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ASIMET sensors

 On this page:
-ASIMET Sensors
-Barometric pressure sensor
-Relative humidity and air temperature sensor
-Sea surface temperature and salinity
-Longwave radiation
-Shortwave radiation
-Wind speed and direction
-Precipitation
-Sources
  
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The ASIMET system consists of seven modular sensors that can be mounted on ocean buoys or ships. (Photo by Sean Whelan, Woods Hole Oceanographic Institution)
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Barometric pressure is measured using a Heise DXD (Dresser Instruments)sensor. (Courtesy of Robert Weller, Woods Hole Oceanographic Institution)
Relative humidity and air temperature sensor attached to WHOTs 2006 buoy. (Photo by Sean Whelan, Woods Hole Oceanographic Institution)
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Relative humidity measurements are made with a Rotronic MP-101A sensor. (Courtesy of Robert Weller, Woods Hole Oceanographic Institution)
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Sea Surface Temperature and Salinity: The SBE 37-IM MicroCAT is a high-accuracy conductivity and temperature sensor/recorder. (Courtesy of Robert Weller, Woods Hole Oceanographic Institution)
Longwave and shortwave radiation sensors attached to WHOTS 2006 buoy. (Photo by Sean Whelan, Woods Hole Oceanographic Institution)
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Shortwave radiation is measured with a modified Eppley Precision Spectral Pyranometer (PSP) mounted on an aluminum base. (Courtsey of Robert Weller, Woods Hole Oceanographic Institution)
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Precipitation sensor (right) and Humidity sensor (left). (Photo by Theresa Smith, Woods Hole Oceanographic Institution)
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Precipitation Sensor (Courtsey of Robert Weller, Woods Hole Oceanographic Institution)
Related Links
» http://uop.whoi.edu/
Upper Ocean Processes home page, with links to current and completed projects and data archives
» UOP Technical Documents
The Upper Ocean Processes Group archive of technical notes and technical reports.
» WHOI UOP IMET sensor specifications
Improved METeorological (IMET)sensor specifications, accuracy and sampling.
» ASIMET Documentation Page
Technical specifications for ASIMET sensors and data handling. From the WHOI Design Group for Electronics.
» UOP instrument glossary page
Glossary page for UOP instruments, abbreviations and parameters measured.
  
ASIMET Sensors

ASIMET sensors provide data which are used to calculate the exchange, or flux, of energy between the atmosphere and the ocean. Complex calculations are developed that help us understand the earth’s climate and are used to build models which predict future climactic conditions. ASIMET data provide high-quality meteorological data at specific locations, and can be used to help modelers check their predictions against reality. 

ASIMET home



Barometric pressure sensor

What is it and why do we use it?

Barometric pressure is the measure of the weight of the atmosphere, or atmospheric pressure. It enters the flux calculations indirectly. For example, momentum flux and sensible heat flux depend on the density of air, which is a function of temperature, humidity and pressure. Pressure is also needed to compute the saturation specific humidity (water vapor at the sea surface) which is used in determining the air-sea humidity difference.

How does it work?

Unlike traditional barometers in which a column of mercury is used to detect air pressure, ASIMET sensors determine atmospheric pressure by measuring resistance in an electrical current. Within the sensor housing, there is a metal disc called a resistive strain gauge. On one side of the disc, the cylinder is sealed and calibrated with a known pressure. The other side is open to the atmosphere. When atmospheric pressure increases, it pushes on the disc creating an electrical signal. Thus, the electrical output, or voltage from the strain gauge is proportionate to pressure being exerted on it. Changes in voltage create a signal which is converted to a digital value using an analogue to digital (A/D) converter) which is collected by an ASIMET data logger.

The Barometric sensor used by ASIMET is the Heise DXD sensor made by Dresser Instruments

Technical specifications for this instrument.



Relative humidity and air temperature sensor

What is it and why do we use it?

