Oceanography

NISKIN WATER SAMPLER

Niskin bottles. Open Niskin bottles are attached to a cable and lowered to water depths where seawater samples are to be obtained for chemical analysis. A metal messenger “trips” each bottle on the cable individually, causing it to fill with water and close securely.

A rosette cluster. Water collecting bottles are arranged around a rigid, circular frame in a rosette pattern. Technicians are able to close the bottles individually as the array is lowered or raised through the water column.

Sampling Depths

Chemists must establish the exact sampling depth for each bottle. Otherwise, the analytical work, no matter how accurate, is of limited use in determining the exact chemical structure of the water column. A common technique is to measure the length of the cable between the ocean surface and the depth at which the bottle was triggered by the messenger. However, the cable rarely hangs straight down, because of the drift of the ship relative to the bottles on the cable. Depth corrections are applied by measuring the angle of the cable and by noting the difference between the temperature readings on the pressure-protected and unprotected thermometers mounted on the sampling bottles. (Temperature discrepancies are indicators of water pressure,which is a function of water depth). When near-bottom water samples are collected, it is customary to attach a pinger(a pulsing sound source) to the free end of the cable. Sound signals reflected off the sea floor and transmitted to the ship are used to determine the distance between the pinger and the bottom to within a meter or so.

Analytical Procedures

Analytical procedure reveal temperature and salinity of water. recorded in the reversing thermometers, which are fastened to water-sampling bottles, Better precision (up to 0.0001°C) is obtained by using temperature-sensitive materials, such as quartz crystals, which vibrate at frequencies that depend on temperature. These signals are transmitted electronically to the ship. This allows the temperature of the water to be monitored continuously as the instrument is lowered.

Because the composition of seawater is constant, chemists traditionally have determined water salinity by chemical titration-the process of standardizing silver nitrate against a normal seawater sample of known chemical composition.The electrical conductivity of seawater, which is proportional to the total concentration of dissolved ions, is now used routinely to determine salinity rapidly. The salinometer compares the electrical conductivity of an unknown sample with that of a known, standard sample of seawater, and converts the difference into a salinity value after correcting for temperature effects. An important instrument called the CTD (conductivity, temperature, depth) consists of a salinometer, an electronic thermometer, and a pressure sensor. As it is lowered through the water column, the CTD transmits electronic signals to the ship, where they are stored in a shipboard computer for analysis later.