                  CTD CALIBRATION: DISCOVERY CRUISE 169                    
                                                                           
1. CTD/Transmissometer (N. Hooker)                                         
1.1 Operation                                                              
The IOS Neil Brown deep CTD system gave some early problems.  At the  trial
Station (11639) the pressure calibration adopted was found to be incorrect,
and  conductivity was erratic.  The former was amended after an exchange of
telex  messages,  and the conductivity behaviour vanished after  inspection
but  was  eventually  (after  Station  11642) found to be  due  to  a  poor
connection  in  the sea unit.  A second similar fault was  corrected  after
Station 11654.                                                             
                                                                           
For  the  majority of the casts however,  the system worked  correctly  and
well.  Data  were  recorded  on the Digidata  mostly  for  down-runs  only,
recording   continuing  on  the  level  C  for  the  full  dips.  A   small
transmissometer  drift  was observed with the up-runs  having  consistently
higher measured values of light transmittance than the down-runs,  but  the
main  variations  were clearly reproduced.  A pressure-operated pinger  was
attached  to the CTD to enable the system to be profiled to within  10m  of
the sea bed.                                                               
                                                                           
2. Calibration                                                             
2.1 Salinometer, Thermometers and Rosette Multisampler (J. Moorey)         
The rosette multisampler was used as a calibration check on every CTD  dip.
Six  water bottles were used and were fired in pairs,  the second bottle of
each  pair having a thermometer frame.  After each pair there was  a  blank
position  on  the multisampler to ensure that thermometer frames could  not
touch each other when in the horizontal position.  (In the past we have had
`hang-ups'  on thermometer frames which were only two spaces apart.  It  is
thought that the frames may swing violently even past the horizontal as the
CTD  is  hauled and,  if the frames are free to swing quickly back  to  the
reversed position, the temperature recording is not lost.  However,  if the
frames  are close enough to jam each other beyond the horizontal  position,
then  one or both sets of thermometers drain and the temperature recordings
are lost.)                                                                 
                                                                           
The  multisampler generally operated satisfactorily,  but on the  last  few
CTDs  the deck unit occasionally had to be fired twice before it  indicated
that  a bottle had fired.  Another problem was the occasional  snagging  of
thermometer frame lanyards.  This problem was almost completely overcome by
taping  lanyards to minimise loops and to fit spare water bottles  (without
end caps) in the blank positions to avoid lanyards snagging on the  rosette
frame.                                                                     
                                                                           
There  were  two sets of water bottles with thermometer frames,  and  these
were  used on alternate stations.  This allowed more time for the  incoming
thermometers to equilibrate to laboratory temperature before reading.  (The
forward  hydro  lab  is less well fitted for such work than  it  once  was,
presently housing the cooling system for the computer room). Each frame had
a  pair of protected thermometers and the `deepest' two frames each had  an
unprotected thermometer.  Some thermometers are readable to 0.002 deg C (-2
to  6  deg  scale) and some to 0.005 deg C (-2 to  13  scale).  Since  each
thermometer  was  read  by two people,  there were four readings  for  each
`frame'.  These  were  `corrected'  for the in-situ  values  and  the  four
readings averaged and quoted to 0.001 deg. Another advantage of two sets of
thermometers  was  that  a possible reading error could be checked  if  the
thermometers were still in the hydro lab.                                  
                                                                           
Two  salinity samples were taken from each bottle and measured on the  SUDO
Guideline  Salinometer.  The salinometer functioned well.  It was run at  a
bath temperature of 24 deg.  It was used and standardised 11 times from  15
to  30 August.  The standardised dial (Rs) setting ranged from a maximum of
688 to a minimum of 663, equivalent to a salinity change of 0.002 psu.  The
salinometer's  very  good stability is indicated by the  consistently  good
agreement of duplicates,  the second of the pair always being measured on a
separate day and hence separate standardisation.  Also, the salinities from
adjacent  water bottles showed good agreement indicating that there are  no
measurable leaks from the water bottles.                                   
                                                                           
