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.
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:
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
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.
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.
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.
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.
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.