Intelligent sensors for
easy pH loop management

Intelligent sensors are sensors that are equipped with diagnostic capabilities to monitor their performance and function. Many pH electrodes in critical processes must operate around the clock without interruption to assure product quality, protect equipment and provide a safe operating environment.

A self diagnostic pH sensor will provide the operator with real time fault and warning information. This put the operator in control of the sensor, and keeps the sensor in control of the process. Reduced maintenance costs and preventing unnecessary line or plant shutdowns are two of the main advantages.

pH sensor diagnostic functions

glass impedance

Several diagnostic variables allow you to monitor the condition of your pH electrode. The implementation of the self-diagnosis function and which variables are used differs between different pH equipment manufacturers. However, the most important diagnostic variables, for an intelligent pH sensor, are:

  • Glass resistance (impedance)
  • Reference resistance (impedance)
  • Slope and Offset

Glass resistance

The electrical resistance of the glass electrode increase as the pH electrode ages. An increase in the glass resistance will give a more sluggish response and an electrode that is unable to respond effectively to a changing pH environment.

At some point the glass resistance will suddenly drop, which indicates a cracked or broken pH electrode. The output signal will be 0 mV, which will be displayed as a pH value of 7.0, see Nernst equation. A cracked or broken pH electrode displaying a "stable" pH value of 7.0 will not be easy to distinguish from a working electrode that is supposed to display 7.0. This can be a catastrophic failure in many applications that works close to a pH value of 7.0. For these applications intelligent sensors will be of great help and give an alarm if the glass resistance suddenly changes.

Reference resistance

The resistance of the reference electrode will increase as the reference becomes fouled by either coating or contamination of the reference junction. An increasing reference resistance value, causes pH drift.

Slope and Offset

Unlike glass and reference resistance, the slope and offset variables is not a continuous measurement. Both slope and offset are calculated during calibration.

The sensor life and maintenance requirements can be estimated by comparing the deterioration of the slope and offset values between calibration times. A slope or offset value out of the normal range indicate a sensor failure or a calibration procedure error.

Resistance (impedance) measurement

intelligent sensors

To be able to measure the electrical resistance of the glass electrode and the reference electrode you need a differential pH sensor with a solution ground. The solution ground is needed as an extra electrode for these measurements, see the figure to the right.
The resistance measurement is performed by injecting a known current into the electrode and measuring the voltage level across the sensor. According to Ohm's law the ratio of voltage and current gives the resistance.

It is worth noting that because of the electrical characteristics of a pH electrode, a resistance measurement requires a certain wait time. During this measurement time, no pH readings are updated. A long update rate (many seconds) can be a problem in many applications. Before a purchase, check with the distributors of smart sensors, what update rate is specified.

The concept of intelligent sensors is in constant development

Manufacturers of pH equipment are developing new software applications that make the diagnostic data more accessible and easier to apply. This allows for even more effective maintenance schemes that help the operator keep the pH control loop and process running with minimum downtime.




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smart sensors