Also on board was the IOS Guideline serial No. 42508. This was not used for
any  of the rosette samples but a set of various salinities was made up  by
diluting  with  distilled  water.  The conductivities were  such  that  the
salinity range was from circa 6.0 to 36.6 psu,  in the oceanographic  range
(31  to 36.6 psu).  The SUDO read higher than the IOS by only 0.001 psu (at
both 31 and 36.6).  In the low salinity ranges (down to circa 6.0 psu)  the
SUDO  read  higher  than  the IOS by only 0.004.  After 15  hours  the  two
salinometers were compared by using water of `about 35' psu.  The SUDO read
lower than the IOS by 0.001 psu.  This shows that the two salinometers  are
consistent  with each other over a wide range of conductivities although we
do not know the absolute values of those conductivities.                   
                                                                           
2.2 CTD Calibrations for pressure, temperature and salinity (S. Boxall and 
J. Taylor, SUDO)                                                           
a) Pressure - Initial values of pressure as read by both the main shipboard
computer  and the BBC micro were low by a factor of 10,  though correct  on
the CTD deck unit.  This was due to necessary software corrections for both
machines  to  take account of the deep pressure sensor fitted to  the  Neil
Brown CTD. This problem was resolved after Station 11640.                  
                                                                           
Pressure  data  from  the   unprotected   reversing  thermometers  gave  no
indications  of  any  problems  with the pressure  transducer.  However,  a
constant  offset of approximately 8 db is present in the IOS CTD such  that
at 10m wire out, a reading of about 2 db was given.  This offset drifted by
about 3 db during the cruise.                                              
                                                                           
b)  Temperature - The temperature sensors on the CTD performed well,  apart
from an intermittent fault following Station 11654 which was remedied. This
fault did not seem to affect the calibration of that Station.              
                                                                           
The  trend of the difference between CTD and thermometer values is  linear,
increasing  with  increasing temperature,  with a spread of values about  a
line of 0.007 deg C. Most points lying outside this limit can be counted as
suspect due to samples being taken in a high gradient of temperature or  to
inconsistent readings on paired reversing thermometers. A linear correction
(estimated by eye) of                                                      
                                                                           
   T(correct) = 1.0065T - 0.002 deg C                                      
                                                                           
where T is the CTD uncorrected temperature                                 
                                                                           
is recommended from this data set at the present time.                     
                                                                           
This was later compared to two other calibrations. The first, of unknown   
origin, is                                                                 
                                                                           
   T(correct) = 1.0063T - 0.0015 deg C                                     
                                                                           
which is in good agreement with the above,  giving no resolvable difference
in  the deeper water (up to approximately 5 deg C),  and the difference  of
0.002 - 0.003 in the intermediate water masses at about 10 deg C.          
                                                                           
The value used on the computer was                                         
                                                                           
   T(correct) = 1.0074T - 0.0081 deg C                                     
                                                                           
This  does not fit the data set as well,  giving errors of 0.005 deg  C  in
deeper waters and shallower waters alike.                                  
                                                                           
There were no noticeable trends with time.                                 
                                                                           
c)  Salinity - In all salinity calculations the temperature  correction  of
1.0065T - 0.002 was used.                                                  
                                                                           
Some problems were experienced with the conductivity measurements from  the
CTD.   Readings  over  1ppt  high  were  initially  given  and  there  were
significant  jumps  in values.  The problem was traced to a  faulty  solder
joint  on  the conductivity interface board on the under-water  unit.  This
meant  that  any calibration of salinity was impossible  prior  to  Station
11644.                                                                     
                                                                           
Stations  11644-11655 demonstrated a better performance but also  showed  a
slow  calibration drift in time,  from approximately 0.24ppt low at Station
11644 up to 0.40ppt low at Station 11654.  The drift was neither linear nor
consistent,  and it is uncertain whether applying an individual  correction
for each station will produce reliable results.  Though this may have to be
done  as  a  last  resort,  a better suggestion would be to  correct  to  a
standard water mass within each profile.                                   
                                                                           
After Station 11655 the CTD was stripped down, checked and the conductivity
cell  thoroughly cleaned.  Results from 11661 (the next CTD  Station)  were
good. A plot of sample minus CTD salinity vs station number shows the above
problems  and  the consistency of calibration with time for  Station  11661
onwards  very  clearly.  A  plot of delta  s  (Bottle  salinity  minus  CTD
salinity) vs salinity showed that with temperature there is a linear trend,
though  it  is  not quite so clearly defined as that  of  temperature.  The
recommended calibration for salinity, based on these data, is therefore    
                                                                           
   S(correct) = 0.989S + 0.590                                             
                                                                           
There are no salinity corrections implemented on the shipborne computer.   
