RSKderiveA0A uses the A-zero-A technique to correct drift in the Digiquartz® pressure gauge(s). This is done by periodically switching the applied pressure that the gauge measures from seawater to the atmospheric conditions inside the housing. The drift in quartz sensors is proportional to the full-scale rating, so a reference barometer – with hundreds of times less drift than the marine gauge – is used to determine the behaviour of the marine pressure measurements.
The A-zero-A technique, as implemented in RSKderiveA0A, works as follows. The barometer pressure and the Digiquartz® pressure(s) are averaged over the last 30 s of each internal pressure calibration cycle. Using the final 30 s ensures that the transient portion observed after the valve switches is not included in the drift calculation. The averaged Digiquartz® pressure(s) are subtracted from the averaged barometer pressure, and these values are linearly interpolated onto the original timestamps to form the pressure correction. The drift-corrected pressure is the sum of the measured Digiquartz® pressure plus the drift correction.