       CTD SENSOR CALIBRATION CORYSTES 2C/1993 (JONUS EXERCISE 14)         
                                                                           
This  cruise  used the Guildline CTD s/n 45056 at the JONUS  stations.  The
following is a presentation of the data returned from stations 7-71.       
                                                                           
Twenty-one  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 36 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 %                                    
      0.00           1          3                                          
      0.01          15         44                                          
      0.02          20        100                                          
                                                                           
Thus,  100%  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. The distribution of the differences is as follows:  
                                                                           
   Difference        No.        Accumulative %                             
0.000 - < 0.003       29               71                                  
0.003 - < 0.006       11               98                                  
0.006 - < 0.009        1              100                                  
                                                                           
Thus, 98% of the differences lie within 0.006.                             
                                                                           
                                                                           
(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/equal to 0.03 deg C (36 occasions). The mean difference calculated for
the  36 instances when the left and right thermometers themselves  differed
by  less  than/equal  to  0.02  was 0.016  deg  C.  However,  10  of  these
differences  appear on Fig.  1 in a group of higher difference values,  and
the mean of the other 26 was .010.                                         
                                                                           
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.  2  shows  the  differences  between  these  36  thermometer  and  CTD
temperatures  after the latter have been corrected.  The mean difference is
0.009 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.                               
                                                                           
If  the ten large differences at stations 1 to 26 are ignored,  all of  the
remaining differences (26 total) are less than/equal to .025 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 3  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, and omitting the suspect CTD values for station 45 because it
was close inshore.                                                         
                                                                           
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 71 bottom  had
duplicate salinity samples which differed by greater than 0.006 and has not
been used to determine the conductivity calibration;  also stations 7 and 9
surface, and station 49 bottom, appeared suspect and were 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 = 4.02815256e-4                                                      
    b = -1.09064574e-6                                                     
    c = 0.997198294                                                        
                                                                           
RMS salinity difference between water sample and corrected CTD   0.005     
number of data values     36                                               
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 0.003  and  the  CTD
salinity to 0.01, then differences between +/- 0.013 are acceptable.  35 of
36 values lie within this range after the calibrations have been applied.  
                                                                           
                                                                           
(4) 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/l) by comparing  the  logged
%transmission with laboratory determinations of suspended load.            
                                                                           
suspended load = a*loge(%Trans) + b                                        
                                                                           
where a = -14.0                                                            
      b = 50.3                                                             
                                                                           
                                                                           
Sue Norris                                                                 
4 June 1993                                                                
