HTB weather and precipitation sensors

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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 humidity 3. rain fall 4. wind speed 5.
wind direction 6. barometric pressure
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WXT510 Weather Transmitter
Piezoelectric rain sensor
Ultrasonic wind sensor
CPU board
Pressure, temperature and humidity module
Screw terminal
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Weather Transmitter WXT510 - Basic Specs
Wind Speed Range 060 m/s Accuracy 0...35 m/s
0.3m/s or 3, whichever is
greater 35...60 m/s 5 Wind Direction Range
0360 Accuracy 3 Liquid Precipitation Accumul
ation Accuracy 5 Intensity Range 0200 mm/h

• Relative Humidity
• Range 0100
• Accuracy 3 (0-90)
• 5 (90-100)
• Air Temperature
• Range -5260C (-60140F)
• Accuracy (at 20C) 0.3C (0.5F)
• Barometric Pressure
• Range 6001100 hPa
• Accuracy 0.5 hPa (030C)
• 1.0 hPa (-5260C)

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
for sensor element
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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
  doesnt degrade with wear nor is affected by
  natural contaminants such as salt, dust or
  sand.
• Vaisalas proprietary equilateral triangle
  design solves the turbulence problem. (1
  redundant measurement path)

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Equilateral triangle design vs. orthogonal design
Vaisala proprietaryequilateral triangle design
Orthogonal design
Reliable measuring paths
Turbulence
Turbulence
Unreliable measuring path
Reliable measuring paths
Unreliable measuring path
Wind direction
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 S f(Ui)
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RAINCAP? calibration
Rainfall R S f(Ui)

• 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

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Helsinki Testbed Weather Transmitter network
Currently 62 stations with 112 WXT510 weather
transmitters. Average distance lt 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
Middle level Weather Transmitters h 20-30 m
Lower Weather Transmitter h 2 m GPRS
communications unit
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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

• 1Lock
• 2Collecting funnel
• 3Side guide plate
• 4Container (volume 30 liters)
• 5Faucet
• 6Collector tray
• 7Spirit level
• 8Load cell and electronics
• 9Base plate
• 10Rim
• 11Top cone
• 12Enclosure

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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 gt 0.5
  mm
• Temperature range -40 ... 60 C (-40 - 140
  F)
• Output Serial RS485/RS232
• Pulse (tipping bucket emulation)
• Power Consumption lt 30 mW (without heating)

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Problems with Liquid Precipitation Evaporation
Error
Problem Evaporation from the container
Evaporation of collected water
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VRG101 Solution Evaporation Error
rainfall mm
corrected
uncorrected
time

• VRG101 software filters out negative rainfall
  due to evaporation
• Use of anti-evaporation oil is not required

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Problems with Liquid Precipitation Wetting Loss
Problem Wetting loss on the gauge inlet
... evaporate and are never measured
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 its 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
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

• Solution The mass of the funnel element is also
  measured. Snow accumulation on the funnel
  surfaces does not introduce error.

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VRG101 Solutions for Solid Precipitation
Problem 3 Outblowing of snow
Solution Optimized gauge geometry for solid
precipitation. Deep container and funnel-shaped
inlet orifice minimize outblowing of snow.
Problem 4 Evaporation error caused by heating
Problem 5 Large heating power consumption

• 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.

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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