Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawai'i at Manoa

Sensor Correction & Calibration

SBE 3plus

Sea-Bird Electronics

FTP View Data
To assist in the interpretation of the data, it can be displayed using the Hawaii Ocean Time-series Data Organization & Graphical System (HOT-DOGS©).

Pressure

The 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 originally calibrated against a primary standard. Subsequent recalibration was performed in May 2020 at Fluke. Calibrations before 2016 were conducted with a Paroscientific Model 760 pressure gauge which was in service between 1988 and 2014 (Fujieki et al., 2020).

CTD pressure transducer bench tests were done using an Ametek T-100 pump and a manifold to apply pressure simultaniously to the CTD pressure transducer and to the transfer standard. All these tests had points at six pressure levels between 0 and 4500 dbar, increasing and decreasing pressures. Pressure sensor #75434 (CTD #91361) failed and displayed bad data during one of the bench tests, and the sensor’s card was replaced at Sea-Bird in April 2016, modifying the sensor’s characteristic slope and offset.

A correction of 0.861 dbar was applied to the pressure offset at 0 dbar during data collection for casts conducted with sensor #75434. 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 2019 and decreased slightly between January 2019 and the August 2019 calibration. These pressures are smaller than the maximum difference between before-cast and after-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 that a slow sensor stabilization accounts for the observed differences.

The 0-4500 dbar pressure offset and hysteresis from the bench tests have been near-constant and within expected values. A linear pressure-dependent offset was applied during data collection for sensor #75434 to correct the 0-4500 dbar span offset of about 0.27 dbar from the September 2017 bench test.

Temperature

Five Sea-Bird SBE-3-Plus temperature transducers #1416, #2454, #2907, #4448, #5519 were used during the 2019 HOT and WHOTS cruises. 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., 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). Calibration coefficients obtained at Sea-Bird for these sensors after 2018 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 2019 was less than 1.1 x 10-4 Deg C for the data collected with these sensors. The baseline calibration is selected as the one 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 2019 cruises. The temperature differences between sensor pairs were calculated for each cast to evaluate the data's quality and 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 2019 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.

Conductivity

Four conductivity sensors were used during the 2019 cruises, including #2218, #2959, #3984, and #4687. Sensor #2218 was sent to Sea-Bird for evaluation in May 2019 after showing large differences against its sensor pair during the deep casts of HOT-309, but nothing wrong was found with the sensor. The sensor was used during HOT-313 and again showed large differences against its sensor pair. This sensor will be retired. Sensor #4687 showed an offset against its sensor pair during HOT-316 and it was sent to Sea-Bird for evaluation in December 2019, but nothing wrong was found with the sensor. The history of our sensors is well documented in previous HOT Data Reports (Fujieki et al., 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). Dual sensors were used during each of the 2019 cruises.

For each sensor, the nominal calibrations were used for data acquisition, and a final calibration was determined empirically from salinities of discrete water samples acquired during each cast. Before empirical calibration, conductivity was corrected for 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-309 through -317, the standard deviation cutoff values for screening of bottle salinity samples were: 0.0034 (0-150 dbar), 0.0049 (151-500 dbar), 0.0019 (501- 1050 dbar), and 0.0009 (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 for 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 as 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.

Oxygen

During the 2019 cruises, our four Sea-Bird SBE-43 oxygen sensors were used: #43262, #431601, #43918, #43982, and a new sensor #433761 acquired in November 2018. The history of our sensors is documented in previous HOT Data Reports (Fujieki et al., 2019, 2018, 2017, 2016, 2015, 2014, 2013, 2012, 2011, 2010, 2008, 2007, 2006, 2005). All these sensors have been calibrated annually at Sea-Bird, and did not show any problems during the 2019 cruises.

Water bottle oxygen data were screened and the oxygen sensors were empirically calibrated following procedures described previously (Winn et. al. 1991; 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). No oxygen samples were collected during cruise WHOTS-16 which used the same sensors used during HOT-316, therefore, coefficients from this cruise were used to calibrate WHOTS- 16 CTD oxygen data.

Dual sensors were used during cruises, but only the sensor whose data were deemed more reliable is reported.