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

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 lab 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 at the time that 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 Paroscientific Model 760 pressure gauge equipped with a 10,000-PSI transducer. This instrument was purchased in March 1988, and was originally calibrated against a primary standard. Subsequent recalibrations have been performed every 2.5 years on average either at the Northwest Regional Calibration Center, at the Scripps Institute of Oceanography or at Fluke Electronics (DH Instruments Division). The latest calibrations were conducted at the Scripps Institute of Oceanography in April 1999, May 2001, May 2003, and July 2005; and at Fluke in July 2009 and November 2012. The standard stopped working soon after the January 2014 bench tests, and was replaced by a new Digiquartz portable standard from Paroscientific SN 136923 in May 2016. There was no other standard available to conduct bench tests during the February 2014 to April 2016 period.

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 tests were not conducted between February 2014 and April 2016 because the transfer standard malfunctioned and could not be repaired.

Temperature

Five Sea-Bird SBE-3-Plus temperature transducers #1416, #5519, #4448, #5554, and #5443 were used during 2015 HOT and WHOTS cruises. These and our other two transducers #2454 and #2907 were calibrated at Sea-Bird.

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., 2016, 2015, 2014, 2013, 2012, 2011, 2010, 2008, 2007, 2006, 2005, 2004, 2002, Santiago-Mandujano et al., 2000, Tupas et al., 1993, 1994a, 1995, 1997, 1998, 1999, Karl et al. 1996). Sensors #5543 and #5554 belong to the UH Ocean Technical Group (OTG), and were used together with their CTD during HOT-270 after our primary and secondary CTDs failed. Calibration coefficients obtained at Sea-Bird for these sensors after 2014 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 2015 was less than 1.6 x 10-3 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 2015 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 2015 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

Six conductivity sensors were used during the 2015 cruises, #3162, #2218, #2959, #3984, #4074 and #3876. Sensor #2959 was sent to Sea-Bird for evaluation in November 2015 after failing during the first deep cast of HOT-277. The instrument's cell was found cracked by the center electrode on the cell, and the cell was replaced. The history of the sensors is well documented in previous HOT Data Reports (Fujieki et al., 2016, 2015, 2014, 2013, 2012, 2011, 2010, 2008, 2007, 2006, 2005, 2004, 2002, Santiago-Mandujano et al., 2000, Tupas et al., 1993, 1994a, 1995, 1997, 1998, 1999, Karl et al. 1996). Sensors #4074 and #3876 belong to the OTG, and were used together with their CTD during HOT-270 after our primary and secondary CTDs failed. Dual sensors were used during each of the 2015 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. Prior to 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-269 through -279, the standard deviation cutoff values for screening of bottle salinity samples were: 0.0034 (0-150 dbar), 0.0050 (151-500 dbar), 0.0019 (501- 1050 dbar), and 0.0010 (1051-5000 dbar).

A least squares fit (ΔC = b0 + b1C + b2C2) to the CTD-bottle conductivity differences was used. None of the 2015 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 as 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 2015 cruises six Sea-Bird SBE-43 oxygen sensors were used: #43262, #43982, #43918, #431601, #43019 and #43224. The history of these sensors is documented in previous HOT Data Reports (Fujieki et al., 2016, 2015, 2014, 2013, 2012, 2010, 2008, 2007, 2006, 2005).

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 consists of fitting 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 together 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-12 which used the same sensors used during HOT-274, therefore HOT-274 coefficients were used to calibrate WHOTS- 12 oxygen data.

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