       CTD SENSOR CALIBRATION CIROLANA 4/1993, JONUS Exercise 15           
                            Stations 33 to 217                             
                                                                           
For  station 33 onwards of this cruise,  the Guildline CTD (s/n 45056)  was
used.  The  following  is a presentation of the data  returned  from  those
stations.                                                                  
                                                                           
Fifty  CTD  profiles were obtained on this part of  the  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  all  stations
(total  96  pairs).  Eighteen pairs of thermometer results  were  not  used
because the differences between them were > 0.02.                          
                                                                           
All the thermometers were graduated at 0.02deg C intervals and a comparison
of the differences follows:                                                
                                                                           
Difference deg C      No.      Cumulative %                                
  0.00 - 0.01          47           60                                     
  0.01 - 0.02          31          100                                     
                                                                           
                                                                           
2. Sample salinity data                                                    
Duplicate water samples were collected on all stations for analysis with  a
Guildline  salinometer.  None of the differences between samples was >0.012
psu, and the distribution is as follows:                                   
                                                                           
Difference (psu)        No.        Cumulative %                            
      zero               32             35                                 
 0.001 - 0.006           57             99                                 
 0.007 - 0.012            1            100                                 
                                                                           
Note that 99% of the differences lie within 0.006 psu.                     
                                                                           
                                                                           
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 less than
or  equal to 0.02deg C (77 occasions).  The mean difference calculated  for
these was 0.007deg C.                                                      
                                                                           
This  compares  with  the laboratory calibration  of  February  1993  which
estimated  the  CTD  correction  to  be approximately  0.007deg  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 = 3.1834419E-03                                                   
                                                                           
Fig  2  shows  the  differences   between  these  77  thermometer  and  CTD
temperatures  after the latter have been corrected.  The mean difference is
0.001deg  C  and  all the differences lie with 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.  Only 5 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 10 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 stations 33 to 217, 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 = 0.754170854E-04                                              
          b = 0.267130949E-05                                              
          c = 0.998625054E+00                                              
                                                                           
RMS salinity difference between water sample and corrected CTD = 0.008  and
the number of data values = 77.                                            
                                                                           
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
88%  of  the values lie within this range after the calibrations have  been
applied.                                                                   
                                                                           
                                                                           
4. Transmission and 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  in
coincident water samples.                                                  
                                                                           
Suspended load = a*loge (%trans) + b                                       
                                                                           
Because  of  the varying type and quantity of sediment loads in  the  water
encountered on this cruise, data were grouped into areas. This enables more
realistic  calibrations  to be obtained than would be the case if  all  the
data were considered in one batch.  For the stations on this second part of
the cruise, the coefficients are as follows:                               
                                                                           
 Station         a          b                                              
  33 - 36      -9.53      31.90                                            
  55 - 65      -16.3      60.10                                            
 66 - 109      -10.6      43.83                                            
111 - 217      -11.5      43.88                                            
                                                                           
Stuart Jones                                                               
September 1993                                                             
