        CTD SENSOR CALIBRATION CORYSTES 9/1993 (JONUS EXERCISE 19)         
                                                                           
This cruise used the FSI CTD s/n 1322 at the JONUS stations.  The following
is a presentation of the data returned from stations 5-102.                
46 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 45 occasions.  3
pairs of thermometer results were not used, as their difference was > .04. 
                                                                           
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          35           78                                    
   0.01 - 0.02           5           89                                    
   0.02 - 0.03           1           91                                    
   0.03 - 0.04           1           93                                    
   0.07 - 0.08           1           96                                    
      >0.1               2          100                                    
                                                                           
Thus,  89%  of  the  differences  lie within the  range  0.02  deg  C,  the
thermometer scale graduation.                                              
                                                                           
                                                                           
Sample salinity data                                                       
Duplicate water samples were collected on 90 occasions for analysis with  a
salinometer.                                                               
                                                                           
7  pairs of duplicates were deleted since their difference was > .006.  The
distribution of the differences is as follows:                             
                                                                           
  difference         No.     Accumulative %                                
0.000 - 0.001         78            87                                     
0.001 - 0.002          3            90                                     
0.002 - 0.003          2            92                                     
    >0.006             7           100                                     
                                                                           
Thus, 92% of the differences lie within 0.003.                             
                                                                           
                                                                           
Sensor Calibration for FSI CTD                                             
(a) Pressure                                                               
                                                                           
The following calibration was used to correct the FSI CTD pressure sensor: 
                                                                           
P(cor) = P(ctd)                                                            
                                                                           
(b) Temperature                                                            
Fig. 1 shows the differences between the (mean) thermometer and uncorrected
CTD temperature when the left/right thermometer difference was <0.04 deg C 
(42 occasions).                                                            
The mean difference calculated for the 42 instances when the left and right
thermometers themselves differed by 0.04 was -0.003 deg C.                 
                                                                           
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  42  thermometer  and  CTD
temperatures  after the latter have been corrected.  The mean difference is
0.001 deg C and 40 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  Guildline salinometer and that derived from the CTD  before  any
calibrations have been applied to the latter's sensors.                    
                                                                           
Fig. 3 suggests that the CTD has a tendency to under-estimate salinity.    
                                                                           
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 4 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.  Station 35 was  in  a
river  estuary;  stations  78 and 95 were close inshore  and  not  used  to
determine the conductivity calibration.                                    
                                                                           
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 = -3.57869208e-05                                                    
    b = -3.44717322e-06                                                    
    c =  1.00109059                                                        
                                                                           
rms salinity difference between water sample and corrected CTD = 0.005     
number of data values = 79                                                 
Fig.  5 illustrates how effective the CTD conductivity and derived salinity
have  been corrected.  The tendency of the CTD to under- estimate  salinity
has been overcome.                                                         
                                                                           
The  histograms in Fig.  6 show how well the CTD conductivity is  corrected
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.  100%
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.                        
