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Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawai'i at Manoa |
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PressureThe pressure calibration strategy employed a high-quality quartz pressure transducer as a transfer standard. Periodic recalibrations of this laboratory standard were performed with a primary pressure standard. The transfer standard was used to check the CTD pressure transducers. The corrections applied to the CTD pressures included a constant offset determined when the CTD first enters the water on each cast, and a pressure-dependent offset, obtained from semi-annual bench tests between the CTD sensor and the transfer standard. The transfer standard is a Digiquartz portable standard Paroscientific SN 136923 pressure gauge equipped with a 10,000-PSI transducer. This instrument was purchased in May 2016 and was initially calibrated against a primary standard. A subsequent recalibration was performed in May 2020 at Fluke. CTD pressure transducer bench tests were done using an Ametek T-100 pump and a manifold to apply pressure simultaneously to the CTD pressure transducer and the transfer standard. All these tests had points at six pressure levels between 0 and 4500 dbar, increasing and decreasing pressures. Pressure sensor #75434This CTD was serviced and recalibrated at SeaBird in May 2021 due to damages when the winch wire parted, and the CTD fell and hit the ship's deck during the March 2021 HOT-328 cruise. The September 2021 test showed that the sensor’s characteristics changed after this incident A correction of 0.137 dbar (from the May 2021 calibration) was applied to the pressure offset at 0 dbar during data collection for casts conducted with sensor #75434 during the HOT-335 through HOT-339 cruises. However, a more accurate offset was later determined when the CTD first enters the water on each cast. On-deck CTD pressures are regularly recorded during cruises at the beginning, and the end of each CTD cast. The 0-dbar pressure for sensor #75434 was near constant during 2021 through 2023. These pressures are smaller than the before-cast on-deck pressure because during bench tests the CTD is powered on at least 12 hours before testing to allow the pressure sensor to stabilize, while during cruises, the CTD is powered on only about 15 minutes before each cast. The bench tests show a slow sensor stabilization account for the observed differences. The 0-4500 dbar pressure offset and hysteresis from the bench tests have been near-constant and within expected values, although the first has shown a slight regular increase. No linear pressure-dependent offset was applied during data collection for sensor #75434 to correct the 0-4500 dbar span offset. Pressure sensor #53702Sensor #53702 (CTD #1487) was acquired in 2022. It was factory-calibrated on January 5, 2022, and bench tested once in May 2022. The test showed a large 0-dbar offset. A correction of -0.98 dbar (from its original sensor calibration) was applied to the pressure offset at 0 dbar during data collection for casts conducted with this sensor during the WHOTS-18 cruise. However, a more accurate offset was later determined when the CTD first entered the water on each cast. A linear pressure-dependent offset was applied during data collection for this sensor to correct the 0-4500 dbar span offset. The hysteresis from the bench test was within expected values. TemperatureFour Sea-Bird SBE-3-Plus temperature transducers, #4448, #5519, #6672, and #6631, were used in 2022. The history of the sensors, as well as the procedures followed to obtain the sensor drift from the Sea-Bird calibrations, are well-documented in previous HOT Data Reports (Fujieki et al., 2023, 2021, 2020, 2019, 2018, 2017, 2016, 2015, 2014, 2013, 2012, 2011, 2010, 2008, 2007, 2006, 2005, 2004, 2002, Santiago-Mandujano et al., 2000, Tupas et al., 1993, 1994, 1995, 1997, 1998, 1999, Karl et al. 1996). Sensors #6672 and #6631 are new sensors acquired in February 2022. Calibration coefficients obtained at Sea-Bird and used in the drift estimates were used in the following formula that gives the temperature (in Deg C) as a function of the frequency signal (f):
temperature = 1/{a + b[ln(fo/f)] + c[ln2(fo/f)] +
d[ln3(fo/f)]} - 273.15
For each sensor, the final calibration consists of two parts: first, a single "baseline" calibration is chosen from among the ensemble of calibrations during the year; second, for each cruise a temperature-independent offset is applied to remove the temporal trend due to sensor drift. The offset, a linear function of time, is calculated by least-squares fit to the 0-30 Deg C average of each calibration during the year. The maximum drift correction in 2022 was less than 2.0 x 10-4 Deg C for the data collected with these sensors. The baseline calibration is selected for which the trend-corrected average from 0-5 Deg C is nearest to the ensemble mean of these averages. A small residual pressure effect on the temperature sensors documented in Tupas et al. (1997) has been removed from measurments obtained with our sensors. Another correction