Underway CO2 System Parameters
The temperature of the water inside the equilibrator was monitored with a Rosemont ultralinear platinum resistance thermometer (PRT). The PRT was calibrated in March 1994, prior to the first leg of the survey, by the SIO Ocean Data Facility (ODF) using standard CTD calibration techniques. Estimated accuracy was ±0.003°C on the ITS90 scale. A secondary check on the accuracy of the equilibrator temperature readings was made by frequently comparing temperature readings from a mercury thermometer, located in the equilibrator, to values recorded from the PRT.
Temperature readings from the Li-Cor detector were not explicitly calibrated for this survey because the final results are only a function of the relative changes in temperature between the standard gases and the sample.
The sensor used to monitor the system pressure (Setra Systems Inc.) was factory-calibrated prior to the survey in August 1994 against NIST-traceable primary standards. Estimated accuracy was ±0.05%.
All system inputs were read into the computer as voltages using a National Instruments Lab- PC+ A/D board. Accuracy of the board's readings was confirmed with a Fluke model 8840A 5½-digit voltmeter prior to the survey. The resolution of the readings was a function of the voltage range being measured, but in all cases was at least an order of magnitude smaller than the estimated precision of the measurement.
Data directly recorded by the underway system were tagged with a time based on the internal clock of the PC running the instrument. This clock was manually reset to Greenwich Mean Time (GMT) at the beginning of each leg. The IMET and navigation data recorded by the ship's system were tagged with GMT recorded from the GPS satellite data. A test of how closely the data were in sync was performed on every leg by examining the time offset between the observation of temperature fronts seen in the IMET sea surface temperature versus the equilibrator temperature. Despite the resetting of the PC clock, the equilibrator temperatures lagged the sea surface temperatures by 3.6 min at the beginning of every leg. This offset most likely represented the real time for the water to travel from the pump to the equilibrator (i.e., the residence time of the water in the ship). The offset generally decreased with time to near zero by the end of the longer cruises. The changing offset was attributed to the notoriously bad clocks used in personal computers, which could easily lose more then one minute per month. Under the assumption that the satellite time was correct, all of the xCO2 data were synchronized to the IMET data before they were merged on the basis of a linear interpolation of the time offsets at the beginning and the end of each leg.
