adjustment is critical for many processes, maintaining the accuracy of your pH measurement devices is vital. Proper maintenance and calibration on a regular basis are required ensure accurate measurements and increase the lifetime of your pH electrode.
Maintaining your pH electrode
A pH meter measures the potential differences between a reference electrode and a pH electrode that is exposed to the measured media. The lifetime of the exposed pH electrode is not very long, but if well-maintained it can be expected to function effectively for 6 to 12 months in a typical application. A pH electrode that is not maintained and cleaned regularly will become coated and reference junctions may become contaminated, causing it to become sluggish or non-responsive. A simple rinse with clean water after use goes a long way to keeping your pH electrode functioning properly, but a cleaning agent may be required. Check with your manufacturer or supplier for a cleaning solution that is appropriate for your particular pH electrode and the type of contamination.
Calibration is key
Because a pH electrode will gradually change or degrade over time due to aging and coating of the membrane (even with proper maintenance), regular calibration of your pH meter is required to ensure accuracy. Frequency of calibration depends on a number of factors. In a laboratory setting, your pH equipment should be calibrated each time it is used. For online pH measurement in a process environment, frequency can vary considerably depending on the application, its unique conditions, and the level of accuracy required.
The pH calibration procedure requires the use of at least two pH buffer solutions (pH 4 and pH 10 are most common) that span the range of pH values to be measured, but a multi-point calibration method should be used for greater accuracy. In a three-point calibration, the meter is first calibrated at pH 7 (neutral), then to the pH closest to the pH desired level. The third calibration point allows you to calibrate to both sides of the zero point (pH 7) by expanding your measurement range and providing greater linear accuracy.
The glass membrane at the probe tip of your pH electrode must be kept moist. If it is allowed to dry out, it will not function properly. To maximize performance, store your pH electrode in a solution recommended by the manufacturer. Never store your electrode in water. If your pH electrode dries out or is damaged after a rough cleaning procedure, there are regeneration procedures that can be attempted to restore the membrane.
Omni Instrumentation & Electrical Services, Inc., located in New Jersey and Maryland, is a premier instrumentation and electrical contractor, providing superior total care solutions since 1986. Services include instrument installation, tubing installation, instrument calibration, control loop testing, startup and commissioning, power and lighting, process control wiring, BMS wiring, telecommunications and data wiring, fire alarm wiring, security wiring, process network wiring, and control panel fabrication. Omni Instrumentation & Electrical Services, Inc., is certified in Foundation Fieldbus, Profibus, DeviceNet and ASI-Bus installation.