New technology enhances pH sensors usage because of the Smart software that’s implemented in advanced pH sensors and instruments. Never before has it been so easy to calibrate, plug-n-play, and evaluate pH monitoring.
Before the Smart technology emerged, the only way to calibrate the 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 was done on-site at a location closest to the sensor installation. So come rain or shine, sleet or snow, hot or cold weather conditions, the technician had to maintain the sensor in even the worst environmental conditions.
Smart technology changes all that. Smart pH sensors have a memory which holds calibration information, so there is no need to carry equipment to field.
The sensors can be calibrated in a controlled environment such as a laboratory or maintenance shop and the calibration information is uploaded into the sensor.
The sensor can then be taken to the field and installed on-site or it can be stored on shelf (keeping sensors wetted) until it’s time to replace one in the field.
Many of the sensors implemented with Smart technology also use a special cable-to-sensor VP connector system. This allows for plug-n-play capability so that the user can plug the pre-calibrated sensors into field equipment and the sensor is ready to measure.
This is especially advantageous for facilities with remote locations or multiple installations. It also gives the added benefit that the sensors can be rotated in and out of process as needed with minimal downtime. This quick and easy sensor exchange keeps the process up and running.
Using the Smart information that is stored in the sensor will help the user evaluate the state of the sensor’s health.
When operators understand how to interpret the basic information for the pH slope, glass impedance, reference offset, and reference impedance values, they gain a deeper understanding of how the information contained in the Smart software can prolong sensor life.
The four main concepts for using the Smart sensor information during calibration are:
- 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
The pH sensor slope information indicates the health of the pH sensitive glass membrane. The slope naturally decreases as the sensor ages. It also will decrease faster with elevated temperatures. Viewing the slope trends information allows users to make informed decisions.
- Good sensors that can be confidently used have a slope value of 54 mV/pH to 59.16 mV/pH.
- Bad sensors have a slope value of 48 to 50 mV/pH, which is too low and the sensor should be replaced.
Glass impedance is another indicator of the pH glass health. Typical pH sensors have an impedance value of 50 to 200 MOhm; some speciality pH glass sensors used for higher temperatures have a maximum glass impedance value of 1,000 MOhm.
Viewing the glass impedance information allows users to make good decisions about the glass health, and glass impedance values trending up to 600 – 1,200 MOhm may indicate one of the following issues:
- The glass is getting old due to high temperature exposure or normal aging; in this case, the sensor needs to be replaced soon.
- 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.
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 offset indicates the health of the reference electrode. New sensors placed in pH 7 buffer solution will have an ideal output of 0 mV. An acceptable offset is 60 mV maximum.
Viewing the reference offset trends allows the user to make informed decisions regarding sensor calibration or replacement.
- A reference offset of less than 60 mV can be adjusted by standardising.
- A reference offset of 60 mV or higher means that the reference electrode is spent or dirty. This indicates that it is time to replace or recharge the sensor.
The reference impedance value is another indicator of the health of the reference electrode. The normal reference impedance value on a new pH sensor is between 10 and 60 kOhm. Viewing the reference impedance information allows the user to make good decisions about sensor maintenance and replacement.
A high impedance value is 140+ kOhms and indicates one of the following:
- The reference is coated from a dirty process liquid and needs to be cleaned.
- The reference junction is clogged and the sensor needs to be replaced.
- The reference electrolyte is depleted and the sensor needs to be replaced.
This extensive diagnostic data is possible through today’s advanced pH sensors with Smart technology that enables plants to lower maintenance requirements and costs and decrease the risk of downtime, while also reducing calibration and installation time.
[Linda Meyers is senior pH product manager, Emerson Process Management, Rosemount Analytical.]