to our temperature measurements was for the viscous heating of the sensor tip due to the water flow. This correction is thoroughly documented in Tupas et al. (1997). Dual sensors were used during each of the 2022 cruises. The temperature differences between sensor pairs were calculated for each cast to evaluate the quality of the data, and to identify possible problems with the sensors. Means and standard deviations of the differences in 2-dbar bins were calculated from the ensemble of all casts at Station ALOHA for each cruise. Both sensors performed correctly during the 2022 cruises, showing temperature differences within expected values. The mean temperature difference as a function of pressure was typically less than 1 x 10-3 Deg C, with a standard deviation of less than 0.5 x 10-3 Deg C below 500 dbar. The largest variability was observed in the thermocline, with standard deviation values up to 5 x 10-3 Deg C. ConductivityFive conductivity sensors were used during the 2022 cruises, #3162, #3984, #4939, #6052, and #6056. The history of our sensors is well documented in previous HOT Data Reports (Fujieki et al., 2023, 2021, 2020, 2019, 2018, 2017, 2016, 2015, 2014, 2013, 2012, 2011, 2010, 2008, 2007, 2006, 2005, 2004, 2002, Santiago-Mandujano et al., 2000, Tupas et al., 1993, 1994, 1995, 1997, 1998, 1999, Karl et al. 1996). Sensors #6052, and #6056 are new sensors acquired in February 2022 and were used during the WHOTS-18 cruise. The water pump for sensor #6052 had problems during this cruise and calibrations against salinity samples could not be conducted. For each sensor, the nominal calibrations were used for data acquisition, and a final calibration was determined empirically from the salinities of discrete water samples acquired during each cast. Before empirical calibration, conductivity was corrected for the thermal inertia of the glass conductivity cell as described in Chiswell et. al. (1990). Procedures for preliminary screening of bottle samples and empirical calibration of the conductivity cell are described in Tupas et al. (1993, 1994a). For cruises HOT-335 through -339, the standard deviation cutoff values for screening of bottle salinity samples were: 0.0034 (0-150 dbar), 0.0049 (151-500 dbar), 0.00185 (501- 1050 dbar), and 0.00092 (1051-5000 dbar). A least squares fit (ΔC = b0 + b1C + b2C2) to the CTD-bottle conductivity differences was used. None of the cruises required a quadratic calibration. The calibrations were best below 500 dbar because the weaker vertical salinity gradients at depth lead to less error when the bottle and CTD pressures are slightly mismatched. The final step of conductivity calibration was a cast-dependent bias correction described in Tupas et. al. (1993) to allow for drift during each cruise or sudden offsets due to fouling. Note that a change of 1 x 10-4 Siemens m-1 in conductivity is approximately equivalent to 0.001 in salinity. Conductivity differences between sensor pairs were calculated the same way for the temperature sensors. The range of variability as a function of pressure was about ± 1 x 10-4 Siemens m-1, with a standard deviation of less than 0.5 x 10-4 Siemens m-1 below 500 dbar, from the ensemble of all the cruise casts. The largest variability was in the halocline, with standard deviations reaching up to 5 x 10-4 Siemens m-1 between 50 and 300 dbar. OxygenDuring the 2022 cruises, our three Sea-Bird SBE-43 oxygen sensors were used: #43262, #433761, and #43918. Two new sensors #434232 and #434246 acquired in February 2022 were used during the WHOTS-18 cruise. Sensor #433761 was found with a punctured membrane during the March 2022 Sea-Bird calibration, and its membrane was replaced. The electrolyte housing of sensor #43262 was found cracked during the March 2022 calibration and it was fixed. The history of our sensors is documented in previous HOT Data Reports (Fujieki et al., 2023, 2021, 2020, 2019, 2018, 2017, 2016, 2015, 2014, 2013, 2012, 2011, 2010, 2008, 2007, 2006, 2005, 2004, 2002, Santiago-Mandujano et al., 2000, Tupas et al., 1993, 1994, 1995, 1997, 1998, 1999, Karl et al. 1996). All these sensors have been calibrated annually at Sea-Bird. Water bottle oxygen data were screened and the oxygen sensors were empirically calibrated following procedures described previously (Winn et. al. 1992; Tupas et. al., 1993). The calibration procedure follows Owens and Millard (1985) and fits a non-linear equation to the CTD oxygen current and oxygen temperature. The bottle values of dissolved oxygen and the downcast CTD observations at the potential density of each bottle trip were grouped for each cruise to find the best set of parameters with a non-linear least squares algorithm. Two sets of parameters were usually obtained per HOT cruise, corresponding to the casts at Station 1 and 2 (calibration coefficients from cast 2 are also used to calibrate the cast at station 6, 50 and 52). The calibration procedure for the Sea-Bird SBE-43 sensors is documented in Santiago-Mandujano et. al. (2001). Dual sensors were used during cruises, but only the sensor whose data were deemed more reliable is reported. | ||||||||
