       CTD SENSOR CALIBRATION CIROLANA 9B/1993 (JONUS EXERCISE 20)         
                                                                           
This  cruise used the FSI CTD s/n 1322 at all 62 CTD stations.  The  logger
failed during one (station 161); 61 profiles are available.                
                                                                           
Reversing  thermometers were used to measure water temperature and  samples
were  collected for salinity analysis using the recently acquired  portable
salinometer.                                                               
                                                                           
                                                                           
Thermometer data                                                           
Left and right thermometer temperatures were available on 51 occasions, but
one  pair  was subsequently rejected because both had  been  influenced  by
direct  sunshine.  Note that temperature differences are not available  for
all stations because it was seen that water temperatures were varying  over
the four minutes allowed for acclimatisation; thermometers were not read in
these circumstances.                                                       
                                                                           
All thermometers were graduated at 0.02 deg C intervals and a comparison of
the differences is as follows:                                             
                                                                           
difference deg C     No.     Accumulative %                                
     0.00             12           24                                      
     0.01             23           70                                      
     0.02             14           98                                      
     0.03              1            2                                      
                                                                           
Thus,  98%  of  the differences lie within the range +/- 0.02  deg  C,  the
thermometer scale graduation.                                              
                                                                           
                                                                           
Sample salinity data                                                       
Duplicate water samples were collected on 21 occasions for analysis with  a
portable salinometer. The distribution of the differences is as follows:   
                                                                           
  difference         No.     Accumulative %                                
0.000 - <0.003       20            95                                      
0.003 - <0.006        1           100                                      
                                                                           
                                                                           
Sensor Calibration for FSI CTD                                             
(a) Pressure                                                               
A laboratory calibration during October 1993,  albeit at room  temperature,
suggested that no correction to the recorded CTD pressure was required,    
                                                                           
i.e. P(cor) = P(ctd)                                                       
                                                                           
(b) Temperature                                                            
Fig. 1 shows the differences between the (mean) thermometer and uncorrected
CTD temperature. The mean of the 50 differences is - 0.005 deg C.          
                                                                           
This is an excellent agreement with the laboratory post-cruise  calibration
of 13 October, which estimated the CTD correction to be -0.004 deg C.      
                                                                           
The  sensor  was  therefore  corrected  using  the  laboratory  calibration
coefficients:                                                              
                                                                           
T(cor) = T(ctd) + dT                                                       
    dT = -0.004                                                            
                                                                           
The  differences between thermometer and CTD temperatures after the  latter
have been corrected is shown in Fig. 2. The mean difference is -0.001 deg C
and  all differences lie well within +/- 0.03 deg C,  supporting the belief
that  if  the  thermometers are accurate to +/- 0.02 deg  C  then  the  CTD
temperatures are accurate to +/- 0.01 deg C.  In fact,  all differences are
within  +/-  0.015 deg C suggesting that the above tolerances may be  over-
estimated.                                                                 
                                                                           
(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 a few data values have
been  removed prior to plotting Fig.  3 because they were not suitable  for
determining calibration coefficients:                                      
                                                                           
(i)  stations 202 to 232 were from the Bull Anchorage in the River  Humber,
whilst  Cirolana  was anchored.  Throughout the JONUS exercise it has  been
found  that the CTD salinity estimates from estuaries are suspect,  perhaps
because  of  high  sediment loads influencing the  instrument  conductivity
measurement, and CTD data from these stations have not been used;          
                                                                           
(ii)  the  conductivity  recorded by the CTD at station  178  was  variable
during  the time that the calibration sample was collected and the CTD data
have not been used.                                                        
                                                                           
Fig.  3 suggests that the CTD has a tendency to over-estimate  salinity  by
0.007 (standard dev. 0.005,89 data values).                                
                                                                           
Fig.  4 shows the difference between salinometer and CTD salinity after the
temperature  and  pressure sensors have been corrected.  The  `offset'  has
increased to 0.010 (0.005,89).                                             
                                                                           
Fig.  5 shows the ratio of CTD:Water sample conductivity ratio,  also after
the  CTD  pressure and temperature sensors have been  corrected  using  the
above coefficients. The CTD is clearly over- estimating the conductivity.  
                                                                           
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 = -1.787654e-5                                                       
    b = -5.632200e-7                                                       
    c =  0.999984                                                          
                                                                           
rms salinity difference between water sample and corrected CTD is 0.005 for
89 data values.                                                            
                                                                           
Figs.  6  and 7 illustrate how effective the CTD conductivity  and  derived
salinity  have been corrected.  The propensity of the CTD to  over-estimate
salinity has been effectively compensated for by the calibration.          
                                                                           
The  histograms in Fig.  8 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. Only 1
of  89  values  lies outside this range after the  calibrations  have  been
applied; 2 of 89 lie outside +/- 0.013.                                    
                                                                           
                                                                           
Transmission/suspended load                                                
Two  transmissometers  were  used during the cruise,  mounted  on  the  CTD
rosette. At the Bull Anchorage instrument s/n 479 (with a 5 cm path length)
was  used  whilst  s/n 198 (25 cm path length) was used at  the  other  CTD
stations.                                                                  
                                                                           
Each  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 = -58.91 & b=263.63 for s/n 479                                    
and   a = -12.42 & b=54.61  for s/n 198                                    
