An analyzer should be calibrated (or recalibrated):
- upon initial installation,
- following physical relocation,
- after any repairs or service that might affect its calibration,
- following an interruption in operation of more than a few days,
- upon any indication of analyzer malfunction or change in calibration, and
- at some routine interval
When calibration relationships are applied to analyzer responses to determine actual concentrations, it is suggested that the analyzer be recalibrated periodically to maintain close agreement.
The frequency of this routine periodic recalibration is a matter of judgment and is a tradeoff among several considerations, including: the inherent stability of the analyzer under the prevailing conditions of temperature, pressure, line voltage, etc., at the monitoring site; the cost and inconvenience of carrying out the calibrations; the quality of the ambient measurements needed; the number of ambient measurements lost during the calibrations; and the risk of collecting invalid data because of a malfunction or response problem with the analyzer that wouldn’t be discovered until a calibration is carried out.
When a new monitoring instrument is first installed, zero/span and one point QC checks should be very frequent, perhaps daily or 3 times per week, because little or no information is available on the drift performance of the analyzer.
With the advancement in data acquisition system technology, many monitoring organizations are running these QC checks daily. However, the QC checks are required to be implemented every two weeks. Information on another unit of the same model analyzer may be useful; however, individual units of the same model may perform quite differently.
After enough information on the drift performance of the analyzer has been accumulated, the calibration frequency can be adjusted to provide a suitable compromise among the various considerations mentioned above.
To facilitate the process of determining calibration frequency, it is strongly recommended that control charts be used to monitor the zero/span and one-point QC drift performance of each analyzer.
Control charts can be constructed in different ways, but the important points are to visually represent and statistically monitor drift, and to be alerted if the drift becomes excessive so that corrective action can be taken. Such control charts make important use of the unadjusted zero and span response readings.
NOTE: Many newer technology analyzers have an “auto-zeroing” function incorporated in the instrument that can be implemented at user defined frequencies. Use of internal auto-zero functions typically does not need any post-processing of the data.
Auto or manual zero adjustment acceptable, but does not recommend making automatic or manual adjustments (corrections) to the span until drift is unacceptable and warrants a calibration.
In continuous monitoring, the total cumulative drift, average of the absolute values of the individual drifts, and the standard deviation of the individual drifts should be calculated on a running basis over the last 100 or so days.