***  Documentation for the CTD data collected on RRS Discovery  Cruise  180                                                         
(Jan-Feb 1989) by the Scottish Marine Biological Association, Oban, Argyll,                                                         
Scotland, UK, under the direction of D. J. Ellett ***                                                                               
                                                                                                                                    
The instrument used was a Bissett Berman 9040 CTD system and the data  were                                                         
logged  on  a  Hewlett  Packard  9820 and  stored  in  an  integer  format.                                                         
Instrument  lowering  and raising speeds were between 0.5m/s and  1m/s.  An                                                         
acoustic  pinger  was  placed  above  the CTD to  give  an  accurate  depth                                                         
measurement, this could then be used to check the CTD pressure calibration.                                                         
An  NIO  bottle with reversing thermometers was placed  above  the  pinger,                                                         
within 2m of the CTD system. A bottle sample was taken at the bottom of the                                                         
cast  providing the temperature and salinity are uniform at that point.  If                                                         
large temperature or salinity gradients were present then the bottle sample                                                         
was triggered at a suitable site on the upcast.  A surface salinity  sample                                                         
was also taken at the start of the dip.                                                                                             
                                                                                                                                    
CALIBRATION                                                                                                                         
                                                                                                                                    
The   CTD  was  not  calibrated  in  the  laboratory.  The   manufacturer's                                                         
calibration  was used and water samples taken to check the calibration  and                                                         
apply corrections where necessary.                                                                                                  
                                                                                                                                    
TEMPERATURE:  The  manufacturer's calibration was used to convert  the  raw                                                         
data to to physical units using the equation below:                                                                                 
                                                                                                                                    
  Temperature (deg C)  =  (10**6/Pt -2238.68/55.84)                                                                                 
  where Pt is the temperature period in microseconds                                                                                
                                                                                                                                    
These  values  were then plotted against the water bottle  (i.e.  reversing                                                         
thermometer)  temperatures  and a regression line fitted to the  data  such                                                         
that:                                                                                                                               
                                                                                                                                    
  Temperature(WB)      =  m x Temperature(CTD) + c                                                                                  
                                                                                                                                    
Then the regression coefficients (m and c) were applied to correct the  CTD                                                         
temperature data - these are given in the table below.                                                                              
                                                                                                                                    
CONDUCTIVITY:  The manufacturer's calibration was used to convert  the  raw                                                         
data to to physical units using the equation below:                                                                                 
                                                                                                                                    
  Conductivity (mmho/cm)  =  (10**6/Pc - 4995)/58.12 + 10                                                                           
  where Pc is the conductivity period in microseconds                                                                               
                                                                                                                                    
The  water  bottle salinities and corrected CTD temperatures were  used  to                                                         
calculate  the  water bottle conductivity values.  These values  were  then                                                         
plotted against the CTD conductivities and a regression line fitted to  the                                                         
data such that:                                                                                                                     
                                                                                                                                    
  Conductivity(WB)     =  m x Conductivity(CTD) + c                                                                                 
                                                                                                                                    
Then  the  regression  coefficients   were   applied  to  correct  the  CTD                                                         
conductivity data - these are given in the table below.                                                                             
                                                                                                                                    
PRESSURE:  The depths from the acoustic pinger were noted where the  bottle                                                         
samples  were taken and then used to check the calibration of the  pressure                                                         
sensor  -  unless  calibration values were  available  from  the  reversing                                                         
thermometers. The equation below was used to convert the pressure period to                                                         
physical units.                                                                                                                     
                                                                                                                                    
  Pressure   = (10**6/Pd - 9712)/0.26267                                                                                            
  where Pd is the pressure period in microseconds                                                                                   
                                                                                                                                    
A regression fit was carried out using the calibration values and the slope                                                         
and  intercept  determined.  The pressure values could  then  be  corrected                                                         
using:                                                                                                                              
                                                                                                                                    
  Pressure (CORR) =  m x Pressure(CTD) + c                                                                                          
                                                                                                                                    
                                                                                                                                    
The  fit of the CTD data to the water bottle calibration data is  given  in                                                         
the table below:                                                                                                                    
                                                                                                                                    
Variable                     Slope,m          Intercept,c                                                                           
                                                                                                                                    
Temperature (deg C)          1.0010906          0.024985                                                                            
                                                                                                                                    
Conductivity (mmho/cm)       1.0023117         -0.013908                                                                            
                                                                                                                                    
Pressure (dbar)              1.0019225         13.206975                                                                            
                                                                                                                                    
                                                                                                                                    
DATA PROCESSING                                                                                                                     
                                                                                                                                    
Obvious  wild  points  were  edited out of the  calibration  file  and  the                                                         
calibration programs run to obtain values for the slopes and intercepts for                                                         
temperature,  pressure  and conductivity.  These were then applied  to  the                                                         
uncalibrated data. Conductivities were converted to conductivity ratios and                                                         
then converted to salinities using UNESCO recommended routines and  sigma-t                                                         
was  calculated.  The  data  values were then sieved to  ensure  a  minimum                                                         
separation between pressure values of 1 dbar.  The data were then  visually                                                         
inspected and major spikes flagged.                                                                                                 
                                                                                                                                    
References:                                                                                                                         
                                                                                                                                    
Fofonoff, N.P. and Millard Jr., R.C. (1983). Algorithms for the computation                                                         
   of fundamental properties of sea water. UNESCO Technical Paper on Marine                                                         
   Science 44.                                                                                                                      
