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Sensor Correction & Calibration
PressureThe 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. TemperatureFour Sea-Bird SBE-3-Plus temperature transducers #2700, #2454, #2454, and #4448 were used in 2005 and were calibrated at Sea-Bird. Sensors #2242, and #2700 were not calibrated after October 2005 because they were lost at sea with the CTD during HOT-175 (see Chief scientist report). SBE-3-02/F transducer #1489 belongs to the UH Ocean Technology Group, and it was used in some of the casts during HOT-175. Sensor #4448 is a new sensor acquired in November 2005 and used during HOT-176. SBE-3-02/F sensor #1416 was a backup sensor. 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 2004 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 2005 was less than 0.2 x 10-3 Deg C for the data collected with these sensors, the only exception was sensor #1489 used in some casts during HOT-175, for these casts the temperature drift correction was 0.9 x 10-3 C. 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 2005 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 2005 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 2005 cruises #2725, #2541, #2218, #1178, and #3162. Sensors #2725 and #2541 were lost at sea when the CTD wire broke during HOT-175. Sensor #1178 belongs to the UH Ocean Technology Group, and it was used in some casts during HOT-175. Sensor #3162 is a new sensor acquired in November 2005 and used during HOT-176. 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 2005 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-167 through -176, the standard deviation cutoff values for screening of bottle salinity samples were: 0.0036 (0-150 dbar), 0.0049 (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. One cruise during 2005 required a quadratic calibration (HOT-175). 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. OxygenDuring the 2005 cruises our three Sea-Bird SBE-43 oxygen sensors were used: #43134, #43262, and #43325, in addition sensor #43019 belonging to the UH Ocean Technology Group was also used during HOT-175. Sensors #43134 and #43325 were lost at sea when the CTD wire broke during HOT-175. A new sensor #43918 was acquired in November, 2005 and used during HOT-176. 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 August 2005 and was repaired. Sensor s #43134 membrane was also found punctured in an October 2005 calibration. 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. | ||||
