*** Documentation for CTD data collected on RRS Discovery Cruise 102 (May -                                                         
Jul  1979) by the Institute of Oceanographic Sciences  (Deacon  Laboratory)                                                         
Godalming, Surrey, UK., under the direction of P. M. Saunders. ***                                                                  
                                                                                                                                    
The instrument used was a Neil Brown Instrument Systems CTD, which measured                                                         
pressure, temperature and conductivity.  The CTD was used alongside a 10kHz                                                         
pinger. Lowering and retrieval rates of 0.5 to 1.0m/s were employed and the                                                         
sensors were flushed with distilled water on recovery.  Bottle samples  and                                                         
reversing  thermometer measurements were made on descent and the sea  water                                                         
samples  were  analysed using a Guildline  Autolab  Salinometer.  Reversing                                                         
thermometers were calibrated before and after the cruise.                                                                           
                                                                                                                                    
CTD  unit 1 (used on Leg 1 of the cruise) was interfaced with an  IOS-built                                                         
vector averaging current meter which prevented use of the multisampler.  As                                                         
a  consequence there are relatively few intercomparisons between  reversing                                                         
thermometers  and  the  CTD,  and sample  salinity  comparisons  were  made                                                         
indirectly.                                                                                                                         
                                                                                                                                    
On  Leg  2 of the cruise a different CTD instrument was  used;  this  model                                                         
incorporated  a Beckman polargraphic dissolved oxygen sensor and a  General                                                         
Oceanics Rosette multisampler with 12 bottles was used.                                                                             
                                                                                                                                    
CALIBRATION AND DATA QUALITY - CRUISE 102 (LEG 1)                                                                                   
                                                                                                                                    
PRESSURE: In situ calibration of the pressure sensor was necessary  because                                                         
of  the  utilisation of an uncalibrated electronics board.  26  comparisons                                                         
with unprotected thermometers mounted on NIO bottles clamped to the wire 1m                                                         
above  the  CTD  unit gave a pressure calibration  coefficient  of  0.09355                                                         
(nominal  value  0.1).  The table below shows the differences in  pressures                                                         
determined from pairs of reversing thermometers (protected and unprotected)                                                         
and simultaneous observations of CTD pressure.                                                                                      
                                                                                                                                    
TEMPERATURE:    Comparing   temperature   difference   between    reversing                                                         
thermometers  (at both the top and bottom of the cast) and the  uncorrected                                                         
CTD,   values  revealed  a  difference  of  0.025  degC   increasing   with                                                         
temperature.  Corrected  CTD  values were obtained using the equation  T  =                                                         
0.025  +  0.0005015 * RAWTEMP.  A comparison is given in  the  table  below                                                         
between the CTD temperatures and reversing thermometers.                                                                            
                                                                                                                                    
SALINITY:  Comparisons  made between nominal salinities from  the  CTD  and                                                         
sample  salinities at the same potential temperature but made on a separate                                                         
cast, revealed considerable drift in the CTD conductivity cell calibration.                                                         
The cell factor varied from 0.0009909 to 0.0009925 during the course of the                                                         
cruise and large pressure and temperature dependence was found.  The  drift                                                         
in the calibration corresponded to a salinity change of nearly 0.1ppt.  The                                                         
table  below compares salinities derived from the NIO bottles with the  CTD                                                         
values.                                                                                                                             
                                                                                                                                    
                                                                                                                                    
  Fit of CTD to NIO bottle data                                                                                                     
                                                                                                                                    
               Reversing Thermometers/Sample minus corrected CTD value                                                              
                                                                                                                                    
   Variable        range       mean difference     standard     no. of                                                              
                                                   deviation    values                                                              
                                                                                                                                    
   Pressure,db     700-2000db       0               5             26                                                                
                                                                                                                                    
   Temperature,    2 - 5 degC      +0.003           0.005         17                                                                
          degC     5 -15 degC      -0.008           0.010          8                                                                
                  15 -30 degC      +0.002           0.015         17                                                                
                                                                                                                                    
   Salinity (ppt) at temperatures,                                                                                                  
                   2 - 5 degC      +0.0005          0.004         38                                                                
                   5 -15 degC      -0.003           0.012        179                                                                
                  15 -30 degC      -0.010           0.021         51                                                                
                                                                                                                                    
Oxygen was not measured with the CTD on this leg.                                                                                   
Approximate conversion of temperature to pressure is as follows:                                                                    
                      5 degC  1500 +/-100 db                                                                                        
                     15 degC   175 +/-25  db                                                                                        
                                                                                                                                    
                                                                                                                                    
CALIBRATION  AND DATA QUALITY - CRUISE 102 (LEG 2)                                                                                  
                                                                                                                                    
PRESSURE:  No change was made to the laboratory calibration of  the  sensor                                                         
(made  with  a dead weight tester) except for the offset of 29 +/-1db.  The                                                         
table  below  shows  differences  in pressures  determined  from  pairs  of                                                         
reversing  thermometers  (protected   and   unprotected)  and  simultaneous                                                         
observations of CTD pressure.                                                                                                       
                                                                                                                                    
