BML Offshore Mooring: Seawater Transmittance
Instrument Type: WETLabs C-Star Transmissometer
Description: The C-Star measures light transmittance at a single wavelength over a known path. The LED light source provides light that transmits within a narrow bandwidth. A portion of the transmitted light is monitored by a reference detector and used in a feedback circuit to account for variations in the LED source over time, as well as changes in the instrument’s internal temperature. The light transits the sample volume and enters the receiver optics, where it passes through additional focusing optics and finally strikes a silicon photodiode detector which converts the amount of received light to a corresponding 0–5 V analog output signal which represents the amount of light received. The ratio of light gathered by the receiver to the amount originating at the source is the beam transmittance.
In general, losses of light propagating through water can be attributed to two primary causes: scattering and absorption. Suspended particles, phytoplankton, bacteria and dissolved organic matter, as well as the intrinsic optical properties of the water itself, all contribute to the losses sensed by the instrument.
Location: Suspended from a chain off the BML oceanographic mooring, ~1.2 km off Bodega Head.
Latitude 38° 18' 42.2" N
Longitude 123° 04' 57.4" W
Installed: Deployed intermittently
Specifications:
Wavelength: 660 nm
Pathlength: 25 cm
Sensitivity: 1.25 mV
Beam Divergence: 0.8° in water
Bandwidth: ~ 20 nm
Acquisition Settings:
Sample interval: 300 seconds
Measurements per Sample: 25
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