Stability of Calibration Factors of Survey Meters: A Five-Year Comparative Study

Introduction: Analysis of historical calibration data can reveal a lot about survey meters. The study aims to analyse a five-year historical calibration data of selected survey meters for the stability of indication or otherwise. Material and Methods: A 1000-cc Physikalisch-Technische-Werkstaetten (PTW) spherical ionisation chamber coupled to a PTW UNIDOS Electrometer was used as the reference dosimeter to determine ambient dose equivalent in a 137Cs radiation beam. Ten survey meters were calibrated every year by the substitution method for five continuous years. The yearly calibration results were analysed. These survey meters are used in border and port control, extractive mining industry, non-destructive testing industry, and health care delivery. Results: Analysis revealed that there are deviations in the calibration factors (CFs) from their initial year 2019 values for all devices. The percentage deviations in the CFs in the year 2020 and beyond from year 2019 values ranged from -34% to 24%. Averagely, each device overestimated its indication of ambient dose equivalent by 0.075 ± 0.009 mSv/h within the five years and underestimated the ambient dose equivalent by an average of 0.163 ± 0.019 mSv/h. Conclusion: The stability of calibration factors of the survey meters degraded with time and usage. Survey meters that are out of calibration produce inaccurate measurements. To help detect the instability early, it is suggested that users of survey meters resort to counting statistics on their measured data at regular intervals.