               CTD CALIBRATION: CRUISE JAMES CLARK ROSS 17
                 (M.A. Brandon, P. Woodroffe, T. Marwood)

1) Summary
In  all CTD stations the 2 dbar averages of the downcast data are  reported
as  the final product.  In some cases the 1 db and 3 db levels are  missing
from the final file.  In these cases the shallowest level with data present
was copied to these pressure levels.


2) The CTD equipment
The  CTD  unit used for the measurement program was the BAS Neil  Brown  Mk
IIIb (serial number 01 - 3868 - 2086). The most recent calibration had been
carried  out  by  Chelsea  Instruments on 12 September 1996.  The  CTD  was
mounted in a purpose built frame with a General Oceanics 12 position bottle
multisampler  rosette.  On  each position on the rosette  was  a  10  litre
General  Oceanics  sampling bottle.  For the near bottom CTD  stations  the
package  was  fitted with a 10 kHz pinger to enable  accurate  near  bottom
approach. On three of the 10 L bottles were SIS Temperature Sensors.  These
were  in two pairs,  serial numbers T711 and T713,  and serial numbers T715
and T716, with serial number T717 alone.

Deployment  of the CTD underwater package was from the midships gantry  and
A-frame  on a single conductor,  torque balanced cable.  This CTD cable was
made  by Rochester Cables and was hauled on the 10T traction  winch.  There
were no significant problems deploying the CTD package as close control was
maintained  with  the  gib arm and two hand lines whilst  the  package  was
suspended  above the surface.  On one occasion (station 078) the  CTD  wire
came  off the roller at the top of the gib arm at the start of a cast.  The
package  was  lowered  to  the deck,  the problem  cured  and  the  package
successfully deployed.

CTD  data were logged via a Neil Brown Instrument Systems deck unit,  model
1150,  to a 386 Viglen PC running E.G.  and G.  Marine Instruments CTD data
acquisition  module version 2.02 control software,  and also to the RVS ABC
system through a dedicated microcomputer.  The CTD level A,  mainly through
historical reasons,  averages the data at this point to 1 second values and
passes  the data through a simple editing procedure.  During  this  editing
procedure  pressure  jumps  of  greater than 100 raw units  (e.g.  for  the
pressure  transducer equivalent to 10 db) are removed along with spikes  in
individual channels through a median sorting routine. The rate of change of
temperature change over 1 second is also calculated.  These one second data
are  then  passed  to  the ship's UNIX  system  and  archived.  Calibration
routines are then applied to the data and are described below.

2a) Bottle problems
On coming onto the ship in November it was noted that one handle on the CTD
package was broken.  This handle was changed before the cruise.  During the
cruise one more handle broke and was changed.  Unfortunately,  the changing
of  this handle led to bottle 5 being out of action for three CTD  stations
as Brandon lost a crucial spring whilst changing the handle.  P.  Woodroffe
inspected  the  other  bottle handles and replaced a further  four.  Whilst
completing  this  task  it was noted that he failed  to  drop  any  crucial
springs.  On  a few occasions the bottles did not seal properly and  leaked
when the package was brought on deck. When this occurred the leaking bottle
was  noted on the logsheet and the sample treated as suspect.  At  stations
150, 155 and 157,  the reversing thermometers on bottle 5 (711 and 713) did
not trigger properly. This problem was cured for subsequent stations.

2b) Bottle Pylon misfires
The bottle rosette was controlled via a General Oceanics RMS MKVI 1015 - PM
controlling unit.  There were several misfires indicated on this unit  that
were indicative of the coming failure of the termination.  The full list of
bottle misfires is in Table 1.

2c) Reterminations
There  were  four reterminations on the CTD package.  These are  listed  in
Table  2  and are of two types.  An electrical retermination was  when  the
electrical  part of the connection was remade;  the mechanical  termination
was  left  intact.  Some concern was expressed at what was felt was a  high
number of reterminations being required and rotation of the package  during
deployment was thought to be straining the electrical connection.  In a bid
to  remove this rotation,  after station 237 the conducting cable was  sent
down to 3000 m with a weight attached to try and remove some turns from the
wire.  On the final electrical retermination a potting compound was used to
try and make the electrical termination more robust.