Humidity is the measurement of how much water vapor is in the air. Relative humidity is the amount of water vapor relative to how much it takes to saturate the air. Relative humidity is expressed as a percent, and is calculated in relation to saturated vapor density. ASIMET uses relative humidity to determine the specific humidity of air. Specific humidity is the mass of water vapor divided by the total mass of air. The difference between specific humidity of air and saturation specific humidity of the sea surface is needed to calculate latent heat flux which is used to compute evaporation as a part of the moisture flux.

Air temperature is used to calculate the density of air as well as specific humidity. It also used to determine the difference between the air temperature and sea temperature which is used in sensible heat flux calculations. Because air temperature is a function of humidity, it is also needed to compute the saturation specific humidity.

How does it work?

Air temperature is measured by a resistive temperature device (RTD). As the temperature increases or decreases, the RTD will limit or increase thevoltage from the RTD. Changes in voltage create an analogue signal which is converted to digital form and sent to ASIMET data loggers.

Humidity works in a similar way. It is measured by a capacitive humidity sensor which is composed of two metal plates that are adjacent to each other. The sensor is located in a cylinder that collects moisture in the air. As moisture collects in the cylinder, the two plates interact with each other producing a voltage. Changes in voltage produce an analogue signal which is converted to digital form and sent to ASIMET data loggers.

Because of intense environmental conditions of the open ocean, instruments on buoys and ships require more protection. For ASIMET, the sensor is packaged in a custom housing which is more rugged than the standard housing and with high pressure water seals. A vented plastic housing called a radiation shield covers the sensor and prevents it from producing artificial temperature values caused by solar heating.

The temperature and humidity sensor used by ASIMET is the Rotronic MP-101A sensor

Technical specifications for this instrument.   



Sea surface temperature and salinity

What is it and why do we use it?

Sea surface temperature is used to determine the air-sea temperature difference needed to compute the sensible heat flux.

Salinity is a measure of the quantity of dissolved solids in ocean water. These solids are mostly salts, or halide anions and metal cations. Sea surface salinity is used only indirectly in flux calculations, such as the computation of water density. Salinity is also used as an independent indicator of rain.

How does it work?

Sea surface temperature is measured by a resistive temperature device (RTD). As the temperature increases or decreases, the RTD will limit or increase thevoltage from the RTD. Changes in voltage create an analogue signal which is converted to digital form and sent to ASIMET data loggers.

Sea surface salinity and density are computed by measuring the conductivity of seawater. However, oceanographic sensors cannot measure conductivity directly; instead they measure conductance, which is the voltage produced in response to the flow of a known electrical current. Conductivity is calculated from the conductance measured by the sensor and a cell constant based on the geometric dimensions of the sensor.

The sensor consists of platinum coated electrodes that create an electrical connection with the seawater. The sensor measures the resistance of the seawater which can be converted to a digital signal. Calculations needed to compute salinity are built into the software within the flashcard located within the sensor housing.

The sea surface temperature and conductivity sensor used by ASIMET is the SBE 37 made by Seabird Electronics .



Longwave radiation

What is it and why do we use it?

Longwave radiation is a term used to describe infrared energy received and emitted by the earth and atmosphere. Not visible to the naked eye, infrared radiation has a longer wavelength than visible light and is sensed by humans as heat. When a body such as the ocean absorbs radiation from the sun, it will also emit radiation back into the atmosphere. Measurements of infrared energy radiating downward from the sun (the term is downwelling longwave radiation) are used in conjunction with estimates of upwelling longwave radiation (based on the temperature at the Earth’s surface) to determine net longwave radiation used in calculating heat flux.

How does it work?

The ASIMET Longwave radiation sensor housing is painted with a very flat black paint which absorbs radiation. A small glass dome at the top of the instrument is covered with an “interference coating” which allows only infrared radiation to come through. Coiled strands of very fine wire within the dome produce a voltage when radiation is detected. This voltage is so small; it is measured in micro-volts (1 volt = 1,000,000 micro-volts) and has to be amplified before it is converted to digital form.

The longwave radiation sensor used by ASIMET is the Eppley Precision Infrared Radiometer .

ASIMET Eppley PIR Instrument Module Version 3 Command Sets



Shortwave radiation

What is it and why do we use it?

