**Documentation for CTD data collected on RRS Challenger Cruise 15/87  (May                                                         
-  Jun 1987) 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.                                                                                                                   
                                                                                                                                    
PRESSURE: The calibration equation based on laboratory measurements was                                                             
taken to be:                                                                                                                        
              P = 0.1 * PRAW - 7.0                                                                                                  
No  attempts were made to test these estimates with reversing  thermometers                                                         
on  this  cruise.  A small systematic underestimate of  pressure  at  depth                                                         
arises  from the temperature sensitivity of the strain gauge sensor.  Since                                                         
its  magnitude,  3.5db for 10degC change,  is small no correction has  been                                                         
made.                                                                                                                               
                                                                                                                                    
TEMPERATURE: Laboratory calibrations were carried out after the cruise and                                                          
the following equation obtained:                                                                                                    
              T = 0.00049953 * TRAW + 0.026                                                                                         
This  has remained virtually unchanged since the purchase of  the  platinum                                                         
resistance  sensor  in  1983: thus only a few comparisons  were  made  with                                                         
reversing thermometers - these are shown in the table below.                                                                        
                                                                                                                                    
SALINITY:  Salinity is not calibrated in the laboratory at IOS:  hence  the                                                         
cell  factor is carried forward from cruise to cruise.  A value of  0.99971                                                         
was adopted to calculate salinity on-board ship.  Approximately 380 samples                                                         
of salinity were analysed on the ship using a Guildline Bench  Salinometer,                                                         
standardised  against IAPSO Standard Seawater (batch P105,  K15 = 0.99988).                                                         
This  revealed that the nominal salinity of the CTD needed to be  corrected                                                         
by between 0.009 and 0.015 during the cruise.  The table below compares the                                                         
salinities derived from the rosette sampler with the CTD values.                                                                    
                                                                                                                                    
OXYGEN:  Due  to  a  defective sensor and the lack of  a  spare  no  oxygen                                                         
measurements were made.                                                                                                             
                                                                                                                                    
TRANSMITTANCE: Potential transmittance was computed.  The instrument  SN035                                                         
was  employed  and  corrections   were   made   for  the  air  calibration,                                                         
manufacturer's  provided  sensitivity,  change  in  refractive  index  with                                                         
pressure,  temperature  and salinity and for the increase in mass of  clear                                                         
water in the 1m path of the instrument with increasing pressure.                                                                    
                                                                                                                                    
Fit of CTD Data to Rosette Sample Values                                                                                            
                                                                                                                                    
                          Difference between CTD and Rosette Measurements                                                           
                                                                                                                                    
   Variable          Range          Mean difference     R.M.S.     Number                                                           
                                                                                                                                    
Temperature, degC    < 3 degC           - 0.001         0.002         7                                                             
                     > 3 degC           - 0.005         0.005         4                                                             
                                                                                                                                    
Salinity, PSU        0-2000 db          + 0.0002        0.0024      380                                                             
                                                                                                                                    
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.                                                                                                    
                                                                                                                                    
It  is noteworthy that the provisional salinity data required more  editing                                                         
than  usual.  Very  sharp  temperature gradients occurred at  a  number  of                                                         
stations, especially 15062 and 15063,  where falls of 5degC occurred within                                                         
10m. In these sharp and prolonged gradient regions salinity spikes appeared                                                         
despite  the  algorithms developed to take account of  the  different  time                                                         
constants  of the temperature and conductivity sensors.  These spikes  were                                                         
removed  by  the median sorting technique described by Pollard  (1985)  and                                                         
linear interpolations made. A very small number of poor values survived but                                                         
were picked off when temperature-salinity plots were made.                                                                          
                                                                                                                                    
Derived quantities were computed from algorithms published by Fofonoff  and                                                         
Millard (1983). To remove the effect of ships heave data cycles were sorted                                                         
by  pressure before all values were averaged at 5db intervals,  centred  on                                                         
2.5db, 7.5db...                                                                                                                     
                                                                                                                                    
References.                                                                                                                         
                                                                                                                                    
Saunders, P.M. and Gould, W.J. (1988).  CTD data from RRS Challenger Cruise                                                         
   15/87  around  the Faroe Islands.  Institute of  Oceanographic  Sciences                                                         
   Deacon Laboratory, Report No. 256, 79pp.                                                                                         
                                                                                                                                    
Pollard,  R.T.  (1985).  CTD data from Northeast Atlantic collected on  RRS                                                         
   Discovery  Cruise  132  in  February 1983.  Institute  of  Oceanographic                                                         
   Sciences Deacon Laboratory, Report No. 192, 99pp.                                                                                
                                                                                                                                    
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.                                                                                                                      
