Piezoelectric precipitation sensor from VAISALA

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Piezoelectric precipitation sensor from VAISALA

• Atte Salmi
• Project Manager
• Vaisala Instruments

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Contents

• Construction of the sensor
• Measurement method
• Sensor calibration
• Errors in measurement method
• Field test results
• Conclusions

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Introduction

• Developed for Weather Transmitter
• Durable and maintenance free

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Vaisala RAINCAP?Construction
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Vaisala RAINCAP?Measurement method

• The voltage output U(t) from the piezo detector
  due to a drop impact is proportional to the
  volume of the drop.
• Since, the surface area is known, the drop
  signals can be directly converted to accumulated
  precipitation.

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Terminal velocity
Atlas et. al. (1973) vt (D) 9.65 - 10.30e(0.6D)
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Vaisala RAINCAP?Type calibration
Precipitation P f(U)

• Comparison of detector voltage response with
  precipitation readings from accurate reference
  instruments under different field conditions
• light and moderate rain in Finland
• moderate and heavy rain in Malaysia

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Errors in measurement method

• Vaisala RAINCAP? do not have systematic error
  sources like
• wetting on the internal walls of the collector
  and the container
• evaporation from the container
• splashing of water in and out

• Error sources related to Vaisala RAINCAP? are
  more stochastic than systematic
• variation in the shape and velocity of raindrops
  caused by air movements
• sensitivity variations over the sensor area, due
  to surface wetness

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Results - Kuala Lumpur, Malaysia
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Results - FMI Observatory, Jokioinen
Total accumulations during a four months test
period at Jokioinen observatory.
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Results - Tokyo University, Japan
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Results - characteristic short-interval data
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Conclusions

• Due to the measurement method and construction of
  the sensor, the Vaisala RAINCAP? is virtually
  maintenance free. The sensor does not suffer from
  systematic errors due to wetting, evaporation or
  splashing of raindrops. It is also capable for
  true real time intensity measurement.
• The field results show good comparability of the
  sensor to traditional tipping buckets and
  weighing-recording gauges.
• Because of its robust design with no moving parts
  the Vaisala RAINCAP? is especially suitable for
  dense measurement networks.

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Contact Information

• Atte Salmi
• Project Manager
• Vaisala Instruments
• Phone [phone]
• atte.salmi_at_vaisala.com

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Errors in precipitation measurement
where Pk is the adjusted amount of
precipitation, Pg the recorded precipitation in
the gauge, k and ?P1 - ?P4 the adjustments for
different error components listed in Table below
and Pr random observational and instrumental
error.
World Meteorological Organization, 2000
Precipitation Estimation and Forecasting, Point
Measurement Using Gauges. Operational Hydrology
Report No. 46, WMO-No. 887, Geneva.
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Four operation modes

• Precipitation Start/End mode
• Transmitter sends automatically a precipitation
  message 10 seconds after the recognition of the
  first drop. The messages are sent continuously as
  the precipitation proceeds and stopped when the
  precipitation ends.
• Tipping bucket mode
• This mode emulates tipping bucket type
  precipitation sensors. Transmitter sends
  automatically a precipitation message when the
  counter detects one unit increment (0.1 mm/0.01
  in).
• Time mode
• Transmitter sends automatically a precipitation
  message in the update intervals defined by the
  user.
• Polled mode
• Transmitter sends a precipitation message
  whenever requested by the user.

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Piezoelectric sensor

• When mechanical pressure is applied to the
  sensor, the crystalline structure produces a
  voltage U(t) proportional to the pressure.
  Conversely, when an electric field is applied,
  the structure changes shape producing dimensional
  changes in the material.

where c is a constant dependent on the properties
of the piezoelectric material.
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Vaisala RAINCAP?
Hail
Rain drop
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Drop signal
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Drop signal
t2
t1
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Drop collapse
t2
t1
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Technical data PTU
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Technical data liquid precipitation
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Technical data wind
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Technical data general
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Technical data general
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The Rain Lab
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Photoacoustic principle
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Interface Architecture
SDI-12 v1.3 Receiver
NMEA 0183 v3.0 Talker
Standard ASCII Terminal
system level
data formattransmission
ASCIIPolled
ASCII Polled / Automatic
HW- interface
3-wire SDI-12
RS232, RS485/422
instrument level
External power supply5 - 30 VDC