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Piezoelectric precipitation sensor from VAISALA
Atte Salmi
Project Manager
Vaisala Instruments
©Vaisala | date | Ref. code | Page 2
Contents
Construction of the sensor
Measurement method
Sensor calibration
Errors in measurement method
Field test results
Conclusions
©Vaisala | date | Ref. code | Page 3
Introduction
Developed for Weather Transmitter
Durable and maintenance free
©Vaisala | date | Ref. code | Page 4
Vaisala RAINCAP
Construction
Sensor frame
Sensor cover
Piezo detector
Electronics + Software
©Vaisala | date | Ref. code | Page 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.
pv = mvt
Piezo detector
Electronics Algorithm
U(t) = c(dp(t)/dt)
Precipitation P = f(U)
©Vaisala | date | Ref. code | Page 6
Terminal velocity
Atlas et. al. (1973):
vt (D) = 9.65 - 10.30e(0.6D)
©Vaisala | date | Ref. code | Page 7
Vaisala RAINCAP
Type calibration
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
0 1000 2000 3000 4000 5000 6000 70000
10
20
30
40
50
60
70
80
Voltage sum [V]
Ref
[mm
]
Precipitation P = f(U)
©Vaisala | date | Ref. code | Page 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
©Vaisala | date | Ref. code | Page 9
Results - Kuala Lumpur, Malaysia
0
10
20
30
40
50
60
70
80
90
0311
20
0311
21
0311
22
0311
23
0311
24
0311
25
0311
27
0311
28
0311
29
0312
01
Date
Acc
um
ula
ted
ra
infa
ll [m
m]
WX1
WX2
TB1
TB2
©Vaisala | date | Ref. code | Page 10
Results - FMI Observatory, Jokioinen
0
10
20
30
40
50
60
70
80
90
100
Aug-04 Sep-04 Oct-04 Nov-04
Month
Acc
um
ula
ted
ra
infa
ll [m
m]
WX1
WX2
WG1
WG2
WG3
Gauge WX1 WX2 WG1 WG2 WG3[mm] 222.7 227.5 205.4 203.9 189.6
Total accumulations during a four months test period at Jokioinen observatory.
©Vaisala | date | Ref. code | Page 11
Results - Tokyo University, Japan
Oct 2004 - Feb 2005
0
2
4
6
8
10
12
14
0 2 4 6 8 10 12 14
WXT510 rain accumulation [mm/10 min]
TB
rai
n ac
cum
ulat
ion
[mm
/10
min
]
WXT510 vs TB
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Results - characteristic short-interval data
©Vaisala | date | Ref. code | Page 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.
©Vaisala | date | Ref. code | Page 14
Contact Information
Atte Salmi
Project Manager
Vaisala Instruments
Phone +358 9 8949 2785
©Vaisala | date | Ref. code | Page 15
Errors in precipitation measurement
Term Description of error component Magnitudek wind-field deformation 2 - 10 %P1 + P2 wetting on the internal walls of the collector and the container after emptying 2 - 15 %P3 evaporation from the container 0 - 4 %P4 splashing of water in and out 1 - 2 %
,)( 4321 rgk PPPPPPkP where Pk is the adjusted amount of precipitation,Pg the recorded precipitation in the gauge,k and P1 - P4 the adjustments for different error componentslisted 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.
©Vaisala | date | Ref. code | Page 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.
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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.
,)(
)(dt
tdpctU
where c is a constant dependent on the properties of the piezoelectric material.
©Vaisala | date | Ref. code | Page 18
Vaisala RAINCAP
Rain drop
Hail
©Vaisala | date | Ref. code | Page 19
Drop signal
©Vaisala | date | Ref. code | Page 20
Drop signal
t1
t2
©Vaisala | date | Ref. code | Page 21
Drop collapse
t1
t2
©Vaisala | date | Ref. code | Page 22
Technical data PTU
Barometric pressure
Range 600...1100 hPaAccuracy ± 0.5 hPa at 0...30°C (+32...+86 °F)
± 1 hPa at -52...+60 °C (-60...+140 °F)Output resolution 0.1 hPa, 10 Pa, 0.001 bar, 0.1 mmHg, 0.01 inHgUnits available hPa, Pa, bar, mmHg, inHg
Relative Humidity
Range 0...100 %RHAccuracy ±3 %RH at 0...90 %RH
±5 %RH at 90...100 %RHOutput resolution 0.1 % RH
PTU Measuring IntervalMeasuring interval 3…3600 s (=60 min),
at one second steps
©Vaisala | date | Ref. code | Page 23
Technical data liquid precipitationRainfall cumulative accumulation after the latest auto or
manual resetCollecting area 60 cm2Output resolution 0.01 mm (0.001 in)Field accuracy fordaily accumulation better than 5%*, weather dependentUnits available mm, in
*Due to the nature of the phenomenon, deviations caused by spatial variations may exist in precipitationreadings, especially in short time scale. The accuracy specification does not include possible wind inducederror.
