Instruments and sensors to measure environmental parameters
This article introduces several types of sensors that are described in the Coastal Wiki.
Contents
Measurement of environmental parameters
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.
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
- Temperature sensors
- Salinity sensors
- Turbidity sensors such as
- Oxygen sensors
- Fluorescence sensors
- Multi-probe sensors (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.
Passive samplers
When contaminant concentrations fluctuate on a wide range of time scales, passive sampling can be an adequate monitoring strategy. Passive samplers collect contaminants in situ by retaining contaminants in a suitable medium within the sampler, which can be a solvent, chemical reagent or a porous adsorbent. The accumulated contaminant mass reflects either the concentration with which the device is at equilibrium or the time-averaged concentration to which the sampler was exposed. Passive samplers are designed to maximise the amount of contaminant sampled in order to detect the generally low concentration levels present in water, whilst ensuring a quantitative correlation between the mass of retained chemical and its concentration in the sampled medium. Calibration of passive sampling devices can be performed in the laboratory at known exposure concentrations[1].
Biota can also be used as passive samplers, in particular bivalves. Analysis of the tissues or lipid extracts of the test organism(s) can give an indication of the equilibrium level of waterborne contamination. Bivalves are effective at accumulating hydrophobic compounds (e.g. petrochemicals) but less at detecting hydrophilic contaminants. Bioaccumulation in bivalves is influenced by various biotic factors such as metabolism, depuration rates, excretion and stress and abiotic factors such as temperature, salinity and turbidity. Physical/chemical sampling devices can be influenced by biofouling, dissolved organic matter, and effects of temperature, pH and flow on sampling rates[2].
See also
- European coastal and marine observatories (2020)
- 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
- ↑ Vrana, B., Mills, G.A., Allan, I.J., Dominiak, E., Svensson, K., Knutsson, J., Morrison, G. and Greenwood, R. 2005. Passive sampling techniques for monitoring pollutants in water. Trends in Analytical Chemistry 24: 845-868
- ↑ Fuller, N., Kimbrough, K. L., Davenport, E., Edwards, M.E. and Johnson W. E. 2023. A comparison of dreissenid mussels and passive samplers as monitors of contaminants of emerging concern and polycyclic aromatic hydrocarbons in the great lakes. NOAA Technical Memorandum NOS NCCOS 317 Silver Spring, MD. 40 pp. DOI 10.25923/kk5w-3r94
|
Please note that others may also have edited the contents of this article.
|
