                 CTD CALIBRATION FOR CRUISE CIROLANA 6/1988                
                                                                           
Both the pressure and temperature sensors of the CTD were calibrated  using
the  pre-cruise  laboratory  calibration.  The salinity  of  water  samples
collected  at  thirty-one stations was used to calibrate  the  conductivity
sensor.  After  rejecting a few samples because of salinity  differences  >
0.006,  a total of 162 samples (mostly from duplicates) remained with which
to determine the calibration coefficients.                                 
                                                                           
                                                                           
Pressure                                                                   
In  the past,  we have tried to corroborate the laboratory  calibration  by
comparing  CTD pressure with thermometric pressures from the DSRT.  Because
of inaccuracies in the latter, this method is no longer used.  We have also
considered  previously  how the sample level could be determined  from  PDR
water  depth  and altimeter height above the seabed.  It was impossible  to
reconcile these sample levels with the CTD pressure when the instrument was
used in deep water. Consequently, the pressure sensor was calibrated before
and after the cruise with a dead weight tester. Good agreement was obtained
and the pre-cruise calibration was used to correct pressure values.        
                                                                           
                                                                           
Temperature                                                                
Laboratory calibrations conducted before and after the cruise were in  fair
agreement.  They did suggest a small difference (4 to 10 mK) and an attempt
was  made  to detect a change in the sensor response during the  cruise  by
comparing  CTD  temperature  with  those from  DSRTs.  No  evidence  for  a
systematic  change was found,  but the accuracy of our DSRTs would probably
prohibit  the detection of a response change of the magnitude suggested  by
the laboratory calibration.                                                
Temperatures were corrected using the pre-cruise calibration.              
                                                                           
                                                                           
Conductivity Ratio                                                         
The  method adopted was to first calculate the conductivity ratio that  the
water  sample would have,  if given its measured salinity,  at the pressure
and temperature at which it was collected.  The ratio of this  conductivity
to that recorded by the CTD was then fitted by least squares to            
                                                                           
        CR(WS)                                                             
        -----------     = a x T(CTD) + b x P(CTD) + C                      
        CR(CTD)                                                            
                                                                           
where T(CTD) and P(CTD) are the corrected CTD temperature and pressure.    
                                                                           
The  rms  difference  between water sample salinity and  CTD  salinity  was
0.011.                                                                     
                                                                           
Better  agreement  was obtained if,  for each station,  an  estimated  cell
coefficient, based on the mean CR(WS) :CR(CTD) ratio,  was evaluated before
determining the coefficients a, b and c above.  This gave an rms difference
of  0.005.  In  view  of  the  tolerances  in  CTD  and  bench  salinometer
measurements  this  seems  as good agreement that  could  realistically  be
expected.                                                                  
                                                                           
Fig. 1 summarises the effectiveness of this calibration and the recommended
coefficients are indicated in Table 1.                                     
                                                                           
A  feature  of  this  data  was the apparent  change  in  response  of  the
conductivity  sensor  during the first eight stations (numbers 24  to  71).
This  is shown in Fig.  2 which displays the ratio CR(CTD) :CR(WS) for  all
stations  (numbers  24  to 71 and 72 to 103) a closer look  at  the  former
group,  Fig.  3,  suggests that at the first two stations (24 and  39)  the
ratio  is approximately 1.0015 and that it appears to change when the  four
upper  samples  are  taken  at  the  following  station  (42),  the   ratio
approaching  1.0025.  At the next three stations (44,  56 and 68) the ratio
remains near 1.0025 (and in this respect is similar to stations 72  through
to 103).  During stations 70 and 71, however, the ratio decreases to 1.0015
before increasing to 1.0025.                                               
                                                                           
Subsequent  examination of theta-S plots from adjacent  stations  suggested
that,  in order to obtain consistent data,  it was necessary to assume that
the CR(CTD):CR(WS) ratio at station 42 was similar (i.e. approx. 1.0025) to
that  during  stations  44,  56  and 68.  The  theta-S  distributions  also
indicated  that the cell factor derived for station 39 was too large and  a
better alternative was to assign to this station the cell factor calculated
for station 42.                                                            
                                                                           
Ken Medler                                                                 
7 November 1989                                                            
                                                                           
                                                                           
TABLE 1. CALIBRATION COEFFICIENTS FOR CTD DATA                             
CIROLANA 6/1988                                                            
                                                                           
Pressure                                                                   
        P(COR) = P(CTD) + PA*T(COR) + PB*P(CTD) + PC                       
where   T(COR) = corrected CTD temperature                                 
            PA = 6.9646 E-1                                                
            PB = 3.85187 E-3                                               
            PC = -3.74                                                     
                                                                           
Temperature                                                                
    T(COR) = T(CTD) + TA*T(CTD)*T(CTD) + TB*T(CTD) + TC                    
where   TA = -1.70167 E-5                                                  
        TB = -7.79426 E-3                                                  
        TC = -7.3795 E-3                                                   
                                                                           
Conductivity Ratio                                                         
   CR(COR) = CR(CTD) x {CF x (CA x T(COR) + CB x P(COR) + CC)}             
                                                                           
where   CA = 5.579934 E-5                                                  
        CB = -3.253974 E-8                                                 
        CC = 0.9998239                                                     
                                                                           
and CF is the cell factor for each station:                                
                                                                           
Station No.     Cell Factor                                                
                                                                           
        24      0.9980783                                                  
        39      0.9975758                                                  
        42      0.9975758                                                  
        44      0.9973358                                                  
        56      0.9976255                                                  
        68      0.9972953                                                  
        70      0.9975061                                                  
        71      0.9983974                                                  
        72      0.9976852                                                  
        73      0.9974241                                                  
        75      0.9976903                                                  
        76      0.9976554                                                  
        77      0.9976174                                                  
        78      0.9977275                                                  
        79      0.9977299                                                  
        80      0.9977350                                                  
        81      0.9976013                                                  
        82      0.9977266                                                  
        83      0.9975244                                                  
        84      0.9976596                                                  
        85      0.9976857                                                  
        86      0.9977118                                                  
        87      0.9975132                                                  
        88      0.9975976                                                  
        89      0.9976082                                                  
        90      0.9975300                                                  
        92      0.9976696                                                  
        94      0.9976939                                                  
        96      0.9977111                                                  
        99      0.9975425                                                  
       102      0.9974842                                                  
       103      0.9978446                                                  
