System Sensors
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PARAMETER
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SENSOR TYPE
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Wave Data
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WaMoS (not connected to this system)
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Water Current Data
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ADCP (not connected to this system)
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Air Temperature
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VAISALA PTU200
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Relative Humidity
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VAISALA PTU200
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Barometric Pressure
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VAISALA PTU200
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Wind Speed Sensor
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Vessel Supply
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Wind Direction Sensor
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Vessel Supply
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Cloud Height
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VAISALA CL31
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Horizontal Visibility / Present Weather
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VAISALA PWD22
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Vessel Motion
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Kongsberg Seatex MRU
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GPS
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Vessel Supply
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Gyro
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Vessel Supply
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Lightning Detection
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Boltek LD250
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Helideck Monitoring Software
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Fugro GEOS WEATHER MONITOR
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Glossary
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Wind
All wind parameters are calculated from a 2-minute sample (unless otherwise specified). Speeds are
measured in knots and directions are measured in degrees from true north.
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Wind Speed:
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(Mean wind Speed) The average wind Speed over the averaging
period.
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Wind Gust:
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(Maximum gust Speed) The highest 3-second wind Speed at any
time during the averaging period.
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Wind Direction:
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(Mean wind direction) The average wind direction over the
averaging period.
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Atmospheric Temperature
All temperatures are calculated from a 1-minute sample and are measured in Celsius.
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Dry Air Temp:
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(Dry air temperature) This is the temperature measured by a normal thermometer.
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Wet Air Temp:
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(Wet air temperature) This is the temperature measured by a normal thermometer whose bulb is covered by a wet wick. The difference between wet and dry temperatures is due to evaporation which lowers the temperature reading and is largely dependent upon humidity. The wet air temperature should always be less than or equal to the dry air temperature.
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Dew Point:
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The temperature at which cloud will form in the current atmospheric conditions. Mist or fog is more likely to form when dew point is near the dry air temperature. Dew point should always be less than or equal to wet air temperature.
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Humidity:
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(Relative Humidity) This is a measure of air saturation. As relative humidity approaches 100%, the air becomes fully saturated and water vapour condenses out as cloud, mist or fog.
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Atmospheric BP
All pressures are calculated from a 1-minute sample and are measured in
millibars. Barometric pressure is the pressure at the sensor and is corrected to
provide pressures at sea level and helideck.
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QNH:
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(Air pressure at mean water level) The relationship between
QNH and barometric pressure as illustrated in Figure 2.
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QFE:
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(Air pressure at the helideck). The relationship between
QFE and barometric pressure is illustrated in Figure 2.
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Figure 2 : Relationship between measured pressure, QNH and QFE
The equation used to convert the pressure measured at sensor height, Z, to QNH
is Kempe’s formula.
Equation :
This equation can also be used to convert from sensor height to QFE.
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Cloud and Visibility
All cloud and visibility parameters are calculated from a 10-minute sample.
Cloud is measured in feet and visibility in metres.
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Cloud Base 1:
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The height of the lowest cloud layer above long term mean
water level, measured in feet.
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Cloud Base 2: .
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The height of the bottom of the second distinct cloud layer
above long term mean water level, measured in feet.
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Visibility:
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The horizontal distance at which the light intensity from a
standard source would drop to 5%. This is calculated by measuring the amount
of infra-red scatter in a small volume of air at the sensor location and
assuming this sample has similar properties to the surrounding air mass.
Contaminants in the air will lower the inferred visibility.
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Waves
All wave parameters are calculated from a 34-minute sample. Constants are
measured in metres and periods in seconds.
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Sig Wave_Radar:
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(Significant wave height) This is 4 times the standard deviation of the
sea surface over the averaging period and is approximately equal to the
average of the 1/3 highest waves.
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Max Wave_Radar:
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(Maximum wave height) This is the largest individual wave within the
averaging period. It is defined as the greatest vertical distance between a
wave crest and its succeeding trough.
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Crest Ht:
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(Maximum crest height) This is the highest wave crest above the mean
water level over the averaging period. Since waves are rarely exactly
symmetrical about the mean water level this is not necessarily associated
with the largest wave.
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Mean Period:
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(Mean zero-crossing period) This is the average wave period.
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Water Levels
All water levels are calculated from a 30-minute sample. Constants are measured
in metres.
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Tide Level:
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The mean height of the sea surface over the averaging period with
respect to the long term mean water level.
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Extreme Level:
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The highest individual sea surface height over the averaging period with
respect to the long term mean water level.
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Air Gap:
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The smallest distance between the bottom deck and the sea surface over
the averaging period. It is related to Extreme Level as illustrated in
Figure 1.
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Figure 1 : Relationship between air gap and extreme level
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