       CTD SENSOR CALIBRATION CORYSTES 13/1993 (JONUS EXERCISE 21)         
                                                                           
This cruise used the FSI CTD s/n 1322 at the JONUS stations.  The following
is a presentation of the data returned from stations 45-160.               
40 CTD profiles were obtained using the FSI CTD during the cruise.         
Reversing  thermometers were used to measure water temperature and  samples
were collected for salinity analysis.                                      
                                                                           
                                                                           
Thermometer data                                                           
Left and right thermometer temperatures were available on 37 occasions.  10
pairs  of thermometer results had a difference of > .04 and,  of  these,  1
pair  was not used.  Of the remaining 9,  either left or right  thermometer
only was used.                                                             
                                                                           
All thermometers were graduated at 0.02 deg C intervals and a comparison of
the differences is as follows:                                             
                                                                           
difference deg C     No.     Accumulative %                                
  0.00 - 0.01          8           22                                      
  0.01 - 0.02          9           46                                      
  0.02 - 0.03          4           57                                      
  0.03 - 0.04          6           73                                      
     >0.04            10          100                                      
                                                                           
Thus,  46%  of  the differences lie within the range +/- 0.02  deg  C,  the
thermometer scale graduation.                                              
                                                                           
                                                                           
Sample salinity data                                                       
Duplicate  water samples were collected on 4 occasions for analysis with  a
salinometer.                                                               
All the differences were less than or equal to 0.003.                      
                                                                           
                                                                           
Sensor Calibration for FSI CTD                                             
(a) Pressure                                                               
The  following calibration of 11/11/1993,  was used to correct the FSI  CTD
pressure sensor:                                                           
                                                                           
P(cor) = P(ctd) + 0.1                                                      
                                                                           
(b) Temperature                                                            
Fig. 1 shows the differences between the (mean) thermometer and uncorrected
CTD temperature (36 occasions).                                            
The mean difference calculated was 0.004.                                  
This  compares with the laboratory pre-cruise calibration  which  estimated
the  CTD  correction to be approximately -0.004 deg C at  the  temperatures
encountered  during  the  cruise.  The  sensor  was  corrected  using   the
laboratory pre-cruise calibration coefficients:                            
                                                                           
T(cor) = T(ctd) - 0.004                                                    
                                                                           
Fig.  2  shows  the  differences  between  these  36  thermometer  and  CTD
temperatures  after the latter have been corrected.  The mean difference is
0.008  deg  C  and  81%  of the 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                                                               
Fig.  3 shows the difference between the water sample salinity as  measured
with  the  portable  salinometer and that derived from the CTD  before  any
calibrations have been applied to the latter's sensors.                    
                                                                           
Fig. 4 shows the ratio of CTD:Water sample conductivity ratio after the CTD
pressure  and  temperature  sensors have been  corrected  using  the  above
coefficients, and after the removal of 8 suspicious values.                
                                                                           
A  set of coefficients have been derived to calibrate the CTD  conductivity
sensor,  using a least square fit between the ratio of water sample and CTD
conductivity and the CTD temperature and pressure.                         
                                                                           
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 = -3.02508292e-05                                                    
    b =  1.15875306e-06                                                    
    c =  1.0001132                                                         
                                                                           
rms salinity difference between water sample and corrected CTD = 0.01      
number of data values = 65                                                 
                                                                           
Figs.  5a and 5b illustrate how effective the CTD conductivity and  derived
salinity have been corrected.                                              
                                                                           
The histograms in Fig.  6 show a small correction in CTD conductivity using
the derived coefficients,  since the upper frame has been derived after the
CTD  temperature  and  pressure have been corrected,  but  before  the  CTD
conductivity calibration has been applied.                                 
                                                                           
If  it  is assumed that the salinometer is accurate to 0.006  and  the  CTD
salinity to 0.01, then differences between +/- 0.016 are acceptable. 89% of
the values lie within this range after the calibrations have been applied. 
                                                                           
                                                                           
Transmission/suspended load                                                
No transmission was logged in the CTD decimated files,  or recorded on  the
log sheets. Therefore, no calibration was possible.                        