2d) 10 kHz pinger
The  10 KHz pinger was not fitted until station 17ctd157 (MEB 22)  as  this
was  the first near bottom station.  It worked well throughout the rest  of
the cruise.

3) The calibration of the CTD
As  stated,  the BAS Neil Brown MK IIIb serial number 01 - 3868 - 2086  was
used for all CTD stations. This unit was calibrated on 12 September 1996 by
Chelsea  Instruments  and  we  use values from  this  calibration  for  the
pressure  and temperature.  The conductivity sensor was calibrated  against
in-situ salinity samples from the GO water bottles. We report three sets of
coefficients  for the conductivity and this is described in greater  detail
below.

3a) Temperature calibration
The temperature calibration was derived by Chelsea instruments using  eight
temperatures on the ITS-68 scale between 2 deg and 30 deg C and was applied
to the data through the following equation.

   T = 0.0004955T(raw) - 2.101                                          (1)

To  convert  from  the ITS-68 scale to T90  following  Saunders  (1990)  we
multiplied all temperatures by 0.999760057, so

   T = T x 0.999760057                                                  (2)

To allow for the mismatch in response times between the temperature  sensor
and conductivity sensor, following the standard procedure,  the temperature
was lagged for the salinity calculation.  This lag was achieved by adding a
fraction delta of the rate of change of temperature that is output from the
level A(dT) to the temperature. The temperature is then

   T(new) = T + delta dt                                                (3)

From  experiment  the spiking in the derived salinity  was  minimised  with
delta = 0.15.

3b) Pressure calibration
A  pressure  calibration derived by Chelsea Instruments from  11  pressures
between 0 and 6000 m was applied through the following equation

   P = 0.0998569P(raw) - 12.11238                                       (4)

Following King and Alderson (1994) the pressures were then modified by  the
addition of a factor deltaP, to take into account the effect of temperature
on the pressure sensor so that

   P = P + deltaP                                                       (5)

And deltaP is calculated from

   deltaP = -0.4 x (T(lag) - 20.0))                                     (6)

Here  T(lag) is a lagged temperature in deg C and is constructed  from  the
CTD temperatures.  We use a time constant for the lagged temperature of 400
seconds and update the temperature following the method put forward in King
(1996). If T is the CTD temperature and t(del) the time interval in seconds
over which the temperature is being updated, and T(const) our time constant
of 400 seconds then the factor W is

   W = exp (- t(del)                                                    (7)
              _____
              T(const))

and now

   T(lag)(t=t(o)+t(del)) = W x T(lag)(t=t(o)) + (1-W) x T(T=T(o) + t(del))
                                                                        (8)

We  finally make an adjustment to the upcast pressure to take into  account
hysteresis in the sensor. The extent of the hysteresis was calculated using
a series of laboratory measurements.  The hysteresis after a cast to 5500 m
(which  we  denote  by dp5500(p)) is given in Table 3.  These  values  were
derived from a laboratory calibration at IOSDL in 1994. Intermediate values
are found by linear interpolation.  If the pressure of the cast is  outside
the  values in Table 1 then dp5500(p) is set to zero.  For a cast in  which
the maximum pressure reached is P(max) dbar,  the correction to the  upcast
CTD pressure (p(i)) is

   P(out) = (dp5500(p(i)) - (((P(i)) x dp5500 (p(max)))                 (9)
                              ______
                              P(max)

3c) Salinity (conductivity) calibration
We  first describe the principal of our method and then detail  the  steps.
For  this  cruise  we calibrated the conductivity against  in-situ  samples
collected  with the GO multisampler rosette.  Once the conductivity of  the
CTD  was  calibrated we derived salinity.  A full data processing route  is
detailed at the end of this report.  In brief,  first we applied a  nominal
calibration of the form

   cond = 1 x cond(raw) + 0.0                                          (10)

From  the salinity samples,  once successfully matched,  we calculated  the
bottle  sample conductivity using in-situ temperature and pressure from the
CTD.  From  this in-situ conductivity we calculated the difference  of  the
bottle  conductivity (cond(b)) and CTD conductivity (cond(ctd)) to derive a
value deltaC.  We now plot bottle conductivity (x variable) against  deltaC
(y variable). This should give a straight line wherefrom

   y = mx + c                                                          (11)

