               CTD SENSOR CALIBRATION: CRUISE CIROLANA 4/94                
                                                                           
This cruise used the FSI CTD s/n 1322.  The following is a presentation  of
the data returned from stations 8-41.                                      
                                                                           
11 CTD profiles were obtained using the FSI CTD during the cruise.  Samples
were  collected for salinity analysis.  No thermometers were used  on  this
cruise.                                                                    
                                                                           
                                                                           
(1) Sample salinity data                                                   
Duplicate water samples were collected on 2 occasions,  and also 15  single
water  samples  were analysed with a salinometer.  Both of  the  duplicates
differed by 0.001.                                                         
                                                                           
                                                                           
(2) Sensor Calibration for FSI CTD                                         
(a) Pressure                                                               
The following calibration was used to correct the FSI CTD pressure sensor: 
                                                                           
P(cor) = P(ctd) - 0.1 db                                                   
                                                                           
(b) Temperature                                                            
The  sensor  was corrected using the  laboratory  calibration  coefficients
obtained in October 1993:                                                  
                                                                           
T(cor) = T(ctd) - 0.004                                                    
                                                                           
(c) Salinity                                                               
Figs.  1a and 1b show 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.                    
                                                                           
Fig. 2 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 has 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 9 had  large
differences between the water sample and CTD salinities. Station 15 did not
have water samples and was therefore 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 = 0.385909905e-03                                                    
    b = 0.319279245e-05                                                    
    c = 0.997631527                                                        
                                                                           
rms salinity difference between water sample and corrected CTD is 0.005 for
17 data values.                                                            
                                                                           
Figs.  3a and 3b illustrate how effective the CTD conductivity and  derived
salinity have been corrected.                                              
                                                                           
The  histograms in Fig.  4 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. 
                                                                           
                                                                           
(3)  Transmission/suspended load                                           
A 25 cm path length transmissometer (s/n 198) was mounted on the rosette at
each CTD station.  The instrument was 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 = -10.6 and b =  47.49                                             
                                                                           
Sue Norris                                                                 
06/05/94                                                                   
