1. Piezoelectric precipitation sensor from VAISALA
Atte Salmi
Project Manager
Vaisala Instruments

2. Construction of the sensor Measurement method Sensor calibration
Contents
Construction of the sensor
Measurement method
Sensor calibration
Errors in measurement method
Field test results
Conclusions

3. Introduction Developed for Weather Transmitter
Durable and maintenance free

4. Vaisala RAINCAP Construction

5. 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.

6. Terminal velocity
Atlas et. al. (1973):
vt (D) = e(0.6D)

7. 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

8. 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

9. Results - Kuala Lumpur, Malaysia

10. Results - FMI Observatory, Jokioinen
Total accumulations during a four months test period at Jokioinen observatory.

11. Results - Tokyo University, Japan

12. Results - characteristic short-interval data

13. 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.

14. Contact Information
Atte Salmi
Project Manager
Vaisala Instruments
Phone

15. 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.

16. 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.

17. 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.

18. Vaisala RAINCAP
Hail
Rain drop

19. Drop signal

20. Drop signal t2 t1

21. Drop collapse t2 t1

22. Technical data PTU

23. Technical data liquid precipitation

24. Technical data wind

25. Technical data general

26. Technical data general

27. The Rain Lab

28. Photoacoustic principle

29. Interface Architecture
Standard
ASCII
Terminal
NMEA
0183 v3.0
Talker
SDI-12
v1.3
Receiver
system level
data format transmission
ASCII
Polled / Automatic
ASCII Polled
HW- interface
RS232, RS485/422
3-wire SDI-12
instrument level
External power supply VDC