              CTD CALIBRATION: CRUISE CORYSTES 3B/1992                     
                                                                           
                                                                           
CTD: S/N 45056                                                             
                                                                           
(1) 50 CTD profiles were in the CTD cell and therefore the salinity was U/S
and  set  to  -9.0  in these profiles obtained  during  the  cruise.  Three
stations, 12, 15 and 32,  had the bung.  Reversing thermometers were fitted
to the Niskin used to collect samples from close to the seabed. Samples for
salinity  analysis  were collected from both near the surface and close  to
the seabed.                                                                
Not all the calibration data collected were used to calibrate the CTD      
sensors:                                                                   
   (i)  Thermometer differences too large, >0.03;                          
   (ii) It was clear from the profiles obtained at the time of station that
some  T and/or S values measured by the CTD were variable,  believed  real,
not an instrument malfunction, and unsuitable for calibration use.         
                                                                           
                                                                           
(2)  The pressure sensor was corrected using the laboratory calibration  of
25/03/1992 at T = 6 deg C.                                                 
                                                                           
P(cor) = P(ctd) - 1.6                                                      
                                                                           
A  check of comparing CTD pressure with that derived from water  depth  and
altimeter height showed good agreement.                                    
                                                                           
                                                                           
(3)  The temperature sensor was corrected using the laboratory  calibration
of 24/03/1992.                                                             
                                                                           
T(cor) = T(ctd) + 0.011                                                    
                                                                           
One  temperature  difference   between   thermometer  and  uncorrected  CTD
temperature  was  untypically  high and therefore not  used.  A  comparison
between  thermometer and uncorrected CTD temperatures is shown in  Fig.  1.
The  uncorrected CTD temperature is likely to be in error by 0.01 (from the
laboratory calibration,  suggesting a shift of 9 m deg C since cruise 1/92)
and  removing temperature differences between thermometers and  uncorrected
CTD  of 0.03 deg C is not entirely justified;  0.04 is probably  acceptable
(0.02  from the "uncertainty" of the thermometers,  0.01 from CTD and  0.01
which the laboratory calibration tells us to expect).                      
                                                                           
                                                                           
(4)  40  salinity samples were used to calibrate the  conductivity  sensor.
Seven  pairs  of salinity samples showed a larger salinity value  than  the
rest  of the data and were deleted before obtaining the conductivity  ratio
calibration.  All  the  remaining  calibration samples  have  a  difference
between water sample and corrected CTD salinity of < = 0.01,  which is well
within the permitted difference of < = 0.016. The calibration derived was: 
                                                                           
     CRFACT = a*T(ctd) + b*P(ctd) + c                                      
and                                                                        
     CR(cor) = CR(ctd) * CRFACT * CELLFACTOR                               
                                                                           
where     a = 0.684543828e-03                                              
          b = 0.202726470e-04                                              
          c = 0.995339800e+00                                              
          CELLFACTOR = 1.0                                                 
                                                                           
The  rms  difference  between salinometer and corrected  CTD  salinity  was
0.004.                                                                     
                                                                           
                                                                           
Fig.  2  shows how effective these coefficients are in correcting  the  CTD
salinity estimates for the calibration samples.                            
                                                                           
Fig. 3 displays the spread in the ratio of conductivity-ratio of the CTD to
that of the water samples prior to applying the correction.                
                                                                           
Fig.  4  shows  the  difference  between  water  sample  salinity  and  the
uncorrected CTD equivalent.                                                
                                                                           
Fig. 5 shows the difference between water sample salinity and the corrected
CTD equivalent.                                                            
