Difference between revisions of "Instruments and sensors to measure environmental parameters"
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Revision as of 11:19, 28 November 2007
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This article explains why instruments are needed to investigate oceanographic processes. It also explains which instruments and sensors are available to do these measurements.
Contents
Measurement of environmental parameters
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.
Oceanographic instruments can contain data loggers to store measurement data for readout after the deployment. See also application of data loggers to seabirds.
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 (such as 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 (such as in a fluorometer that sends out a light pulse (stimulus) that 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. See also wireless sensor networks
Types of sensors
There are numerous sensors in oceanographic work. Some of the most commonly used are sensors for:
- temperature sensors (under construction)
- salinity sensors (under construction)
- turbidity sensors (under construction, see also turbidity)
- oxygen sensors (under construction)
- fluorescence sensors (under construction)
- multi-probe sensors (under construction)
Less common are sensors for:
- pH Sensors (under construction)
- Optical sensors (under construction, see also Optical remote sensing)
- Observation of algal groups (under construction)
- Observation of algal shapes with flow cytometers (under construction)
- Observation of algal activity with fast repetition fluorometers (under construction)
- pCO2 sensors (under construction)
Examples of specialized sensor systems are:
- nutrient analysers
- trace metal analysers (under construction)
- biochemical or genetic sensors (under construction)
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
- definition of sensors
- definition of oceanographic instruments
- wireless sensor networks
- biofouling
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