                  CTD CALIBRATION: CRUISE CIROLANA 9/1981                  
                                                                           
                                                                           
Calibration  coefficients  for the ten CTD profiles from this  cruise  have
been  calculated  and  applied to the logged data.  A comparison  with  the
theta-S  regression  suggested by Peter Saunders for the area  suggests  an
effective calibration of the conductivity cell.                            
                                                                           
Pressure                                                                   
Logged pressure was corrected by applying the coefficients derived from the
laboratory calibration of April/May 1983.                                  
                                                                           
   P(COR) = P(CTD) + deltaP                                                
   deltaP = a * T(COR) + b * P(CTD) + c                                    
                                                                           
where   a = 0.857466                                                       
        b = 0.33198 E-2                                                    
        c = -11.96                                                         
                                                                           
                                                                           
Temperature                                                                
In the absence of a suitable laboratory calibration,  a comparison was made
between  the CTD temperature sensor and reversing thermometers used  during
the cruise. This suggested that a suitable correction would be             
                                                                           
   deltaT = a * T(CTD) + b                                                 
                                                                           
where   a = -0.34585 E-2                                                   
        b = 0.5425 E-1                                                     
                                                                           
and     T(COR) = T(CTD) + deltaT                                           
                                                                           
                                                                           
Conductivity Ratio                                                         
A least square fit to the equation                                         
                                                                           
        CR(WS)                                                             
        -----------     = a * T(COR) + b * P(COR) + C                      
        CR(CTD)                                                            
                                                                           
was  solved  to evaluate calibration coefficients a,  b and c.  CR(WS)  was
derived  from  the UNESCO algorithm and represents the  conductivity  ratio
that  the  water  sample  would have,  at  the  corrected  temperature  and
pressure, to maintain the salinity as measured with a bench salinometer.   
                                                                           
The corrected conductivity ratio is then calculated from                   
                                                                           
  CR(COR) = CR(CTD) * (a * T(COR) + b * P(COR) + c)                        
                                                                           
where   a = -0.82835 E-4                                                   
        b = -0.29297 E-7                                                   
        c = 1.000926                                                       
                                                                           
A total of 48 water sample salinity values were used to derive a,  b and c.
An rms difference between corrected CTD salinity and water sample  salinity
amounted to 0.009.                                                         
                                                                           
Of  the  ten  profiles,  eight extended to abyssal  depths.  For  these,  a
comparison  was made between corrected CTD salinity and the linear  theta-S
relationship suggested by Saunders for theta < 2.7degC; see attached plots.
This indicated agreement to +/- 0.004.                                     
                                                                           
Ken Medler/Sue Norris                                                      
10 October 1989                                                            
