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PIEZOELECTRIC VAISALA RAINCAP RAIN SENSOR APPLIED TO DROP SIZE DISTRIBUTION MONITORING
Atte Salmi, Lasse Elomaa, Panu Kopsala and Emmi LaukkanenVaisala Oyj, Helsinki, Finland
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Contents
Vaisala RAINCAP® rain sensor
DSD measurements in laboratory
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Disdrometer needs
Radar adjustements (DSD) (Z – R relation)
Soil erosion (KE flux) agricultury (soil splash erosion, seal formation, soil aggregates
brekdown) hydrology (infiltration, evporation, surface runoff)
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Low-cost disdrometer
low purchase price
low maintenance costs
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VAISALA RAINCAP® rain sensor
Developed for VAISALA Weather Transmitter
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Construction of the sensor
Robust sensor with negligible maintenance needs
Simple design without any moving parts
Sensor frame
Sensor cover
Piezo detector
Electronics + Software
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Measurement principle
The drop impact generates elastic waves to the sensor plate, and further on to the piezoelectric sensor.
The resulting mechanical stresses in the piezoelectric material causes a voltage U(t) between the sensor electrodes.
The voltage output U(t) from the piezo detector due to a drop impact is proportional to the drop size.
pv = mvt
Piezo detector
Electronics Algorithm
U(t) = c(dp(t)/dt)
DSD output
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Sensor output
The instrument divides the measured data into eight drop-size classes and normalizes the drop diameters with a weighted equivalent drop diameter.
As an example, all data in the class 1.795-2.244 mm are normalized to 2.0 mm in the number of drops. Therefore, the number of drops in a class can be expressed with a decimal point.
Size class Weighted diameter [mm] Range [mm]
1 1.00 - 1.122
2 1.25 1.122 - 1.403
3 1.60 1.403 - 1.795
4 2.00 1.795 - 2.244
5 2.50 2.244 - 2.895
6 3.20 2.896 - 3.591
7 4.00 3.591 - 4.489
8 5.00 4.489 -
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Experimental arrangements: Vaisala Rain Laboratory
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Experimental arrangements: Drop velocity and shape measurements
• The converted voltage signal, was directly proportional to the area of the laser beam intercepted by the raindrops. Every drop fell through both beams producing two sequential voltage signals. By comparing the resulting signal pairs, we ensured that no acceleration occurred. From the time difference, Δt, between the peak values of the voltage signals, speed of the drop could be calculated.
• Vertical radius a was calculated from the width of the voltage pulse produced by the parallel beam linear sensor, horizontal radius b from the voltage amplitude.
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Experimental arrangements:Vaisala Rain Laboratory
Since, the physical process behind the raindrop impact is a function of drop size, shape and impacting velocity. It was important to verify the functionality of the laboratory before beginning the calibration measurements. The verification included the determination of fall velocity and the shape of falling raindrops in the laboratory. The work was reported by Salmi and Elomaa (2007).
0 1 2 3 4 5 60
1
2
3
4
5
6
7
8
9
10
Velo
city
[m/s
]
Drop diameter D [mm]
Gunn & Kinzer (1949)Salmi & ElomaaPresent empirical formula
1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 60.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
1.05
1.1
Axi
s ra
tio
Drop diameter D [mm]
Pruppacher & Beard (1970)Andsager et al. (1998)Salmi & ElomaaPresent experiment
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Results
The table shows median value of terminal velocity, measured with parallel beam linear sensor and standard deviation of three measurement instances. From which we have calculated drop sizes and compared them against median values of measured drop size. Also standard deviation of measured drop size is shown. All data values contain about 2000 individual measurements.
Velocity measured [m/s] Diameter [mm] Diameter measured [mm]
v(median) v(std) D D(std) D(median) D(std)
6.7808 0.1116 2.09 0.03 2.09 0.3906
8.0406 0.0572 3.01 0.03 2.99 0.8374
8.7417 0.0498 3.99 0.055 3.97 1.2882
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Results
A typical example of measured DSD with drops ranging from 2.98-3.04mm in size.
0
200
400
600
800
1000
1200
1400
1600
1800
1 1.25 1.6 2 2.5 3.2 4 5
Drop diameter [mm]
Num
ber of dro
ps
.
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Conclusions
The STD of measured data is significant. This reflects very well the characteristic behavior of the instrument namely: sensitivity variations over the sensor area (due to surface wetness and construction of the sensor itself), and the production of statistical error (seen particularly in the short integration time).
Vaisala RAINCAP rain sensor cannot detect drop sizes below ~0.8mm. Radar reflectivity is proportional to D6, bigger drops have more importance in calculations.
Applying the technology used in the Vaisala RAINCAP rain sensor, we have a great possibility of developing an affordable disdrometer with negligible maintenance.
Further study is still needed to clarify the ability to adjust Z - R relation in radar application.
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Contact information
Atte Salmi
Product Development Manager
Vaisala Oyj
Phone +358 9 8949 2785