              CTD SENSOR CALIBRATION: CRUISE CORYSTES 1B/1994              
                                                                           
This cruise used the FSI ICTD s/n 1322 at 6 CTD stations.                  
                                                                           
Reversing  thermometers were used to measure water temperature and  samples
were collected for salinity analysis using the portable salinometer.       
                                                                           
                                                                           
(1) Thermometer data                                                       
Reversing thermometers were read on just two occasions and these  indicated
a difference between thermometer and CTD of 0.00 deg C.                    
                                                                           
                                                                           
(2) Sample salinity data                                                   
Water  samples were collected on 10 occasions for analysis with a  portable
salinometer.   Only  one  sample  had  duplicate  salinity  estimates.  The
instrument was standardised with Standard Seawater of batch P123.          
                                                                           
                                                                           
(3) Sensor Calibration for FSI CTD                                         
                                                                           
(a) Pressure                                                               
A  laboratory  calibration  during  October  1993,  at  room   temperature,
suggested that no correction to the recorded CTD pressure was required, but
it  was  noticed that when on deck the CTD displayed a value of 0.1 db  and
this was taken as an offset to be corrected.                               
                                                                           
i.e. P(cor) = P(ctd) - 0.1 db                                              
                                                                           
(b) Temperature                                                            
The  comparison of thermometer and CTD temperatures was in  good  agreement
with  the laboratory calibration of 13 October 1993,  although  thermometer
readings were limited to just two.                                         
                                                                           
The  sensor  was  therefore  corrected  using  the  laboratory  calibration
coefficients.                                                              
                                                                           
T(cor) = T(ctd) + dT                                                       
    dT = -0.004                                                            
                                                                           
(c) Salinity                                                               
Fig.  1 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.                                 
                                                                           
Fig.  1 suggests that the CTD has a tendency to over-estimate  salinity  by
0.006 (standard dev. 0.003, 10 values).                                    
                                                                           
Fig.  2 shows the difference between salinometer and CTD salinity after the
temperature  and  pressure sensors have been corrected.  The  `offset'  has
decreased to 0.002 (0.003, 10).                                            
                                                                           
Fig.  3 shows the ratio of CTD:Water sample conductivity ratio,  also after
the  CTD  pressure and temperature sensors have been  corrected  using  the
above coefficients.                                                        
                                                                           
A  least square fit was used to determine calibration coefficients for  the
CTD conductivity ratio:                                                    
                                                                           
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 = -2.647128e-4                                                       
    b = -6.9316356-6                                                       
    c =  1.001472                                                          
                                                                           
rms salinity difference between water sample and corrected CTD is 0.002 for
10 data values.                                                            
                                                                           
Figs.  4  and 5 illustrate how effective the CTD conductivity  and  derived
salinity  have  been corrected.  The tendency of the CTD  to  over-estimate
salinity has been effectively compensated for by the calibration.          
                                                                           
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.  All
ten  values  lie well within this range after the  calibrations  have  been
applied,  although in this instance this applies to the estimates prior  to
any sensors being calibrated (see Fig. 1).                                 
                                                                           
                                                                           
(4)  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 = -12.84 and b =  55.16                                            
                                                                           
                                                                           
Ken Medler                                                                 
14 April 1994                                                              