Shortwave radiation is radiant energy emitted by the sun in the visible and near-ultraviolet wavelengths. Downwelling shortwave radiation is used directly flux calculations once it is corrected for surface albedo, which is light that is reflected off the surface of the earth.

How does it work?

The ASIMET shortwave radiation sensor is similar to the longwave sensor. It is housed in a weatherproof titanium canister that has been painted with a very flat black paint that absorbs radiation. A small glass dome at the top of the instrument is allows light to enter the sensor. Within it are coiled strands of very fine wire within the dome that produce a miniscule voltage when radiation is detected. This voltage is so small; it is measured in micro-volts (1 volt = 1,000,000 micro-volts) and has to be amplified before being sent through the A/D converter.

The shortwave radiation sensor used by ASIMET is the Eppley Precision Infrared Radiometer

Technical Specifications for this instrument.  



Wind speed and direction

What is it and why do we use it?

Wind speed, after adjustment to a standard reference height of 10 m, is used to compute the momentum flux or “wind stress” at the sea surface. The direction of the wind is taken to be the direction of the stress. Wind speed is also needed to compute sensible and latent heat fluxes.

How does it work?

ASIMET wind sensors are composed of a weather vane, shaft and propeller. Wind direction is measured by an encoderthat has a printed bar code for each direction. As the wind pushes the vane, the encoderdisc rotates and the bar code is read optically. A magnetometercompass is used to provide the north reference for use on buoys.

To measure wind speed, a six-pole magnet is mounted on the back of the propeller shaft. As the propeller rotates, the magnet rotates over a pick up coil. Electronic pulses from the magnetic poles passing over the coils are counted. One revolution of the propeller equals six pulses. The shape of the propeller is calibrated so scientists know that a known volume of air will cause the propeller to rotate one revolution. This constant value is used to calculate wind speed.

The wind speed and direction sensor used by ASIMET is the R.M. Young model 05103 wind monitor.

Technical specifications for this instrument.    



Precipitation

What is it and why do we use it?

Precipitation in millimeters is converted to a rate (mm/hr) and is compared to the estimated evaporation rate (related to the latent heat flux) to determine the moisture flux (evaporation minus precipitation). Precipitation rate may also be used to estimate the contribution of rain to the heat and momentum fluxes.

How does it work?

To measure precipitation, this sensor uses a capacitive technique to record the volume of rain water deposited inside a collection chamber. This is basically a tube with a conductive wire. As the chamber fills will water the voltage increases. It automatically empties when the chamber is full. The sensor housing is designed to collect rain coming in at many different angles.

A new sensor is also being used by ASIMET. This sensor optically reads water droplets channeled within the collection chamber by a tiny funnel which has been calibrated to allow only a certain amount of water to pass through it. The optical sensor counts the droplets which are calculated into volume measurements.

Rainfall is measured by an R.M. Young model 050201 self-syphoning rain gauge or a Eigenbrodt / Heisse Dual Face Rain Gauge / 450/3.

Technical specifications for R.M. Young model 050201 self-syphoning rain gauge  



Sources

Bob Weller, Senior Scientist, Physical Oceanography Department, Woods Hole Oceanographic Institution

Al Plueddeman, Associate Scientist with Tenure, Physical Oceanography Department, Woods Hole Oceanographic Institution

David Hosom, Principal Engineer, Physical Oceanography Department, Woods Hole Oceanographic Institution

Frank Bahr, Research Specialist, Physical Oceanography Department, Woods Hole Oceanographic Institution (Volunteer Observing Ships program)

Jason Smith , Engineer Assistant III, Physical Oceanography Department, Woods Hole Oceanographic Institution

George Tupper, Research Associate II, Physical Oceanography Department, Woods Hole Oceanographic Institution

Hosom, D. S., R. A. Weller, R. E. Payne and K. E. Prada. The IMET (improved meteorology) ship and buoy systems. Journal of Atmospheric and Oceanic Technology 12:527-540. (1995)

Colbo, K., and R. A. Weller. The accuracy of the IMET sensor package. Journal of Atmospheric and Oceanic Technology, submitted 2005.

WHOI Upper Ocean Processes Group, Technical Notes archive