HTB weather and precipitation sensors
Mesoscale Atmospheric Network Workshop
University of Helsinki, 12 February 2007
Heikki Turtiainen
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Contents
WXT510 Weather Transmitter
WXT network implementation
VRG101 Weighing Precipitation Gauge
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WXT510 Weather Transmitter
• Compact (only 9.5” / 24 cm tall)
• No moving parts - durable and requires minimum maintenance
• Optimal life time cost
• Easy to use purchase, install and use
6 MEASUREMENTS IN 1 INSTRUMENT
1. temperature2. relative humidity3. rain fall4. wind speed5. wind direction6. barometric pressure
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WXT510 Weather Transmitter
CPU board
Pressure, temperature and humidity module
Screw terminal
Ultrasonic wind sensor
Piezoelectric rain sensor
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Weather Transmitter WXT510 - Basic Specs
Relative HumidityRange: 0…100%
Accuracy: ± 3% (0-90%)
± 5% (90-100%)
Air TemperatureRange: -52…+60°C (-60…+140°F)
Accuracy* (at 20°C): ± 0.3°C (0.5°F)
Barometric PressureRange: 600…1100 hPa
Accuracy: ± 0.5 hPa (0…30°C)
± 1.0 hPa (-52…+60°C)
Wind SpeedRange: 0…60 m/s
Accuracy:
0...35 m/s ± 0.3m/s or ± 3%, whichever is greater
35...60 m/s ± 5%
Wind DirectionRange: 0…360°
Accuracy: ± 3°
Liquid PrecipitationAccumulation Accuracy: 5%**
Intensity Range: 0…200 mm/h
* for sensor element
** Due to the nature of the phenomenon, deviations caused by spatial variations may exist in precip. readings, especially in short time scale. The accuracy specified does not include possible wind induced error
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Windcap - ultrasonic wind sensor
Durable and maintenance free
•Zero starting thresholds or distance constants virtually zero.
• No moving parts - thus sensor performance doesn’t degrade with wear nor is affected by natural contaminants such as salt, dust or sand.
• Vaisala’s proprietary equilateral triangle design solves the turbulence problem. (1 redundant measurement path)
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Equilateral triangle design vs. orthogonal design
Reliable measuring pathsUnreliable
measuring path
Wind direction
TurbulenceUnreliable measuring path
Reliable measuring paths
Vaisala proprietaryequilateral triangle design
Orthogonal design
Turbulence
Wind direction
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RAINCAP measuring principle
A raindrop hitting the piezoelectric detector generates a voltage pulse Ui, whose amplitude is a function of the drop volume Vi. Consequently, drop size can be estimated from the measured voltage:
Vi = f(Ui)
Accumulated rainfall is sum of the individual drops
R [mm] = Σ f(Ui)
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RAINCAP calibration
Type calibration is based on comparison with accurate reference instruments under different field conditions:
• light and moderate rain in Finland
• moderate and heavy rain in Malaysia
Individual production calibration using highly repeatable laser pulse equipment
0 1000 2000 3000 4000 5000 6000 70000
10
20
30
40
50
60
70
80
Voltage sum [V]
Ref
[mm
]
Rainfall R = Σ f(Ui)
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Helsinki Testbed Weather Transmitter networkCurrently 62 stations with 112 WXT510 weather transmitters.
Average distance < 10 km. Data interval 5 min.
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Cell phone base station masts utilized as meteorological towers
Upper Weather Transmitter
h = 40...100 m
Lower Weather Transmitter
h = 2 m
GPRS communications
unit
Middle level Weather
Transmitters h = 20-30 m
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Installation examples
Temporary battery operated WXT station near Hietaniemi Beach.
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All-weather Precipitation gauges
Weighing rain gauges • 5 pcs VRG101 all-weather
precipitation gauges– Lahti– Nummi-Pusula, Loukku– Vihti, Maasoja– Nurmijärvi, Röykkä– Helsinki, Malmi Airport
• measure both liquid and solid precipitation
• heated rim• Tretyakov-type wind shield• communications: GPRS
Lahti 5.12.2005
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VRG101 - Vaisala All Weather Precipitation Gauge
FEATURES
• Weighing principle applying high accuracy, temperature compensated load cell
• Simple, robust design
• All weather operation with heating option
• High capacity up to 650 mm (25”)
• Large collecting area to enhance performance in light rain and snow
• Selection of optional features for enhanced performance and extended service interval
• Field-removable measurement unit, enabling use of pre-calibrated measurement units. Field check with dedicated weight.
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VRG101 Components
1=Lock
2=Collecting funnel
3=Side guide plate
4=Container (volume 30 liters)
5=Faucet
6=Collector tray
7=Spirit level
8=Load cell and electronics
9=Base plate
10=Rim
11=Top cone
12=Enclosure
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Specifications of VRG101
• Capacity 650 mm (25 in) without automatic draining pump antifreeze charge included
• Collecting area 400 cm2 (62 in2)
• Resolution 0.1 mm (0.005 in)
• Accuracy ± 0.2 mm (± 0.01 in) of measured amount during a rain event > 0.5
mm
• Temperature range -40 ... +60 °C (-40 - +140 °F)
• Output Serial RS485/RS232Pulse (tipping bucket emulation)
• Power Consumption < 30 mW (without heating)
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Problems with Liquid Precipitation: Evaporation Error
Evaporation of collected water
Problem: Evaporation
from the container
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VRG101 Solution: Evaporation Error
• VRG101 software filters out “negative rainfall” due to evaporation• Use of anti-evaporation oil is not required
uncorrected
corrected
time
rainfall mm
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Problems with Liquid Precipitation: Wetting Loss
... evaporate and are never measured
Problem: Wetting loss on the gauge
inlet
Raindrops sticking on the inlet tube...
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VRG101 Solution: Wetting Loss
The orifice/inlet geometry minimizes wetting loss:
• Funnel shaped inner orifice element is resting on the collector container so that it’s mass is measured together with the container.
• Water sticking on the funnel surface will be measured and included in the cumulative rainfall before it evaporates.
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VRG101 Solutions for Solid Precipitation
Solution: The mass of the funnel element is also measured. Snow accumulation on the funnel surfaces does not introduce error.
Problem 1: Uneven snow distribution in the container
Solution: The load cell technology used measures only forces along the vertical axis. Eccentric snow accumulation is not a problem.
Problem 2: Snow deposit on the inlet funnel surface
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VRG101 Solutions for Solid Precipitation
Solution: Intelligent heating control by software • Heating is applied only when necessary, using control algorithm based on air temperature and precipitation amount.• Evaporation loss caused by heating is minimized.
• Compared to continuous heating, power consumption on a typical winter day is decresed from over 2 kWh to 0.1 - 0.2 kWh.
Problem 4: Evaporation error caused by heating
Problem 5: Large heating power consumption
Problem 3: Outblowing of snow
Solution: Optimized gauge geometry for solid precipitation. Deep container and funnel-shaped inlet orifice minimize outblowing of snow.
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VRG101 Wind Shields XRS111 / XRS121 / XRS131
• Stabilizes the wind conditions over the gauge
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Installation examples
WXT + VRG101 weighing precipitation gauge at Malmi Airport