PIEZOELECTRIC VAISALA RAINCAP RAIN SENSOR APPLIED 压电维萨拉雨帽雨量传感器的应用

1、PIEZOELECTRIC VAISALA RAINCAP RAIN SENSOR APPLIED TO DROP SIZE DISTRIBUTION MONITORINGAtte Salmi, Lasse Elomaa, Panu Kopsala and Emmi LaukkanenVaisala Oyj, Helsinki, FinlandPage 2 / date / name / Vaisala ContentsVaisala RAINCAP rain sensorDSD measurements in laboratoryPage 3 / date / name / Vaisala

2、Disdrometer needsRadar adjustements (DSD) (Z R relation)Soil erosion (KE flux) agricultury (soil splash erosion, seal formation, soil aggregates brekdown) hydrology (infiltration, evporation, surface runoff)Page 4 / date / name / Vaisala Low-cost disdrometer low purchase price low maintenance costsP

3、age 5 / date / name / Vaisala VAISALA RAINCAP rain sensor Developed for VAISALA Weather TransmitterPage 6 / date / name / Vaisala Construction of the sensor Robust sensor with negligible maintenance needs Simple design without any moving partsSensor frameSensor coverPiezo detectorElectronics + Softw

4、arePage 7 / date / name / Vaisala 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

5、) from the piezo detector due to a drop impact is proportional to the drop size.pv = mvtPiezo detectorElectronicsAlgorithmU(t) = c(dp(t)/dt)DSD outputPage 8 / date / name / Vaisala Sensor output The instrument divides the measured data into eight drop-size classes and normalizes the drop diameters w

6、ith 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 classWeighted diameter mmRange mm11.00 - 1.12221.251.122 - 1.40331.601.4

7、03 - 1.79542.001.795 - 2.24452.502.244 - 2.89563.202.896 - 3.59174.003.591 - 4.48985.00 4.489 -Page 9 / date / name / Vaisala Experimental arrangements: Vaisala Rain LaboratoryPage 10 / date / name / Vaisala Experimental arrangements: Drop velocity and shape measurements The converted voltage signal

8、, 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, t, between the peak values

9、 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.Page 11 / date / name / Vaisala Experimental arrangements:Vaisala Rain Labo

10、ratory 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 s

11、hape of falling raindrops in the laboratory. The work was reported by Salmi and Elomaa (2007). 0123456012345678910Velocity m/sDrop diameter D mmGunn & Kinzer (1949)Salmi & ElomaaPresent empirical formula11.522.533.544.555.560.60.650.70.750.80.850.90.9511.051.1Axis ratioDrop diameter D mmPruppacher &

12、 Beard (1970)Andsager et al. (1998)Salmi & ElomaaPresent experimentPage 12 / date / name / Vaisala 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

13、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/sDiameter mmDiameter measured mmv(median)v(std)DD(std)D(median)D(std)6.78080.11162.090.032.090.39068.0406

14、0.05723.010.032.990.83748.74170.04983.990.0553.971.2882Page 13 / date / name / Vaisala ResultsA typical example of measured DSD with drops ranging from 2.98-3.04mm in size. Page 14 / date / name / Vaisala Conclusions The STD of measured data is significant. This reflects very well the characteristic

15、 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 disdrome