Feature ArticleBringing Laboratory Salinometry To Modern Standards
By Gereon Th. Budéus
Alfred Wegener Institute for Polar and Marine Research
Precise, reliable and stable laboratory salinometers are the key element for quality salinity measurements. They serve effectively as part of the salinity standard in oceanography, as standard seawater (SSW) cannot be prepared without using them (see Ridout, Sea Technology, June 2011). But they are also indispensable on board research vessels for immediate quality assessment of field data. While measurement techniques have improved constantly with respect to in-situ CTD instruments, progress on laboratory salinometers has stood still despite the fact that improvements were possible and necessary. Improved independence from environmental conditions (e.g., electromagnetic vulnerability, thermal fluctuations), full documentation of the measurement procedure, a clear indication of the instrument's status and automated sample processing have been on oceanographers' wish lists for many years.
With these needs in mind, a work group consisting of researchers from Optimare Sensorsysteme GmbH & Co. KG (Bremerhaven, Germany) and the Alfred Wegener Institute for Polar and Marine Research recently developed the Optimare Precis'ion Salinometer (OPS). First production samples of the OPS were tested for more than a year both at sea and in the laboratory.
Principles of Operation
Independent of actual and future developments of the definition of salinity, measuring electrical conductivity remains the standard method for salinity determination. The comparison between the conductivity of a well-defined potassium-chloride solution and the conductivity of seawater is the basis for the production of SSW. The OPS 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 the United Nations Educational, Scientific and Cultural Organization.
A technician works on a prototype of the salinometer. The automated intake is seen at the left, and the touch-screen display is integrated in the front.
The temperature of the sample water cannot be measured directly: Only small sample volumes are available, so a direct measurement would interfere with the sample's temperature by thermal interactions with the sensor. A further consideration is that it is not sufficient to know the temperature at one point of the conductivity cell—homogeneous temperature distribution must exist throughout the cell.
To achieve these requirements, the OPS uses a controlled and isolated immersion bath with an integrated conductivity cell and temperature sensor. Water is the most convenient liquid for the bath, as it shows a high thermal capacity, is readily available, and components can be disassembled without greasy residues.
A key factor to accuracy is superior thermal control. For in-situ ocean measurements, the most adverse effects are high pressures, rough handling, potential sensor fouling and temperatures far from laboratory specifications. For a laboratory salinometer, thermal effects on the immersion bath present the most severe problem.
The OPS's thermal management includes several innovative aspects. The first is the prebath, 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 its temperature can be kept very homogeneous and stable.
The second thermal management tool is the heater and cooler in the baths. There is no electrical heater in the main bath or prebath: A continuously running stirring element serves as the heat source in both. This method distributes the heat input rapidly and evenly throughout the entire bath volume by dissipation of mechanical energy. If more or less heat is required, the rotation frequency of the stirrer is increased or decreased. The reaction of this system can be very fast, and it does not possess a thermal mass. Cooling is achieved through a Peltier device with a specially shaped water contact surface.
The third tool is an extremely precise and fast detection of the thermal drift in the main bath. This allows a very fast reaction using only small amounts of heat input. The fourth tool is the elimination of uncontrolled heat input into the bath, for example, the radiant energy of a light source. Even an apparently modest amount of radiant light energy is adverse to the measurements at the high levels of precision the OPS achieves. Therefore, lights in the bath are switched off during the measurements.
The fifth aspect involves measuring the temperature of the main bath with millikelvin accuracy and submillikelvin short-term precision. The use of a thermometer with frequency output (a Wien bridge, i.e., a resistance-dependent oscillator) allows greater precision while maintaining a low noise level by simply evaluating a longer time interval. The temperature measurement is used together with the conductivity measurement of the water sample to calculate salinity according to the PSS78. The assembly described here is patented by inventor Klaus Ohm from Alfred Wegener Institute for Polar and Marine Research.
In order to determine the temperature of a water sample indirectly by measuring the temperature of the surrounding water bath, a maximum drift of the bath temperature must not be exceeded. This drift determines whether measurements are permitted. The threshold of the permitted temperature drift in the main bath depends on the thermal time constants of the conductivity cell and temperature sensor. In practice, the thermal mass of the conductivity cell is the limiting element. The OPS does not attempt to keep the temperature of the bath constant, though in practice the drift is smaller than 1 millikelvin over a few hours.
All important parameters, including the actual bath temperature and its drift are recorded together with conductivity and salinity of an evaluated water sample. This assures, and for the first time proves, that measurements of the salinometer are valid. Automated sample processing eliminates the common operator-specific noise and offset in previous salinity determinations.
Main Salinometer Elements
Sample treatment is highly automatized in the OPS. The bottle containing the water sample to be analyzed (or, during calibration, the SSW) is located underneath the intake. The intake is inserted into the bottle automatically when a new measurement starts. The filling pump sucks the sample water through a heat exchanger inside the prebath. The sample water then leaves the prebath with a temperature very close to that of the main bath. A Peltier device is located at the bottom of the prebath. The temperature sensor is a platinum thermometer. To continue this article please click here.
Gereon Th. Budéus has been involved in high-latitude oceanography and instrument development at Alfred Wegener Institute for Polar and Marine Research since 1992. He received his Ph.D. from the University of Hamburg and leads the Alfred Wegener Institute team developing the Optimare Precision Salinometer, which is partnered with Optimare.