PIEZOELECTRIC VAISALA RAINCAP



RAIN SENSOR APPLIED TO DROP SIZE DISTRIBUTION MONITORING

Atte Salmi, Lasse Elomaa, Panu Kopsala and Emmi Laukkanen Vaisala Oyj, Helsinki, Finland

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

Piezo detector Electronics + Software Sensor cover Sensor frame  Robust sensor with negligible maintenance needs  Simple design without any moving parts

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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.

p v

= mv

t

Piezo detector U(t) = c(dp(t)/dt) Electronics Algorithm DSD output

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Sensor output

Size class

1 2 3 4 5 6 7 8

Weighted diameter [mm]

1.00

1.25

1.60

2.00

2.50

3.20

4.00

5.00

Range [mm]

- 1.122

1.122 - 1.403

1.403 - 1.795

1.795 - 2.244

2.244 - 2.895

2.896 - 3.591

3.591 - 4.489

4.489  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.

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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). 10 7 6 9 8 5 4 3 2 1 0 0 Gunn & Kinzer (1949) Salmi & Elomaa Present empirical formula 1 2 3 Drop diameter D [mm] 4 5 6 1.1

1.05

1 0.95

0.9

0.85

0.8

0.75

0.7

0.65

0.6

1 Pruppacher & Beard (1970) Andsager et al. (1998) Salmi & Elomaa Present experiment 1.5

2 2.5

3 3.5

4 Drop diameter D [mm] 4.5

5 5.5

6

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Results

Velocity measured [m/s]

v(median) v(std) 6.7808

8.0406

8.7417

0.1116

0.0572

0.0498

Diameter [mm]

D 2.09

3.01

3.99

D(std) 0.03

0.03

0.055

Diameter measured [mm]

D(median) D(std) 2.09

2.99

3.97

0.3906

0.8374

1.2882

 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.

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Results

1800 1600 1400 1200 1000 800 600 400 200 0 1 1.25

1.6

2 2.5

Drop diameter [mm]

3.2

4 5 A typical example of measured DSD with drops ranging from 2.98-3.04mm in size.

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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 D 6 , bigger drops have more importance in calculations.

 Applying the technology used in the Vaisala RAINCAP a great possibility of developing an affordable disdrometer with negligible maintenance.  rain sensor, we have  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 [email protected]

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