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Instruments and sensors to measure environmental parameters

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Versie door MaartenDeRijcke (Overleg | bijdragen) op 3 aug 2011 om 15:43

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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 simplest way in which on can measure the environmental parameters of water, is to take samples and then analyze them after returning to the laboratory. It is a powerful approach since specialized laboratory equipment can be used to analyze a multitude of parameters. The main shortcomings of this approach are that only a limited number of measurements (samples) can be processed and the time 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. As a result, 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 of time and space (see Figure 1). To allow for the investigation of these processes, a large volume of data must 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 as well as a signal processing unit that converts the sensor signal and carries out scaling and conversion to engineering units and to 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 unit time (e.g. measurements/hour)
  • Power consumption: mean of electrical power uptake during deployment (usually measured 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 interact either directly with the detector (e.g. in a temperature, pressure or light sensor) or a stimulus can be exerted by the instrument. The stimulus is then modified by the property to be measured and then interacts with the detector, such as 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 interacts with the detector.

If the detector signal is of a property (such as color) it can be converted to an electrical signal by a not an electrical signal (e.g. an optical signal or the change transducer). The sensor is made up of both the detector and the transducer.

Types of sensors

There are numerous sensors in oceanographic work:

Some of the most commonly used are

Secchi disk
Optical backscatter point sensor (OBS)
Optical transmissiometers (Theme 9 wanted page)

Less common are

Examples of specialized sensor systems are

Important properties

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

See also

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.