        CTD SENSOR CALIBRATION CORYSTES 5A/1993, JONUS Exercise 16         
                             Stations 2 to 132                             
                                                                           
For this cruise, the Guildline CTD (s/n 45056) was used. The following is a
presentation of the data returned from all stations.                       
                                                                           
Fifty-nine   CTD   profiles  were  obtained  on  this   cruise;   reversing
thermometers  were  used  to  measure water temperature  and  samples  were
collected for salinity analysis.                                           
                                                                           
                                                                           
1. Thermometer data                                                        
Left  and right hand thermometer readings were available on  most  stations
(total 58 pairs). Initially, six pairs of thermometer results were not used
because the differences between them were > 0.025.                         
                                                                           
All the thermometers were graduated at 0.02degC intervals and a  comparison
of the differences follows:                                                
                                                                           
Difference deg C    No.    Cumulative %                                    
  0.00 - 0.01        25         48                                         
  0.01 - 0.02        21         88                                         
  0.02 - 0.03         6        100                                         
                                                                           
                                                                           
2. Sample salinity data                                                    
Water samples were collected on all stations for analysis with a  Guildline
salinometer. Duplicate samples were taken on only one station.             
                                                                           
                                                                           
3. Sensor calibration for Guildline CTD                                    
a) Pressure                                                                
The laboratory calibration of March 1992 with the sensor at 6deg C was used
to correct the pressure sensor:                                            
                                                                           
P(cor) = P(ctd) - 1.6db                                                    
                                                                           
b) Temperature                                                             
Fig. 1 shows the differences between the (mean) thermometer and uncorrected
CTD temperature when the left/right thermometer differences were <  0.03deg
C (52 occasions). The mean difference calculated for these was 0.017deg C. 
                                                                           
This  compares  with  the laboratory calibration  of  February  1993  which
estimated  the  CTD  correction  to  be approximately  0.009deg  C  at  the
temperatures encountered on this cruise. The sensor was corrected using the
laboratory calibration coefficients:                                       
                                                                           
T(cor) = T(ctd) + dT                                                       
                                                                           
where dT = a*T(ctd)*T(ctd) + b*T(ctd) + c                                  
and a = 1.21101E-05                                                        
    b = 5.92251E-04                                                        
    c = 3.18442E-03                                                        
                                                                           
Further examination of the data led to the removal of a further eleven  CTD
or  thermometer  pairs  and  Fig.  2 shows the  differences  between  these
remaining  41 pairs of good thermometer and CTD data after the latter  have
been  corrected.  The mean difference is 0.001deg C and all the differences
lie  within 0.03deg C,  supporting the belief that if the thermometers  are
accurate  to 0.02deg C then the CTD temperatures are accurate to 0.01deg C.
Ten differences are outside +/- 0.012deg C.                                
                                                                           
c) Salinity                                                                
Fig.  3 shows the difference between the water sample salinity as  measured
with  the salinometer and that derived from the CTD before any calibrations
have  been applied to the latter's sensors.  Note that 6 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. The plot
in 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 coefficients
for temperature and pressure above.                                        
                                                                           
For  this  data,  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 respectively                                                      
and where a = 0.154477980E-04                                              
          b = 0.197859277E-05                                              
          c = 0.999379414E+00                                              
                                                                           
RMS salinity difference between water sample and corrected CTD = 0.011  and
the number of data values = 102.                                           
                                                                           
Fig.  5  illustrates  how  effectively the  CTD  conductivity  and  derived
salinity  have  been corrected.  The tendency of the CTD  to  over-estimate
salinity has been removed.                                                 
                                                                           
The  histograms in Fig.  6 show how well the CTD conductivity is  corrected
since the upper one 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.  Some
78%  of  the values lie within this range after the calibrations have  been
applied.                                                                   
                                                                           
                                                                           
4. Transmission and suspended load                                         
Because of a fault in the equipment, no transmission data was logged during
the  whole of this cruise.  Full scale voltages were  recorded  throughout.
Consequently, no calibration of this sensor is possible.                   
                                                                           
                                                                           
Stuart Jones                                                               
October 1993                                                               
