Instruments and sensors to measure environmental parameters

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This article explains why instruments are needed to investigate oceanographic processes. It also explains the properties of available oceanographic instruments and sensors.

Measurement of environmental parameters

Figure 1 Temporal and spatial scales of ocean processes

The most simple approach to measure environmental parameters of water is to take samples and analyse them after returning to the laboratory. It is also a powerful approach as specialised laboratory equipment can be used to analyse a multitude of parameters. The main shortcoming of this approach is that only a limited number of measurements (samples) can be processed and the interval between samples taken at the same location (to gain information about the temporal variation) usually spans from weeks to months. Processes that occur on time-scales shorter than weeks or episodic and transient events are therefore not captured and the importance of these processes and events for the distribution of parameters cannot be assessed.

In oceanography there is a vast range of processes spanning many orders in time and space (see Figure 1). To investigate this range of processes a large volume of data has to be gathered on the appropriate time and space scales. To achieve this task instruments are needed that measure environmental parameters automatically in situ.

Oceanographic instruments

Introduction

An oceanographic instrument generally consists of one or more sensors and a signal processing unit that converts the sensor signal and carries out scaling and conversion to engineering units and the output data protocol. Figure 2 shows a schematization of an oceanographic instrument. The analyte (property to be measured) interacts with the detector (in some cases after a stimulus has been exerted by the instrument). The detector produces a signal, that is transformed into an electrical signal by the transducer. Detector and transducer together constitute the sensor. The electrical signal is fed to the signal processing (and conditioning) unit that creates the signal output of the instrument.

Figure 2 Schematization of a generalised oceanographic instrument

Oceanographic instruments can contain data loggers to store measurement data for readout after the deployment.

Important properties

• Accuracy: deviation of the measured value from the true value
• Precision: deviation of a measured value from another measured value of the same quantity (but at different environmental conditions (e.g. the two measurements taken at different temperatures)
• Resolution: smallest change in the measured quantity that can be detected by the instrument
• Measurement rate: number of measurements that can be carried out per time unit (e.g. measurements/hour)
• Power consumption: mean of electrical power uptake during deployment (usually in Watts [W])
• Deployment time: time period for which the instrument can be deployed (usually depends on environmental conditions, such as biofouling, or on stored energy and power consumption)

Sensors

Introduction

In an oceanographic instrument the stimulus can either interact directly with the detector (e.g. in a temperature, pressure or light sensor) or a stimulus is exerted by the instrument, then is modified by the property to be measured and the modified stimulus then interacts with the detector. For example, a fluorometer that sends out a light pulse (stimulus), which is transformed by chlorophyll fluorescence in the water (modification of stimulus); the transformed light (modified stimulus) then is interacting with the detector.

If the detector signal is of a property (such as colour) it can be converted to an electrical signal by a not an electrical signal (e.g. an optical signal or the change transducer). Detector and transducer together form the sensor.

Types of sensors

There are numerous sensors in oceanographic work:

Some of the most commonly used are

• Temperature sensors (under construction)
• Salinity sensors (under construction)
• Turbidity sensors such as

Secchi disk

Optical backscatter point sensor (OBS)

Optical transmissiometers (Theme 9 wanted page)

• Oxygen sensors
• Fluorescence sensors
• Multi-probe sensors (Theme 9 wanted page)

Less common are

• pH sensors
• Optical Laser diffraction instruments (LISST)
• Flow cytometers
• pCO2 sensors
• Acoustic point sensors (ASTM, UHCM, ADV)
• Acoustic backscatter profiling sensors (ABS)

Examples of specialized sensor systems are

• Nutrient analysers
• Trace metal analysers (Theme 9 wanted page)
• Measuring instruments for fluid velocity, pressure and wave height
• Measuring instruments for sediment transport
• Instruments for bed level detection
• Waverider buoys (under construction)
• Underwater video systems

Important properties

• Sensitivity: The smallest change in the property to be measured that leads to a measurable change in the detector signal.
• Selectivity: In how far the change of other properties than the one to be measured leads to a change in the detector signal. High selectivity sensors exhibit little influence of detector signal from changes in properties other than the one to be measured.
• Range: The span between the extremes of the property to be measured at which no further change in detector signal occurs.
• Linearity: A measure that represents in how far equal amounts of change in the property to be measured lead to equal amounts of change in detector signal.

See also

• Wireless sensor networks
• Coastal observatories
• Ships of opportunity and ferries as instrument carriers
• General principles of optical and acoustical instruments
• Currents and turbulence by acoustic methods
• Manual Sediment Transport Measurements in Rivers, Estuaries and Coastal Seas
• Light fields and optics in coastal waters
• Differentiation of major algal groups by optical absorption signatures
• Optical remote sensing
• Optical measurements in coastal waters
• Real-time algae monitoring
• The Continuous Plankton Recorder (CPR)
• ALGADEC - Detection of toxic algae with a semi-automated nucleic acid biosensor
• Application of data loggers to seabirds
• Acoustic monitoring of marine mammals
• Sampling tools for the marine environment

References

The main authors of this article are Schroeder, Friedhelm and Prien, Ralf Please note that others may also have edited the contents of this article. For other articles by this author see Category:Articles by Schroeder, Friedhelm For other articles by this author see Category:Articles by Prien, Ralf For an overview of contributions by this author see Special:Contributions/Wikischro For an overview of contributions by this author see Special:Contributions/Ralfprien