TEMPERATURE:    Comparing   temperature   difference   between    reversing                                                         
thermometers and the uncorrected CTD value revealed an offset of 0.065 degC                                                         
increasing  slightly  with temperature.  The instrument was  calibrated  in                                                         
November  1978.  Corrected CTD values were obtained using the equation T  =                                                         
0.063  +  0.0005005  * RAWTEMP.  A comparison is given in the  table  below                                                         
between the CTD temperatures and reversing thermometers.                                                                            
                                                                                                                                    
SALINITY:  From  on board determinations of salinity  using  the  Guildline                                                         
Autolab  Salinometer  sample conductivities at corrected CTD  pressure  and                                                         
temperature  (Cs) were calculated and compared with in situ  conductivities                                                         
(C).  Using  Cs/C  =  CCR(1+aT+bp)  a least squares  fit  of  297  sets  of                                                         
observations gave CCR = 0.000999737, a = 0.952E-05 per degC,  b = 0.809E-07                                                         
per  db.  The table below compares the salinities derived from the  rosette                                                         
sampler with the CTD values - the conductivity cell was very stable on this                                                         
leg.                                                                                                                                
                                                                                                                                    
OXYGEN:  From  a sample value of oxygen concentration  (determined  by  the                                                         
Winkler  method)  the  fractional   saturation   content  was  obtained  by                                                         
normalising it by the appropriate saturation value. The Beckman oxygen cell                                                         
also measures fractional saturation given by the equation:                                                                          
                                                                                                                                    
      ln F = ln I + ln C + aT + bp                                                                                                  
                                                                                                                                    
where I is the oxygen sensor raw current                                                                                            
      T is a weighted temperature - (0.25 x T(ambient) + 0.75 x T(oxygen))                                                          
      p is pressure                                                                                                                 
                                                                                                                                    
From the 297 sets of data on leg 2 of this cruise a least squares fit  gave                                                         
C = 1.43E-03,  a = -0.0323 per degC,  b = 1.263E-04 per db.  The comparison                                                         
between  sample oxygen concentrations and CTD oxygen values is shown in the                                                         
table below.                                                                                                                        
                                                                                                                                    
Fit of CTD to Rosette Sample data                                                                                                   
                                                                                                                                    
               Reversing Thermometers/Sample minus corrected CTD value                                                              
                                                                                                                                    
   Variable         range      mean difference     standard     no. of                                                              
                                                   deviation    values                                                              
                                                                                                                                    
   Pressure, db    300-2000db       2               3             63                                                                
                                                                                                                                    
   Temperature,    2 - 5 degC      -0.002           0.005         50                                                                
          degC     5 -15 degC      -0.004           0.007         65                                                                
                  15 -30 degC      +0.001           0.012         48                                                                
                                                                                                                                    
   Salinity (ppt) at temperatures,                                                                                                  
                   2 -15 degC       0               0.003        311                                                                
                  15 -30 degC       0               0.007        107                                                                
                                                                                                                                    
   Oxygen (ml/l) at temperatures,                                                                                                   
                   2 -15 degC      -0.05            0.22         216                                                                
                  15 -30 degC       0.13            0.39          81                                                                
                                                                                                                                    
Approximate conversion of temperature to pressure is as follows:                                                                    
        5 degC   1500 +/-100 db         15 degC    175 +/-25 db                                                                     
                                                                                                                                    
                                                                                                                                    
DATA PROCESSING - LEGS 1 AND 2                                                                                                      
                                                                                                                                    
Original  values  were  averaged  over   an  interval  of  one  second  and                                                         
calibration  coefficients and correction factors applied.  A time  constant                                                         
correction algorithm was employed to compensate for the slower response  of                                                         
the platinum resistance thermometer.  Differences between successive values                                                         
of  each  parameter  were examined;  the mean difference and  its  standard                                                         
deviation  calculated and values greater than several  standard  deviations                                                         
from  the  mean difference were checked.  Genuinely suspect data were  then                                                         
replaced  by  interpolated values.  Derived quantities were  computed  from                                                         
algorithms  published  by Fofonoff et al (1974).  To remove the  effect  of                                                         
ships  heave  data cycles were sorted by pressure before  all  values  were                                                         
averaged at 2db intervals, centred on 1db, 3db...                                                                                   
                                                                                                                                    
References:                                                                                                                         
                                                                                                                                    
Saunders, P.M. (1980). CTD data from the western equatorial Indian Ocean 10                                                         
   May  -  6  July 1979: Discovery Cruise 102.  IOS  Data  Report  No.  23.                                                         
   Unpublished manuscript.                                                                                                          
                                                                                                                                    
Fofonoff, N.P., Hayes, S.P.  and Millard Jr., R.C.  (1974).  WHOI/Brown CTD                                                         
   microprofiler:  methods  of calibration and data  handling.  Woods  Hole                                                         
   Oceanographic Institution WHOI-74-89.                                                                                            
