The optical refractometer project OPRA-a further development of an in situ going optical density measuring sensor

Currently there are several initiatives to look for alternative methods to determine the salinity or density of seawater especially for long term unattended measurements. EUROGOOS, the European initiative of the world-wide observation program GOOS, has established a working group devoted to evaluate different approaches. One way that has been considered is the refractometer principle. The attractive feature of this principle lies in the fact that there is no direct interaction like in the case of conductivity measurements of the sensing elements with the surrounding fluid and the sensor head can be streamline shaped. This allows for a better flushing of the measuring volume. On the other hand due to the high demands in the design of the optical imaging system and the photodetector module most designs are still in an experimental phase or just exist as an idea. Furthermore a now equation of state has to be developed as there are up to now no reliable reference data. Facing this problems a redesign of an existing in situ going refractometer has been started that shows new solutions to the above mentioned problems. The basic principle that has been described in earlier OCEANS papers is still the same. But the redesign of the sensor head has gone through some major changes. The new geometry allows measurements in water and in air without replacing any components. This is of utmost importance for the calibration procedure. Furthermore dedicated modules as for example a hybrid photodetection unit has been developed to ease a production of the sensor by a company. In this paper the performance in laboratory and under field conditions of the redesigned sensor is described