       CTD SENSOR CALIBRATION CORYSTES 6A/1993 (JONUS EXERCISE 17)         
                                                                           
This  cruise  used the Guildline CTD s/n 45056 at the JONUS  stations.  The
following is a presentation of the data returned from stations 7-70.       
                                                                           
38 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.                                      
                                                                           
                                                                           
Thermometer data                                                           
Left  and right thermometer temperatures were available on 28 occasions.  4
pairs of thermometer results were not used, as their difference was <.04. 3
pairs  of  thermometer  results were not used,  as the stations were  on  a
thermocline.                                                               
                                                                           
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         6           21                                       
  0.01 - 0.02        15           75                                       
  0.02 - 0.03         6           96                                       
  0.03 - 0.04         1          100                                       
                                                                           
Thus,  75%  of  the  differences  lie within the  range  0.02  deg  C,  the
thermometer scale graduation.                                              
                                                                           
                                                                           
Sample salinity data                                                       
Duplicate water samples were collected on 73 occasions for analysis with  a
Guildline  salinometer.  3 pairs of duplicate salinities were not used,  as
their difference was >.012.                                                
                                                                           
On  2  other  stations,  only 1 water sample salinity  was  available.  The
distribution of the differences is as follows:                             
                                                                           
  difference        No.     Accumulative %                                 
0.000 - <0.002      70            96                                       
   <0.01             3           100                                       
                                                                           
Thus, 96% of the differences lie within 0.002.                             
                                                                           
                                                                           
Sensor Calibration for Guildline CTD                                       
(a) Pressure                                                               
The laboratory calibration of November 1992,  sensor at 13 deg C,  was used
to correct the Guildline CTD pressure sensor:                              
                                                                           
P(cor) = P(ctd) - 1.4 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 or
equal to 0.04 deg C (27 occasions).  1 pair of thermometer results was  not
used,  as  the difference between the thermometer and CTD temperatures  was
suspect.  The mean difference calculated for the 27 instances when the left
and  right thermometers themselves differed by less than or equal  to  0.04
was 0.022 deg C.                                                           
                                                                           
This compares with the laboratory pre-cruise calibration of May 1993  which
estimated  the  CTD  correction  to be approximately 0.006  deg  C  at  the
temperatures encountered during the cruise.  The sensor was 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 =  5.24713e-5                                                      
      b = -9.60448e-4                                                      
      c =  8.809e-3                                                        
                                                                           
Fig.  2  shows  the  differences  between  these  27  thermometer  and  CTD
temperatures  after the latter have been corrected.  The mean difference is
0.016 deg C and all 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.  Note that 4  data
values  have been removed prior to plotting Fig.  3 because the differences
were  large,  suggesting  either an erroneous CTD  value  or  water  sample
salinity.                                                                  
                                                                           
Fig. 3 suggests that the CTD has a tendency to over-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.                                                              
                                                                           
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.  Stations 46, 69 middle,
and  70 middle and bottom,  were on a thermocline and not used to determine
the conductivity calibration;  also station 48 surface appeared suspect and
was omitted.                                                               
                                                                           
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 = -7.70050692e-05                                                    
    b = -1.06122264e-05                                                    
    c =  1.00106613                                                        
                                                                           
rms salinity difference between water sample and corrected CTD = 0.012     
number of data values = 67                                                 
                                                                           
Fig.  5 illustrates how effective the CTD conductivity and derived salinity
have been corrected. The tendency of the CTD to over- 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. 88% of
the values lie within this range after the calibrations have been applied. 
                                                                           
                                                                           
Transmission/suspended load                                                
A 25 cm path length transmissometer (s/n 198) was fitted to the CTD rosette
and  calibrated in terms of suspended load (mg/1) by comparing  the  logged
%transmission with laboratory determinations of suspended load.            
                                                                           
suspended load = a*loge(%Trans) + b                                        
                                                                           
where a = -4.88                                                            
      b = 19.77                                                            
