htb weather and precipitation sensors mesoscale atmospheric network workshop university of helsinki,...

HTB weather and precipitation sensors

Mesoscale Atmospheric Network Workshop

University of Helsinki, 12 February 2007

Heikki Turtiainen

©Vaisala | date | Ref. code |

Contents

WXT510 Weather Transmitter

WXT network implementation

VRG101 Weighing Precipitation Gauge



• 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



CPU board

Pressure, temperature and humidity module

Screw terminal

Ultrasonic wind sensor

Piezoelectric rain sensor


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


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)


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


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)


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)


Helsinki Testbed Weather Transmitter networkCurrently 62 stations with 112 WXT510 weather transmitters.

Average distance < 10 km. Data interval 5 min.


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


Installation examples

Temporary battery operated WXT station near Hietaniemi Beach.


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


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.


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


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)


Problems with Liquid Precipitation: Evaporation Error

Evaporation of collected water

Problem: Evaporation

from the container


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


Problems with Liquid Precipitation: Wetting Loss

... evaporate and are never measured

Problem: Wetting loss on the gauge

inlet

Raindrops sticking on the inlet tube...


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.


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


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.


VRG101 Wind Shields XRS111 / XRS121 / XRS131

• Stabilizes the wind conditions over the gauge


Installation examples

WXT + VRG101 weighing precipitation gauge at Malmi Airport

htb weather and precipitation sensors mesoscale atmospheric network workshop university of helsinki,...

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