How often should a pH, conductivity, or Ion Selective Electrode (ISE) probe be calibrated?

Document ID

Document ID TE1021

Version

Version 3.0

Status

Status Published

Published Date

Published Date 02/05/2019
Question
How often should a pH, conductivity, or Ion Selective Electrode (ISE) probe be calibrated?
Summary
pH, conductivity and ISE probe calibration frequency.
Answer
Hach does not publish any recommendations on how often a sensor needs to be calibrated.

Typically, pH probes and many conductivity probes are calibrated at the start of each 8 hour shift, but this can vary depending on application.

In theory, a probe is calibrated (or at a minimum controlled with a calibration standard) prior to any measurement series. So, every time a measurement series is started, one would have to:

1) Calibrate the sensor
2) Control the calibration with a standard different from the one(s) used for calibration (this process is also called running a Check Standard).
3) Run the sample series (10 samples is typically considered the maximum, but it can vary)
4) Run another check standard with the standard used in step 2.
5) Repeat step 3.

In practice, only certified labs are running such a procedure. For practical reasons, many labs will calibrate less frequently.

Calibration frequency depends on:
- The environment factors and measuring conditions.
- The likelihood the sensor will change with time.
- The likelihood the sensor will be contaminated by the sample
- How the sensor is maintained by the operator between each measurement.
- The customer experience in regard to samples and application.

In all cases the calibration frequency is a customer experience based decision linked to the sample type measured and the measurement frequency. As a manufacturer, Hach can only give generic advise and recommendations for use which would be:

- Prior to using the sensor, use a check standard.
- If the reading of the check standard is outside of the accuracy acceptance limits, then recalibrate the sensor.
- If the calibration is outside of the acceptance limits, then clean/maintain the sensor according to manufacturer recommendations. If maintenance doesn't help, then replace the sensor.
- This process will give information on how frequently calibration/maintenance/replacement of the sensor needs to be done in regard to the frequency of use and the sample matrix.