Master pH Sensor Calibration: A Guide by Lakewood Instruments

Calibration of pH sensors is of the utmost importance. It helps ensure we are getting accurate data. These sensors are fairly delicate devices. A number of factors can cause their readings to drift. These include aging of the electrode, contamination of the electrode surface and changes in the internal electrolyte solution. All pH sensors have a relatively short shelf life. They begin to decay as soon as they are built. Even with proper care and maintenance, most pH sensors usually last 1 – 3 years. 

The pH Calibration Process Explained

A pH calibration is a process for adjusting your pH meter by measuring solutions of known pH values. The characteristics of your pH sensor will change over time. Therefore, calibrating your meter is necessary to compensate for these changes. Calibration does this by matching your pH meter to the current characteristics of your pH sensor. 

Selecting the appropriate test solutions is very important. Ideally, two-point or multi-point calibration should be used. A single point calibration can be used if you are looking to measure a consistent pH value with minimal or little variation. This method involves using a single buffer solution as the reference for calibration. 

The most common pH meter calibration is the two-point calibration and is best when a range of pH values are to be measure. The buffer solution values should bracket your expected pH value. In this case, the pH meter will determine the slope and the offset error for the actual pH sensor in use.  If a wider range of expected pH values is expected, then a multi-point calibration should be used. This is where 3 or more calibration points are performed giving the pH meter more accurate mV/pH equation over the range of expected pH values.

Understanding pH Sensor Variability

There is inherent variability in pH measurement which can affect pH Sensor Calibration. Selecting a calibration standard close to the expected measured value will help reduce inaccuracies. The further the standard is from the expected value, the lower the data quality will be. Water monitoring equipment and analytes do not possess absolute linear behavior. No instrument is linear over all ranges. There are slight variations from linearity that contribute to difficulty in calibration. If an instrument does not exhibit linear behavior over the desired range, the use of two calibration standards which closely bracket the expected value will help reduce the inaccuracies created by non-linearity.  

Temperature Considerations in Calibration

Temperature can play a significant role in pH measurement. The pH level of a solution is inversely proportional to the temperature. In other words, as temperature increases, the pH reading will decrease. This does not mean the solution is becoming more acidic. When the solution temperature increases, the molecules become more active which results a lower pH reading. Acidity and pH are often confused. A decrease in pH does not mean a given solution has become more acidic at a higher temperature. The solution can only become more acidic when the number of hydrogen ions increases beyond the number of hydroxide ions. In purified water, the concentration of hydrogen and hydroxide ions never changes. Purified water is always neutral and has a pH of 7.0 at room temperature. Heating the water to the boiling point will drop the pH to 6.1. The water is still neutral. Imagine the difference between the winter and summer months pH readings. Incorrect decisions can be made if temperature compensation is not taken into account. Always try to get your buffering solution to the same temperature as your process sample for accurate pH calibration.

Step-by-Step Guide for pH Sensor Calibration

  1. 1. Turn the pH meter/controller on.
  2. Allow a minimum of 15 minutes warm -up to allow the electronics to come up to temperature and stabilize.
  3.  Select two (2) pH buffers that bracket the expected pH measurement range
       a) First buffer should be 7.0 to establish the neutral/zero point
       b) The second should be close to the expected pH sample reading
  4. Prior to calibration, be sure the sensor and buffer solutions are at the same  temperature. Allow time for temperature equilibrium.
  5.  Pour the necessary amount of buffer solutions into individual glass beakers.
       a) Buffer solutions will remain stable in a glass beaker for a maximum of 2 hours.
       b) DO NOT CARRY OUT CALIBRATIONS IN THE BOTTLE OF BUFFER SOLUTION. This will taint the solution and reduce the accuracy.
       c) Do not pour the used buffer solution back into the bottle. Discard the used solution and use fresh with each calibration.
  6. Place the sensor into the pH 7.0 solution. When the reading is stable, set the pH reading to 7.0.
  7. Remove the sensor  and rinse well with distilled water.
  8. Repeat step 6 with your next buffer solution.
  9.  When the pH meter is calibrated, rinse the sensor with distilled water and place back into the plumbing assembly.

Recalibrating a pH Sensor

The frequency of calibrating a pH meter depends on usage and possible contamination. The higher the usage and the amount of contamination, the more often you need to calibrate your pH meter. Perform a pH meter calibration twice a month to avoid measuring errors. Recalibrate more frequently if the variation is significant. It is also advisable to calibrate the pH meter in the following cases:

  • When you install a new electrode
  • When the electrode hasn’t been used for a an extended period of time.
  • After the electrode has been cleaned.
  • After measuring a strong solution
  • When very accurate measurements are required

Many users are not aware of the considerations and reasons behind pH sensor calibration. The calibration process is pretty straight forward. Keep in mind that the output at pH 7.0 will be slightly above or below 0 mV, but within a certain tolerance. The slope may not be exactly 59.14 mV per 1.0 pH. The slope may not be constant across the entire pH scale. Larger deviations at the ends of the scale are to be expected due to the performance curves of the electronic circuits involved. Please note that these variations do not mean the sensor is faulty.  Regular calibration is required for optimum accuracy. This is due to the inherent degradation in pH sensors.  

As a premier water treatment sensor manufacturer, Lakewood Instruments has the right pH sensors for your cooling tower, condensate or waste applications. View our sensors and reach out to us to discuss your water treatment sensor needs.

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