        CTD SENSOR CALIBRATION CORYSTES 2A/1993 (JONUS EXERCISE 12)        
                                                                           
This  cruise  used the Guildline CTD s/n 45056 at the JONUS  stations.  The
following is a presentation of the data returned from stations 1-103.      
                                                                           
Twenty-six  CTD profiles were obtained using the Guildline CTD  during  the
cruise.  Reversing thermometers were used to measure water temperature  and
samples were collected for salinity analysis.                              
                                                                           
                                                                           
(1) Thermometer data                                                       
Left and right thermometer temperatures were available on 37 occasions. All
thermometers were graduated at 0.02 deg C intervals and a comparison of the
differences is as follows:                                                 
                                                                           
Difference deg C     No.      Accumulative %                               
   0.0 - 0.01         20            54                                     
  0.01 - 0.02         16            97                                     
  0.02 - 0.03          1           100                                     
                                                                           
Thus,  97%  of  the  differences  lie within the  range  0.02  deg  C,  the
thermometer scale gradation.                                               
                                                                           
                                                                           
(2) Sample salinity data                                                   
Duplicate water samples were collected on 41 occasions for analysis with  a
Guildline  salinometer.  However,  a comparison between  the  water  sample
salinities  and  CTD  salinities showed the water  sample  analyses  to  be
suspect;  therefore  no  water sample salinity values were  used  for  this
cruise.                                                                    
                                                                           
                                                                           
(3) Sensor Calibration for Guildline CTD                                   
                                                                           
(a) Pressure                                                               
The laboratory calibration of March 1992,  sensor at 6 deg C,  was used  to
correct the Guildline CTD pressure sensor:                                 
                                                                           
P(cor) = P(ctd) - 1.6 db                                                   
                                                                           
(b) Temperature                                                            
Fig. 1 shows the differences between the (mean) thermometer and uncorrected
CTD  temperature when the left/right thermometer difference was  less  than
0.03 deg C (39 occasions). Fig. 2 is similar to Fig. 1 but omitting station
84,  which  only had one thermometer reading and this was  suspicious.  The
mean difference calculated for the 38 instances remaining was .013.        
                                                                           
This  compares with the laboratory pre-cruise calibration of  Feb/March  93
which  estimated  the CTD correction to be 0.007 deg C at the  temperatures
encountered during the cruise. The sensor was therefore corrected using the
laboratory pre-cruise calibration coefficients:                            
                                                                           
T(cor) = T(ctd) + dT                                                       
    dT = a*T(ctd)*T(ctd) + b*T(ctd) + c                                    
                                                                           
where a = 1.21101e-5                                                       
      b = 5.92251e-4                                                       
      c = 3.1834419e-3                                                     
                                                                           
Fig. 3 shows the differences between thermometer and CTD temperatures after
the latter have been corrected.  The mean difference is 0.008 deg C and all
except 2 differences lie within 0.03 deg C,  supporting the belief that  if
the  thermometers are accurate to 0.02 deg C then the CTD temperatures  are
accurate to 0.01 deg C.                                                    
                                                                           
(c) Salinity                                                               
The  water sample salinity results were suspect for this cruise;  therefore
the  CTD  salinities  were  corrected using  the  calibration  coefficients
derived for cruise Corystes 2C/93.                                         
                                                                           
Plots of the suspect water sample data are shown on Figs. 4 and 5.         
                                                                           
CR(cor) = CR(ctd)*{a*T(cor) + b*P(cor) + c}                                
                                                                           
where	T(cor) and P(cor) are the corrected CTD temperature and pressure     
and   a = 4.02815256e-4                                                    
      b = -1.09064574e-6                                                   
      c = 0.997198294                                                      
                                                                           
                                                                           
(4) Transmission/suspended load                                            
A  25  cm  patch length transmissometer (s/n 198) was  fitted  to  the  CTD
rosette  and calibrated in terms of suspended load (mg/l) by comparing  the
logged %transmission with laboratory determinations of suspended load.     
                                                                           
suspended load = a*loge(%Trans) + b                                        
                                                                           
where a = -12.5                                                            
      b = 47.48                                                            
                                                                           
                                                                           
(5) Chelsea Instrument Salinity                                            
Salinity readings were taken with the Chelsea Instrument,  on 13 of the CTD
stations,  for  comparison  with  corrected  CTD  salinities.  One  Chelsea
salinity value, for station 6, was suspect and ignored. The other 12 values
were  plotted,  as shown on Fig.  6,  showing the differences  between  the
Chelsea and CTDcorr. salinities, suggesting a mean difference of:          
                                                                           
S = Schelsea + 0.09            (range of differences .05 to .14)           
                                                                           
                                                                           
Sue Norris                                                                 
9 June 1993                                                                
