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©).

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 or at the Scripps Institute of Oceanography. The latest calibrations were conducted at the Scripps Institute of Oceanography in April 1999, May 2001, May 2003, and July 2005.

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.

Two Sea-Bird SBE-3-Plus temperature transducers #2454, and #4448 were used in 2006 and were calibrated at Sea-Bird. New sensors #4448 and #2907 were acquired in December 2005 and February 2006 respectively to replace sensors #2700, and #2242 which were lost with the CTD during HOT-175 (see Chief scientist report). Sensors #2907 and SBE-3-02/F #1416 were not used during 2006. 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 Santiago-Mandujano et al. (2002, 2001, 1999), Tupas et al. (1993, 1994a, 1995, 1997, 1998) and Karl et al. (1996). Calibration coefficients obtained at Sea-Bird for these sensors after 2005 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):

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 2006 was less than 1.2 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 2006 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 2006 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.

Three conductivity sensors were used during the 2006 cruises #2218, #3162, and #2959. Sensor #2959 was acquired in February 2006 to replace sensors #2725 and #2541, lost at sea during HOT-175 (see Chief scientist report). The history of the sensors is well documented in Santiago-Mandujano et al. (2002, 2001, 1999), Tupas et al. (1993, 1994a, 1995, 1997, 1998) and Karl et al. (1996). Dual sensors were used during each of the 2006 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-177 through -188, the standard deviation cutoff values for screening of bottle salinity samples were: 0.0035 (0-150 dbar), 0.0048 (151-500 dbar), 0.0020 (501- 1050 dbar), and 0.0011 (1051-5000 dbar).

A least squares fit (ΔC = b₀ + b₁C + b₂C²) to the CTD-bottle conductivity differences was used. Two cruises during 2006 required a quadratic calibration (HOT-177 and 188). 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.

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.

During the 2006 cruises three Sea-Bird SBE-43 oxygen sensors were used: #43262, #43918 and #43982. Sensor #43982 is a new sensor acquired in March 2006 to replace sensors #43134 and #43325 lost at sea during HOT-175 (see Chief scientist report). The history of these sensors is documented in Santiago-Mandujano et al. (2002, 2001, 1999). Sensor's #43262 membrane was found punctured during a routine calibration at Sea-Bird in November 2006 and was repaired. Sensor's #43982 membrane was also found punctured during a routine calibration in July 2006. These sensors had their lid and membrane assembly replaced at Sea-Bird, as well as their electrolyte reservoir re-backfilled.

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 51). 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.