CTD cast

Figure 5b. Nitrate concentration (gM) data compiled from transects across warm core Ring 81 G. The data are from continuous analysis of pumped water with an autoanalyzer and from discrete measurements of bottle samples during CTD casts. Note the different shape of nitrate-rich intrusions.

For modern devices, the time of measurement is known very accurately, contrary to earlier observations of which it is not always known whether UTC or local time is reported. Sometimes, only the day of measurement was given. A typical CTD cast in the Baltic Sea takes perhaps 5-20 min. For BALTIC, we have used the station start time for aU samples of... 

Cud) was measured In the water column off the Florida Coast on the SOLARS III cruise. Results from a cast taken at a station west of Tampa are shown In Figure 6. The cast was made at 1400 on a sunny, flat calm day optimal conditions for photochemical activity. Hydrographic data, obtained during a CTD cast Immediately afterwards Indicated a well stratified water column. The profile Indicates that Cu(I) Is present at the surface where It constitutes about 15Z of the total copper. The concentration declines with depth and Is below the limit of detection at 90m. The H 02 profile shows similar characteristics, with a surface maximum. 

As a CTD cast starts, the conductivity cell is the sensor to identify best the transition from air to water because of the large gradient from zero measurement at deck to ocean surface values. However, waves and swell will not allow the CTD to be immersed into the water... 

Salinity-Temperature-Pressure (Depth). Conductivity, temperature and pressure data were collected using a Plessey Systems CTD with model 8400 data logger. These sensors were interrogated five times per second for values of temperature, conductivity (salinity), and pressure (depth). Data were recorded during the down-cast using a lowering rate of 30 m min". Niskin bottle samples were taken on every other cast to provide temperature and salinity calibration. Nominal precision of the salinity, temperature and depth measurments was 0.02 g kg", 0.02 °C and 0.2 m, respectively. 

Rinse both the CTD and the rosette intensively with fresh water to avoid the formation of salt crystals that may falsify the tirst measurements on the next cast or block mechanical parts of the rosette. 

In situ calibration of the conductivity sensor starts with calibration of the temperature and pressure values in the up profile at bottle closing pressures. Next, in situ reference conductivity Cref is derived from bottle salinity 5ref, T and P. ff necessary, Ccro is compensated for pressure and temperature effects according to Eq. (3-14). The final correction Cc=Cref-Ccc depends on the sensor and the type of the CTD and on Ccxi the drift in terms of a group of cast numbers may be included and further pressure correction may be needed. As an example, the corrected conductivity C may be written as ... 

The coefficients in Eq. (3-15) are derived from linear regression. The calibration is applied to the lowering profiles and salinity is calculated. An example of a conductivity ceU calibration according to Eq. (3-15) is shown in Fig. 3-6. The coefficients were calculated for a group of 75 casts. The overall accuraQr is estimated to better than 0.005 in calibrated CTD salinity. It may be improved by splitting into smaller cast groups. 

If the pressure sensor of the CTD responds to fast changes in temperature, it is corrected for according to the results of its dynamic calibration (Section 3.6.3). The correction starts with the pre-cast deck values thus assuming that the sensor is close to thermal equilibrium. 

Next, the required minimiun downward velocity is pre-set, e.g., to half of the lowest nominal lowering speed during the cast, e.g., 0.5 m/s at the beginning of the cast. For each record, the CTD s actual downward velocity is estimated from the difference of the two adjacent pressure values and the corresponding time interval. If it is less the pre-set minimum velocity criterion and if the pressure difference to the last record accepted as good does not exceed one fifth of the interpolation interval, it is flagged. Finally, all flagged records are removed. 

Fig. 3-6. Conductivity/salinity calibration for a CTD (METEOR cruise M39/2). Upper row shows the corrections (CREFC-CTD) which is needed to match the conductivity measurements CCTD to the in situ reference value CREF as a function of the cast number (left) and pressure (right), the second row the residuals (CREF-C) after calibration of conductivity C. The third and fourth rows are corrections in salinity (SREFS-CTD) and residuals (SREF-S), respectively. Accuracy in C (95% confidence level) for all 74 casts is twice the root mean square error of the linear least square approximation, 0.0004 S/m (Siemens/m), corresponding to 0.004 in salinity. The result would be improved if the obvious cast-dependancies (see lower left part of panel) could be removed.

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