The Precision Salinometer calculates the salinity of a water sample from the measurement of the electrical conductivity and the determination of its temperature. The calculations are based on the Practical Salinity Scale 1978 (PSS78) as published by UNESCO.
The temperature of the sample water cannot be measured directly. Since small sample volumes are desired, a direct measurement would interfere with the temperature of the sample by thermal interactions with the sensor. Furthermore, it is essential to assure a homogeneous temperature distribution in the conductivity cell. Therefore, a controlled water bath, integrating conductivity cell and temperature sensor, is required.
A key element to achieve the outstanding accuracy of the Precision Salinometer is thermal control. This includes several innovative aspects. The first is the introduction of a pre bath, which adjusts the temperature of the water sample very close to the temperature of the main bath. By this, the main bath is essentially isolated from the original temperature of the water sample and can be kept very homogeneous and stable. The pre bath is, similar to the main bath, fully PID controlled. The second aspect is the realization of the heater and cooler in the baths. The stirring element serves as the heat source, both in the pre and main bath. This distributes the heat input rapidly and evenly throughout the entire bath volume. If more heat is required, the rotation frequency is increased. The cooling is realized using a Peltier element with a specially shaped surface and a controlled heat resistance across the element. Both measures result in a very homogeneous distribution of temperature in the bath. The third aspect builds on this property. It is an extremely precise and fast detection of the thermal drift in the main bath. This allows a fast reaction using small amounts of heat input. The forth aspect is the elimination of uncontrolled heat input into the bath, as for example the radiant energy of a light source. Even an apparently modest amount of radiant light energy is adverse to the measurements on the precision level achieved here. Therefore, lights in the bath are switched off during the measurements. Aspect five is that the temperature of the main bath is measured at sub-millikelvin accuracy and even better short term precision. This temperature measurement is used together with the conductivity measurement of the water sample to calculate salinity according to PSS78.
In order to determine the temperature of a water sample indirectly by the measurement of the temperature of the surrounding water bath, a maximum drift of the bath temperature must not be exceeded. It is this drift, which determines whether measurements are permitted or not; it is not a deviation from a pre set temperature. The threshold of the permitted temperature drift in the bath depends on the thermal time constants of conductivity cell and temperature sensor. No attempt is made to keep the temperature of the bath constant – even though in practice the drift is smaller than 1 mK over a few hours.
The conductivity cell can and should be calibrated ('standardized', i.e. performing a one point calibration) prior to a measurement session using IAPSO Standard Seawater. The temperature sensor in the main bath shows high accuracy and long term stability. Therefore a calibration is required in long term intervals only.
All important parameters, including the actual bath temperature and its drift are recorded with conductivity and salinity of a water sample. This assures and proves that measurements are valid and at the same time eliminates the common operator specific noise and offsets in salinity determinations.