*** Documentation for CTD data collected on RRS Discovery Cruise 117 (Jan -                                                         
Feb  1981) by the Institute of Oceanographic Sciences  (Deacon  Laboratory)                                                         
Godalming, Surrey, UK., under the direction of P. M. Saunders. ***                                                                  
                                                                                                                                    
The instrument used was a Neil Brown Instrument Systems CTD which  measured                                                         
pressure,  temperature  and  conductivity  and was fitted  with  a  Beckman                                                         
dissolved oxygen electrode.  The CTD was used alongside a General  Oceanics                                                         
Rosette Multisampler with 12 water bottles,  a 10kHz pinger,  a bottom echo                                                         
sounder and a Sea Tech transmissometer. Lowering and retrieval rates of 0.5                                                         
to  1.5m/s were employed and the sensors were flushed with distilled  water                                                         
on  recovery.  Bottle samples and reversing thermometer  measurements  were                                                         
made  on  ascent and the sea water samples were analysed using a  Guildline                                                         
Autolab  Salinometer.  Reversing thermometers were  calibrated  before  and                                                         
after the cruise.                                                                                                                   
                                                                                                                                    
Two CTD units were employed during the cruise, the first for stations 10261                                                         
and  10263  and the second for the remainder.  After 11 stations  with  the                                                         
second  unit  the conductivity cell was replaced with the hope of  reducing                                                         
the calibration drift.                                                                                                              
                                                                                                                                    
On four stations during the cruise (10275, 10276, 10279 and 10285) repeated                                                         
lowerings were made in the bottom 500m of the water column.                                                                         
                                                                                                                                    
PRESSURE:  The  deck  pressure offset was stable and a  value  of  8db  was                                                         
assumed  for all stations.  The table below shows differences in  pressures                                                         
determined from pairs of reversing thermometers (protected and unprotected)                                                         
and simultaneous observations of CTD pressure.  Pressures at the bottom  of                                                         
the  cast  were  converted to depth and added to the CTD height  above  the                                                         
bottom  to  give water depth.  The echo sounder depth was  corrected  using                                                         
Carter's  tables  and the 2 depths compared.  In the measured  depth  range                                                         
5200-5500m  the corrected echo sounder depth exceeded that calculated  from                                                         
the  CTD  for 25 observations by a mean of 6m with a standard deviation  of                                                         
5m.                                                                                                                                 
                                                                                                                                    
TEMPERATURE:  Laboratory calibrations were carried out in  September  1980.                                                         
Corrected  CTD  temperatures were determined from the equation T = 0.044  +                                                         
0.00050042 * RAWTEMP.  A comparison is given in the table below between the                                                         
CTD temperatures and reversing thermometers separating the deep and shallow                                                         
measurements.                                                                                                                       
                                                                                                                                    
SALINITY:  During  the cruise bottle salinities and  reversing  thermometer                                                         
measurements  revealed  a linear potential temperature (POTT)  -  practical                                                         
salinity (S) relationship for POTT less than 2.6deg C of S = 34.698 + 0.098                                                         
*  POTT.  Employing  a  nominal cell factor of  1.0  *  10**-3  provisional                                                         
salinities  were  computed and the multiplicative factor required to  bring                                                         
the  provisional  salinities  into agreement with the  expected  POTT  -  S                                                         
relationship was then determined. For the cell employed on stations 10264 -                                                         
10274  this  factor varied between 0.99951 and 0.99974 (but  not  smoothly)                                                         
corresponding to a salinity change of 0.008.  For the cell used on stations                                                         
10275 - 10294 this factor varied between 1.00027 and 1.00039  corresponding                                                         
to a change of 0.004.  The table below compares salinities derived from the                                                         
rosette sampler and Guildline salinometer with the CTD values.                                                                      
                                                                                                                                    
                                                                                                                                    
Fit of CTD Data to Rosette Sample Values                                                                                            
                                                                                                                                    
                          Difference between CTD and Rosette Measurements                                                           
                                                                                                                                    
   Variable          Range          Mean difference     R.M.S.     Number                                                           
                                                                                                                                    
Pressure, db         0-2000 db           +0.5             2          30                                                             
                  2000-5600 db           +1.0             4          52                                                             
                                                                                                                                    
Temperature, degC    5-23 degC            0              0.006       31                                                             
                     2-5 degC           - 0.001          0.004       59                                                             
                                                                                                                                    
Salinity, PSU        0-2000 db          + 0.002          0.008       58                                                             
                  2000-5600 db          - 0.001          0.0025      70                                                             
                                                                                                                                    
                                                                                                                                    
The depth of the 5 degC isotherm is approximately 1800 db.                                                                          
                                                                                                                                    
Original  values  were  averaged  over   an  interval  of  one  second  and                                                         
calibration  coefficients and correction factors applied.  A time  constant                                                         
correction algorithm was employed to compensate for the slower response  of                                                         
the platinum resistance thermometer.  Differences between successive values                                                         
of  each  parameter  were examined;  the mean difference and  its  standard                                                         
deviation  calculated and values greater than several  standard  deviations                                                         
from  the  mean difference were checked.  Genuinely suspect data were  then                                                         
replaced  by  interpolated values.  Derived quantities were  computed  from                                                         
algorithms published by Fofonoff and Millard (1983). At ten of the stations                                                         
(10268, 10270, 10271, 10272, 10288, 10290, 10291, 10293 and 10294) the data                                                         
cycles  were sorted by pressure to remove the effect of ships heave  before                                                         
all values were averaged at 5db intervals, centred on 2.5db, 7.5db...                                                               
                                                                                                                                    
References:                                                                                                                         
Saunders, P.M. (1981).  CTD data from the north Madeira Basin: 19 January -                                                         
   12 February 1981 Discovery Cruise 117. Unpublished manuscript.                                                                   
                                                                                                                                    
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.                                                                                                                      
