Highly accurate measurement of oxygen using a paramagnetic gas sensor

The amount fraction of oxygen in oxygen–nitrogen gas mixtures has been measured with a relative uncertainty of ±0.02% within the range 100–1000 mmol mol−1. The method employed uses an auto-null 'dumbbell' type paramagnetic oxygen sensor, which has high resolution (10 µmol mol−1) and excellent linearity. The sensor is used to compare the sample gas mixture with pure oxygen and nitrogen gases, which compensates for drift and pressure change, and avoids the uncertainty from using bottled calibration gas mixtures. The method was verified by measuring primary standard gas mixtures at the National Physical Laboratory. To the authors knowledge the uncertainty achieved is better than any other analytical instrument, within the specified range.