                    CTD CALIBRATION: CRUISE CORYSTES 1/1992                
                                                                           
                                                                           
CTD: S/N 45056                                                             
                                                                           
(1)  Eighty-two  CTD profiles were obtained during  the  cruise.  Reversing
thermometers  were fitted to the Niskin used to collect samples from  close
to the seabed.  Samples for salinity analysis were collected from both near
the surface and close to the seabed. Not all the calibration data collected
were used to calibrate the CTD sensors:                                    
   (i)  Thermometer differences too large, > 0.02.                         
   (ii) It was clear from the profiles obtained at the time of station that
some  T and/or S values measured by the CTD were variable,  believed  real,
not an instrument malfunction, and unsuitable for calibration use. Eighteen
data sampling points were not used because of this.                        
                                                                           
                                                                           
(2)  The pressure sensor was corrected using the laboratory calibration  of
September 1991 at T = 12 deg C.                                            
                                                                           
P(cor) = P(ctd) - 1.6                                                      
                                                                           
Normally  this is checked by comparing CTD pressure with that derived  from
water depth and altimeter height, but a fault with the altimeter throughout
this cruise made this impractical.                                         
                                                                           
                                                                           
(3)  The temperature sensor was corrected using laboratory  calibration  of
December 1991.                                                             
                                                                           
  T(cor) = T(ctd) + dT                                                     
                                                                           
      dT = a*T(ctd)*T(ctd) + b*T(ctd) + c                                  
                                                                           
where  a = -2.59480e-6                                                     
       b = -1.60314e-4                                                     
       c =  3.128e-3                                                       
                                                                           
These coefficients are equivalent to a correction of approximately 2 mK for
the range of temperatures encountered during the cruise.                   
                                                                           
A comparison between thermometer and uncorrected CTD temperatures is  shown
in Fig. 1.  If it is assumed that the thermometer temperatures are accurate
to 0.02 deg C and the CTD to 0.01 deg C then all differences to within 0.03
deg  C are acceptable.  Only four out of sixty-nine values are outside this
range and the mean difference is 0.002 deg C.                              
                                                                           
                                                                           
(4)  One hundred and forty two salinity samples were used to calibrate  the
conductivity sensor.                                                       
                                                                           
    CR(cor) = CR(ctd)*[ a*T(cor) + b*P(cor) + c ]                          
                                                                           
where     a = -1.467061e-5                                                 
          b =  2.790880e-6                                                 
          c =  0.999653                                                    
                                                                           
The  rms  difference  between salinometer and corrected  CTD  salinity  was
0.018.                                                                     
                                                                           
Fig.  2  shows how effective these coefficients are in correcting  the  CTD
salinity estimates for the calibration samples.  Note that the CTD salinity
of  the upper histogram was derived after the CTD temperature and  pressure
had  been  corrected  but  prior   to   applying  corrections  to  the  CTD
conductivity  ratio.  The  Figure gives an indication of how well  the  CTD
conductivity is corrected.                                                 
                                                                           
Fig. 3 displays the spread in the ratio of conductivity-ratio of the CTD to
that of the water samples prior to applying the correction.                
                                                                           
Fig.  4  shows  the difference between the water sample  salinity  and  the
uncorrected CTD equivalent before any of the CTD sensors are corrected; the
CTD is generally 0.02 too high.                                            
                                                                           
Fig.  5  shows  the difference between the water sample  salinity  and  the
corrected CTD equivalent after all three sensors (pressure, temperature and
conductivity ratio) have been corrected.                                   
                                                                           
Stations  23  to  45 are at or near the entrance  to  the  Humber  Estuary,
stations  62 to 74 are well into the Wash whilst station 80 is at the outer
Thames Estuary. The differences are relatively larger at these stations.   
                                                                           
If the CTD is assumed to have an accuracy of 0.01 in terms of salinity  and
the  salinometer of 0.006 then the corrected CTD salinity values should lie
within  0.016.  In this instance 28 out of 142 samples (20%)  fall  outside
this  range (see Fig.  2).  If the data from the above stations are removed
from  the calibration this figure is reduced to 6 from 85 (7%) and the  rms
difference between corrected CTD and water sample salinity is 0.011; 7 from
85 lie outside +/- 0.013.                                                  
                                                                           
                                                                           
(5)  The suspended load estimates from %transmission were  calculated  from
the following:                                                             
                                                                           
suspended load (mg/litre) = B * ln (%transmission) + C                     
                                                                           
station number           B          C                                      
                                                                           
    1 to 21            -54.5      215.8                                    
   23 to 36            -69.0      263.8                                    
   37 to 48            -55.0      217.6                                    
   49 to 74            -52.1      205.0                                    
   77 to 83           -103.0      397.4                                    
   84 to 99            -79.1      309.1                                    
                                                                           
These  coefficients  derived from suspended load  measurements  on  samples
collected whilst transmission being logged, two samples per station.       
                                                                           
The transmissometer used during the cruise was s/n 479, with a 5 cm path.  
