*** Documentation for CTD data collected on RRS Discovery Cruise 130 (Jun -                                                         
Aug  1982) 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.                                                                                                                   
                                                                                                                                    
Stations 10560 to 10574 were shared between the CTD/Multisampler and a  14m                                                         
long  hydrophone employed in listening for moored sound sources.  Listening                                                         
was performed on lowering and both CTD and bottle sample data were obtained                                                         
on recovery.  For stations 10555,  10556 and 10575 to 10599 the  hydrophone                                                         
was  removed  and  CTD data were gathered on lowering  and  bottle  samples                                                         
obtained on recovery.                                                                                                               
                                                                                                                                    
On seven stations during the cruise (10575,  10576,  10585,  10587,  10592,                                                         
10593 and 10594) repeated lowerings were made between 4000m and the bottom.                                                         
                                                                                                                                    
PRESSURE:  The pressure sensor was calibrated in a laboratory in  September                                                         
1980  and  repeat calibrations of January and October 1982 showed the  same                                                         
slope.  The deck pressure offset was stable and a value of 10db was assumed                                                         
for all stations.  The table below shows differences between the  pressures                                                         
determined from pairs of reversing thermometers (protected and unprotected)                                                         
and  simultaneous  observations  of CTD pressure.  For stations  where  the                                                         
bottom  is  flat the depth of the CTD can be estimated from  the  corrected                                                         
water  depth minus the height of the echosounder above the bottom.  This is                                                         
converted to pressure and compared with the measured CTD pressure.  11 such                                                         
determinations  gave the mean difference 3db (CTD lower) and an rms of  6db                                                         
very similar to the reversing thermometer values.                                                                                   
                                                                                                                                    
TEMPERATURE:  Laboratory  calibrations  of January 1982  and  October  1982                                                         
showed  the  CTD  temperature sensor to be stable.  By comparison  with  57                                                         
reversing thermometer measurements between 2 and 23 degC the corrected  CTD                                                         
temperatures  were  determined  from the equation T = 0.047 +  0.0005002  *                                                         
RAWTEMP.                                                                                                                            
                                                                                                                                    
SALINITY:  During the cruise,  bottle salinities and reversing  thermometer                                                         
measurements  revealed  a linear potential temperature (POTT)  -  practical                                                         
salinity (S) relationship for POTT less than 2.6 degC of S = 34.698 + 0.098                                                         
* POTT.  The cell factor required to bring provisional CTD salinities  into                                                         
agreement  with  this relation was determined for each station.  The  table                                                         
below  compares salinities derived from the rosette sampler  and  Guildline                                                         
salinometer  with the CTD values.  If a constant cell factor had been  used                                                         
the  deviations  from  the POTT-S relation measured with  the  CTD  may  be                                                         
plotted  - this indicates that the stability of the CTD is good excluding 3                                                         
outliers.  These are stations 10556,  10563 and 10567 and may be the result                                                         
of partial fouling.                                                                                                                 
                                                                                                                                    
OXYGEN:  From  a sample value of oxygen concentration  (determined  by  the                                                         
Winkler  method)  the  fractional   saturation   content  was  obtained  by                                                         
normalising it by the appropriate saturation value. The Beckman oxygen cell                                                         
also measures fractional saturation F, given by the equation:                                                                       
                                                                                                                                    
           ln F = ln I + ln C + aT + bp                                                                                             
                                                                                                                                    
      where   I = oxygen probe current                                                                                              
              T = temperature                                                                                                       
              p = pressure                                                                                                          
                                                                                                                                    
A least squares fit of 46 data sets gave C = 1.47E-03,  a = -0.035 per degC                                                         
and b = 1.425E-04 per db.  The electrode current exhibits a time  dependent                                                         
lag which has been corrected using the expression:                                                                                  
                                                                                                                                    
LAG (secs) = 70 exp -0.1 x T (ambient) degC.                                                                                        
                                                                                                                                    
This  equation is a reasonable fit to the behaviour of the  electrode  from                                                         
3000m to 5000m where the water is nearly isothermal (2.5 degC - 3.0 degC).                                                          
                                                                                                                                    
There  are  some  spikes in the oxygen profiles,  particularly  during  the                                                         
upcasts.  These  are probably the result halting recovery of the CTD  while                                                         
collecting a bottle sample. These spikes have not been flagged.                                                                     
                                                                                                                                    
TRANSMITTANCE:   Potential  transmittance,  which  takes  account  of   the                                                         
increasing  mass  of  clear  water in the 1m path of  the  instrument  with                                                         
increasing pressure, was calculated.                                                                                                
                                                                                                                                    
                                                                                                                                    
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           +3               7          14                                                             
                  2000-5600 db           -2               6          28                                                             
                                                                                                                                    
Temperature, degC    5-23 degC          +0.005          0.012        33                                                             
                     2-5 degC           -0.0005         0.005        24                                                             
                                                                                                                                    
Salinity, PSU        0-2000 db           0.003          0.007        48                                                             
                  2000-5600 db          -0.002          0.003        39                                                             
                                                                                                                                    
Oxygen, ml/l         0-2000 db           0.25           0.30         23                                                             
                  2000-5600 db           0.0            0.16         23                                                             
                                                                                                                                    
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).                                                                                
                                                                                                                                    
                                                                                                                                    
References:                                                                                                                         
                                                                                                                                    
Saunders, P.M. (1984).  CTD data from the north east Atlantic Ocean 31 degN                                                         
   -   46  degN,  July  1982.  RRS  Discovery  Cruise  130.  Institute   of                                                         
   Oceanographic Sciences, Report No. 165                                                                                           
                                                                                                                                    
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                                                                                                                       
