Smarter technology for pH sensors reduce maintenance costs, downtime
pH sensors are a time-honored technology and one that is indispensible in virtually every plant and process known to industry. They can also be a challenge. Ask plant operators about their most time-consuming and burdensome tasks, and chances are they will mention the field calibration of pH sensors.
In addition, pH sensors are often isolated from the central plant information systems, which makes them maintenance nightmares and creates potential risks of downtime. Fortunately, while pH technology is classic, the continuous improvements to pH systems are helping to overcome some of these operational problems for plant engineers.
One of these improvements is making pH sensors “smart”—smart enough to hold calibration and other data and to communicate that information to central control systems. The result is lower cost of operation, substantially reduced maintenance requirements, and reduced downtime in a wide range of applications.
The calibration nightmare
Traditionally, the only way to calibrate a pH sensor was to carry all of the calibration equipment into the field. In many facilities, this meant carrying at least two buffer solution bottles, two beakers, and one rinse bottle to the various installation sites. Then, the calibration is done on-site at a location closest to the sensor installation. So come rain or shine, sleet or snow, hot or cold weather, the technician had to maintain the sensor in even the worst environmental conditions.
New technologies now embed memory in pH sensors, which allows them to hold calibration information. This means a sensor can be calibrated in a controlled environment such as a lab or maintenance shop. The information is then held in the sensor memory as the sensor is taken into the field and installed. Pre-calibrated sensors can even be stored on shelves and then taken into the field to replace a sensor requiring calibration or maintenance. No more bottles and beakers in the snow, plus no downtime.
Because the new sensor technologies store data in the sensor, they also solve another important pH measurement problem—unpredictable failure. While new sensor designs and materials make today’s pH sensors far more durable than previous generations, problems such as clogging and poisoning of the reference electrode or cracks in the glass can cause sudden unreliable measurement or complete failure.
As a result, pH sensors are often “over-replaced” in order to assure that the sensor doesn’t fail unexpectedly when maintenance staff is far from the site—a costly practice. Or maintenance costs can soar as companies cope with temperamental sensors. When sensors store their own diagnostic data, however, operators can learn to use key indicators to assess the health of the sensor and actually predict failure.
The information stored in the sensor that can be used to predict accuracy and sensor life include:
- Slope trends, which normally decrease over time
- Glass impedance trends, which normally increase over time
- Reference offset trends, which normally shift slowly over time
- Reference impedance trends, which normally shift slowly over time.
Analyzing the glass
The sensitive glass membrane on most pH sensors is an area of potential failure. To analyze the condition of the glass, operators can observe the slope trends. The slope naturally decreases as the sensor ages. Elevated temperatures will cause it to decrease more rapidly.
- A slope value of 54 mV/pH to 59.16 mV/pH indicates the sensor is in good condition.
- Sensors should be replaced when they have a slope value of 48 to 50 mV/pH.
A second indicator of the condition of pH sensor glass is glass impedance. Typical pH sensors have an impedance value of 50 to 200 Mohm; some specialty pH glass sensors used for higher temperatures have a maximum glass impedance value of 1000 Mohm. Glass impedance values trending up to 600-1200 Mohm may indicate one of the following issues:
- Glass aging due to normal conditions or excessive exposure to high heat.
- The sensor is not immersed in the process liquid or buffer solution.
- The glass is dirty and should be cleaned before installing it back into the process liquid.
With this type of information in hand, an operator can schedule regular maintenance to clean or replace the sensor in plenty of time to prevent inaccurate operation or complete failure. Glass impedance values of less than 10 Mohm identify cracked glass, excessive exposure to high temperatures, or a high impedance short in the sensor. In any of these scenarios, the sensor should be replaced.
The reference imperative
A large number of pH sensor problems are caused by the easily obstructed reference electrode. Observing and understanding the reference offset can prepare the operator for regular maintenance rather than an emergency situation. New sensors placed in pH 7 buffer solution will have an ideal output of 0 mV. An acceptable offset is 60 mV maximum. Any variation may indicate the need for maintenance.
A reference offset of less than 60 mV can be corrected by a standardizing one-point calibration. A reference offset of 60 mV or higher indicates the need to clean a dirty electrode or to recharge or replace the sensor.
Another live diagnostic now available in the smarter sensor technologies is reference impedance, which together with mV input can indicate that the sensor is coated, poisoned, or overexposed to high temperatures. The normal reference impedance value on a new pH sensor is between 10 and 60 Kohm. Impedance over 140+ Kohms indicates that the reference is coated and needs cleaning; or the reference junction is clogged and the sensor needs to be replaced; or the reference electrolyte is depleted and the sensor needs to be replaced. Operators can observe both reference offset and reference impedance moving to the edge of the normal range and take action in advance of any inaccuracy.
Some sensors that incorporate smart memory technology are set up for plug-and-play operation. Through cable systems or special connectors, the pre-calibrated sensors can be plugged into the field analyzer ready to operate without further setup. This type of connector system reduces downtime, particularly in facilities with remote locations and multiple installations. The sensor exchange can be performed with no downtime.
pH sensors today would barely be recognizable to a plant operator from 20 years ago even though the fundamental science is little changed. Sensors last months instead of days, glass seldom cracks, reference junctions resist poisoning, and the cumbersome job of calibration has now been simplified. pH sensors even help diagnose themselves, saving maintenance time and costs, simplifying installation, and significantly reducing the risk of downtime.
Linda Meyers is the senior pH product manager for Emerson Process Management, Rosemount Analytical. More information can be found at www.RosemountAnalytical.com.