Instruments and sensors to measure environmental parameters
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Measurement of environmental parameters
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.
IntroductionAn 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.
Oceanographic instruments can contain data loggers to store measurement data for readout after the deployment.
- 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)
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
Less common are
Examples of specialized sensor systems are
- 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.
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