Rain duration counting each 10-second increment wheneverdroplet detected
Ouput Resolution 10 s
Rain intensity running one minute average in 10-second steps
Range 0...200 mm/h (broader range with reducedaccuracy)
Units available mm/h, in/h
Hail cumulative amount of hits against collectingsurface
Output resolution 0.1 hits /cm2 (1 hits/ in2), hitsUnits available hits /cm2, hits/ in2, hits
Hail duration counting each 10 second increment wheneverhailstone detected
Ouput resolution 10 s
Hail intensity one minute running average in 10-second steps
Output resolution 0.1 hits/ cm2h (1 hits/ in2h)Units available hits/cm2h, hits/ in2h, hits/h
©Vaisala | date | Ref. code | Page 24
Technical data wind
Wind speedRange 0...60 m/sResponse time 0.25 sAvailable variables average, maximum and minimumAccuracy ± 0.3 m/s or ±2 % whichever is greaterOutput resolution 0.1 m/s (km/h, mph, knots)Units available m/s, km/h, mph, knots
Wind directionAzimuth 0...360°Response Time 250 msAvailable Variables Average, maximum and minimumAccuracy ± 2°Output Resolution 1°
Measurement frame Averaging time 1…600 s (=10 min),
at one second stepson the basis of 0.25 second samples
Measuring interval 1…3600 s (=60 min),at one second steps
©Vaisala | date | Ref. code | Page 25
Technical data general
Operation voltage 5*...30 VDC
*) Below 5.3V the measurement performance for high wind speeds may be degraded.
Average power consumption minimum 0.07 mA @ 12VDC (SDI-12)maximum 13 mA @ 30 VDC (constant measurement of all
parameters)typical 3 mA @ 12 VDC (with default measuring
intervals)
Heating voltage options: DC, AC, full-wave rectified AC
recommended ranges 12 VDC 20 %, 1.1 A max24 VDC 20 %, 0.6 A max68 Vp-p
20 % (AC), 0.6 Arms max34 Vp 20 % (f/w rect. AC), 0.6 Arms max
absolute max 30 VDC84 Vp-p (AC)42 Vp (f/w rect. AC)
Digital outputs SDI-12, RS-232, RS-485, RS-422
Communicationprotocols SDI-12 v1.3, ASCII automatic& polled, NMEA
0183 v3.0 with query option.
©Vaisala | date | Ref. code | Page 26
Technical data general
Operating conditions
Temperature operation -52 …+60 °C (-60...+140 °F)storage -60 …+70 °C (-76...+158 °F)
Relative humidity 0...100 %RHPressure 600...1100 hPaWind 0...60 m/s
Electromagneticcompatibility EN61326: 1997 + Am 1:1998 + Am2:2001
Electrical equipment for measurement, controland laboratory use - EMC requirements;Generic environment
Materials
Radiation shield, top andbottom parts Polycarbonate + 10 % glass fibre
Precipitation sensor plate stainless steel (AISI 316)
Weight 650 g (1.43 lbs)
©Vaisala | date | Ref. code | Page 27
The Rain Lab
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Photoacoustic principle
©Vaisala | date | Ref. code | Page 29
Interface Architecture
SDI-12 v1.3
Receiver
SDI-12 v1.3
Receiver
NMEA0183 v3.0
Talker
NMEA0183 v3.0
Talker
StandardASCII
Terminal
StandardASCII
Terminalsystem level
instrument level
HW- interface 3-wire SDI-12RS232, RS485/422
data formattransmission
ASCIIPolled
ASCII Polled / Automatic
External power supply5 - 30 VDC