Evaluation of Single- Versus Multi-Point Barometer Calibration for Reference Dosimetry
Abstract
Purpose
AAPM WGTG51 Report 374 emphasizes the importance of field barometer cross-calibration versus a reference barometer prior to reference dosimetry measurements. This work evaluates the validity of single-point calibration across altitudes and whether multi-point calibration improves accuracy or identifies faulty barometers for precise pressure-based corrections.
Methods
Three aneroid and one digital, Accredited Dosimetry Calibration Laboratory (ADCL) calibrated barometers, were evaluated against a reference ADCL barometer with measurements performed at eight altitudes spanning approximately 300 feet (Δ10 mmHg) in an elevator assuming 12 feet/floor. Pressure readings were plotted versus altitude, and linear best-fit equations were determined. Pressure readings were corrected versus the reference barometer using single-point (offset) and multi-point (slope and offset) methods. Maximum absolute percent differences relative to the reference barometer were calculated for both calibration methods. Corrected pressure–altitude gradients were compared to theoretical gradients calculated using the barometric formula. Impact on the pressure-temperature correction factor, PTP, was extrapolated to pressures ranging from 740-780 mmHg at a constant 24°C temperature.
Results
All barometers exhibited a linear relationship between pressure and altitude (R² ≥ 0.98). One barometer demonstrated significant non-uniform deviation across the altitude range. Using single-point calibration, maximum absolute deviation across the measured data points was ≤0.65%. Multi-point calibration reduced the deviation to ≤0.1% for all barometers. The maximum absolute deviation between corrected and theoretical pressure–altitude gradients was 0.0015 mmHg/ft. Under single-point calibration, the faulty barometer showed PTP differences of 1.5% and 1.3% at extrapolated pressures of 740 and 780 mmHg, respectively, whereas the others showed differences ranging from −0.41% to 0.37%.
Conclusion
Single-altitude barometer calibration may not detect faulty instruments across varying pressures. Multi-point calibration provides slope and intercept values, enabling accurate corrections at other altitudes. Despite the limited altitude range used, this study demonstrates that multi-point calibration can detect anomalies and complement the single-point calibration method.