A common question I get from customers about pH measurement is “How accurate is my sensor?”. You’d think there would be a straightforward reply for pH sensor accuracy, like 0.25% of range or 0.5 pH of the span. Unfortunately, it’s difficult to provide a definitive answer to this question. As a matter of fact, it’s rare to see an accuracy specification for most process pH electrodes. When you do see one, there are a number of exceptions. This is because there are many uncertainties and potential measuring errors, such as:

• Buffer solution degradation or contamination
• Calibration procedures
• Temperature dependencies
• Junction potential
• Process flow velocity
• Cross-sensitivity to other ions such as sodium

Most manufacturers of pH electrodes build to a quality level that gives an accuracy of approximately 0.1 pH. If the pH sensor is properly maintained, calibrations done correctly, and stored according to manufacturer recommendations, then 0.1 is possible.

Getting the Best Accuracy Out of your pH Sensor

To ensure that you have the best pH sensor accuracy, you need to make certain you concider the following:

• Use fresh buffer solutions for each point of calibration. Buffer quality is important to ensure accuracy.
• The temperature of the buffers and the electrode should be the same. Allow the sensor pH and temperature reading to stabilize during each step of your calibration process. Doing so ensures that you are using an accurate pH value as your standard.
• Always keep the pH glass or “bulb” wet. A pH sensor that has not been properly hydrated will drift after calibration.

Calibration is King

Something else to consider is offline calibrations. By calibrating in the field, you are already dealing with a number of additional challenges such as weather (rain, snow or wind), electrical noise, and potential human error caused by stress.

For a better calibration, consider using smart digital technology. It allows you to bring a pre-calibrated sensor to your measurement point. Because the sensor head stores the calibration, you can “hot swap” the sensor at the transmitter. After installing the pre-calibrated sensor, return the old sensor to the shop for maintenance and calibration in a controlled environment. Place the sensor in a safe location until needed. Offline calibrations are one of the best ways to achieve the highest possible measurement accuracy.

In summary, remember that one of the most important things you can do to ensure an accurate pH measurement is to do a high-quality calibration. Being patient during your calibration is critical. Make sure you don’t rush the calibration steps and allow the pH and temperature reading to stabilize. Finally, start doing offline calibrations with smart digital technology so that you can stop stressing about the accuracy of your pH measurements.