piezoelectric vaisala raincap rain sensor applied to drop size distribution monitoring atte salmi,...

PIEZOELECTRIC VAISALA RAINCAP RAIN SENSOR APPLIED TO DROP SIZE DISTRIBUTION MONITORING

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

/ date / name / ©Vaisala

Contents

Vaisala RAINCAP® rain sensor

DSD measurements in laboratory


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)


Low-cost disdrometer

low purchase price

low maintenance costs


VAISALA RAINCAP® rain sensor

Developed for VAISALA Weather Transmitter


Construction of the sensor

Robust sensor with negligible maintenance needs

Simple design without any moving parts

Sensor frame

Sensor cover

Piezo detector

Electronics + Software


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


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 -


Experimental arrangements: Vaisala Rain Laboratory


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.


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


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


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

.


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.


Contact information

Atte Salmi

Product Development Manager

Vaisala Oyj

Phone +358 9 8949 2785

[email protected]

piezoelectric vaisala raincap rain sensor applied to drop size distribution monitoring atte salmi,...

Documents

vaisala construction

vaisala rain laboratorypage

drop diameters

vaisala rain laboratorysince

function of drop size

dropsize classes

vaisala resultsthe table

piezoelectric sensor