We get

   deltaC = m cond(b) + c                                              (12)

After rejecting suspect bottles we use the pstar programme plreg2 to derive
m and c for deltaC. Now, as

   deltaC = cond(b) - cond(ctd)                                        (13)

the  calibration coefficients for the CTD conductivity are derived  through
substituting equation (13) into (12), the CTD conductivities are now

   cond(ctd) = a + b cond(raw)                                         (14)

and from the m and c in equation (12)

   a =   c                                                             (15)
       _____
       1 - m

and

   b =   1                                                             (16)
       _____
       1 - a

These  values for a and b are entered into the calibration files  for  both
the pstar and RVS system. The processing route is then repeated and the new
graph  of  deltaC  against cond(b) gives the  conductivity  residuals;  the
residuals  should  now  be random with a mean  of  zero.  This  calibration
procedure does have a feature in that,  as we moved south along the section
and  moved  into  waters  where  the  entire  water  column  was  of  lower
conductivity  than  the  station  used for  the  initial  calibration,  the
validity  of  the  original  m and c are called into  question  because  of
extrapolation.  Accordingly, we used three sets of coefficients for a and b
that are detailed in Table 4. After applying these calibration coefficients
to  the  relevant  stations  there is still a  residual  drift  within  the
conductivity signal with time. For each station this drift is

   deltaC = residual drift

From  substitution  into  our original equations we  can  now  remove  this
residual drift.

3d) Salinity Samples
Salinity samples were taken for all of the CTD casts made for the  physical
oceanographic  program.  For  the  22 stations of the  Maurice  Ewing  Bank
section  18  samples  were taken from the GO 10 L bottles.  This  gave  one
sample for each bottle plus six duplicates. For the core boxes around South
Georgia  this  was reduced to nine samples for each station.  This  gave  a
total of 420 samples with 148 duplicates (568 in total).  The samples  were
taken  in  300 ml medicine bottles,  each bottle being rinsed twice  before
being filled to just below the neck.  The rim of the bottle was then  wiped
with  tissue,  a  plastic  lid inserted and the  screw  cap  replaced.  The
salinity  samples  were placed near to a salinometer to  allow  the  sample
temperatures  to  equalise  with the salinometer.  The  samples  were  then
analysed  on  the  BAS  Guildline  Autosal  model  8400  S/N  45363.   This
salinometer  was  serviced and electronically aligned by  Ocean  Scientific
International  In  June  1996.  For each CTD station's  worth  of  salinity
samples (18 samples) one vial of OSIL standard seawater (batch P130,  1996)
was  run  through  the  salinometer to enable a calibration  offset  to  be
derived.  Once analysed,  the conductivity ratios were entered by hand into
an  Apple Macintosh based EXCEL spreadsheet using software written  by  Dr.
Brian  King (SOC) before being transferred to the UNIX system as  described
below.  For the 148 duplicate samples the mean difference was 0.000 and the
standard deviation 0.001.

4) The quality of the conductivity calibration procedure
After applying the calibration coefficients and adjusting for the  residual
offset  deltaC,  the salinity of the bottle sample was differenced with the
derived CTD salinity.  After rejecting,  the mean of the remaining  samples
was 0.0000 with a standard deviation of 0.0018 psu. The drift of the sensor
was small.


Table 1. Misfires on the General Oceanics MK IV 1015 - PM

  Cast     Misfire position     No. of Misfires
   045            12                  1
   110            All              numerous
   125            12                  1
                  11                  1
                  10                  1
   237            12                  3
                  11                  2


Table 2. Reterminations of the CTD conducting cable

  After Station     Type of Retermination
       110                  Full
       125               Electrical
       210                  Full
       261               Electrical


Table 3. Table of hysteresis corrections in the pressure sensor

p(dp)     dp5500(p)db
 0.0          0.0
 100          2.7
 200          3.9
1000          5.9
1500          6.3
2000          5.8
2500          5.7
3000          5.1
3500          4.5
4000          3.7
4500          2.4
5000          1.5
5500          0.0


Table 4. Calibration coefficents used for the conductivity calibration

  calibration number      a             b            from station
          1           0.0146667     0.916304          17 ctd 061
          2          -0.0326956     0.917617          17 ctd 063
          3          -0.080606      0.919089          17 ctd 237
