A full ocean depth UV-spectrophotometer for nitrate measurements

The instrument presented, named SUV-6, uses the well-known method of UV-absorption measurement for the determination of nitrate concentrations in seawater. To account for the presence of other constituents that show absorption in the UV portion of the light spectrum (e.g. bromide and bisulfite) the absorption is measured in six wavelength bands from 205 nm to 280 nm. While the original instrument, developed in the 1990s suffered from drift due to temperature changes and changes in the spectrum emitted by the light source, the new SUV-6 compensates for these drifts by employing an internal reference path. The instrument consists of a Xe-flashlamp as light source, collimating optics, pressure window and retroreflection prism, a ruled grating and a photodiode array. It is microprocessor controlled and allows the user to select either RS-232 or analog outputs. The fast response time of 1 s makes it suitable for use on towed vehicles, such as SeaSoar (at a tow speed of 8 knots 1 Hz results in a spatial resolution of 4 m), autonomous underwater vehicles and for CTD casts. Results of laboratory calibrations and first field tests are presented.