                    CTD CALIBRATION: CORYSTES 11/1992                      
                                                                           
CTD: S/N 45056                                                             
                                                                           
(1)  A  total  of  68 valid profiles  were  collected  during  the  cruise.
Reversing  thermometers  were  used  on only the first 20  (stns  10-  38).
Samples for analysis were collected from the near-surface and near-bed. Not
all the calibration data collected were used to calibrate the CTD:         
   (i)   Thermometer differences > 0.04 were discounted (4 deleted).       
   (ii)  Salinity and CTD differences > 0.03 were discounted (3 deleted).  
   (iii) The first station (10) was at anchor off Killingholme, River      
Humber.  One set of thermometers failed and one set of duplicate salinities
was  in  very poor agreement.  This station was discarded  for  calibration
purposes.                                                                  
                                                                           
                                                                           
(2)  The pressure sensor was corrected using the laboratory calibration  of
August 1992 at T = 16.2 deg C.                                             
                                                                           
P(cor) = P(ctd) - 1.4                                                      
                                                                           
                                                                           
(3)  The temperature sensor was corrected using the laboratory  calibration
of August 1992.                                                            
                                                                           
 T(cor) = T(ctd) + dT                                                      
                                                                           
     dT = a*T(ctd)*T(ctd) + b*T(ctd) + c                                   
                                                                           
where a = -1.18042e-05                                                     
      b =  3.74e-04                                                        
      c =  1.0502e-02                                                      
                                                                           
This amounts to 13 mK for the range of temperatures encountered 16 deg C to
11 deg C.                                                                  
                                                                           
A comparison between thermometer and uncorrected CTD temperatures is  shown
in Fig. 1.  If it is assumed that the thermometer temperatures are accurate
to 0.02 deg C and the CTD to 0.01 deg C then all differences to within 0.03
deg C of the Laboratory calibration are acceptable i.e.  those within 43 mK
and - 17 mK. All of the 37 values were found to inside this range.         
                                                                           
The mean of the 37 differences was 20 mK.                                  
                                                                           
                                                                           
(4) Fig.  2 shows how the uncorrected CTD salinity estimates compared  with
salinity measurements from the salinometer.  (Note that CTD measurements of
pressure,  temperature  and conductivity have not been corrected;  the  CTD
salinity is derived from uncorrected observations). The salinity sensor was
calibrated using the equation:                                             
                                                                           
    CR(cor) = CR(ctd)x[a x T(cor) + b x P(cor) + c]                        
                                                                           
where     a = -0.920897052e-4                                              
          b = -0.227664036e-5                                              
          c =  0.100148999e+1                                              
                                                                           
The  rms difference between salinometer and CTD salinity was 0.011 for  117
observations.                                                              
                                                                           
Fig. 3 shows the ratio of the conductivities of CTD cf water samples before
correction of CTD conductivity ratio.                                      
                                                                           
Fig. 4 shows the differences between water sample salinity and CTD salinity
after all the sensors have been calibrated.                                
                                                                           
Fig.  5  shows the difference between water sample  (salinometer)  and  CTD
salinity  before  and after the calibration of the conductivity sensor  has
been applied.  Note that for the upper histogram the CTD salinity has  been
derived  from  corrected  CTD  pressure  and  temperature  but  uncorrected
conductivity  ratio.  This  figure therefore indicates  how  effective  the
conductivity  calibration has been.  If it is assumed that the CTD salinity
is  accurate  to  0.01  and the salinometer  is  accurate  to  0.006,  then
differences less than or equal to 0.016 are acceptable.  Only 14 of 117 are
outside this range after the sensor calibrations have been applied.        
                                                                           
                                                                           
(5) There were two transmissometers used during this cruise,  the 5 cm  and
25 cm. The readings were converted using the following calibrations:       
                                                                           
Stns 10-92 (5 cm)                                                          
     SUSP.LOAD = -53.5 x LOGe %TRANSMISSION + 209.5                        
                                                                           
Stns 93-194 (25 cm)                                                        
     SUSP.LOAD = -3.77 x LOGe %TRANSMISSION + 16.41                        
                                                                           
                                                                           
(6) Fig. 6 gives the general distribution of the stations on this cruise:  
                                                                           
10           Killingholme, River Humber                                    
                                                                           
11-2, 54     River Humber to The Wash                                      
                                                                           
32-52        The Wash                                                      
                                                                           
56-112       North Sea                                                     
                                                                           
118-194      Coastal waters between River Tees and north of Aberdeen       
