Latest Developments in Rainfall Measurement Sensor Technology - Essay Example

Latest Developments in Rainfall Measurement Sensor Technology TABLE OF CONTENTS 0 INTRODUCTION 2 1 Requirements for Sensors 2 1.1.1Requirement in Relation to Measurement Performance 2 1.1.2 Requirements in Relation to the Maintenance of Traceable Measurements over the Operational Cycle 2 1.1.3 Requirements in Relation to Operational Reliability 2 2.0 TEMPERATURE SENSORS 4 2.1 Electronic 4 2.2 Resistive Devices 5 2.3 Weather Temperature Sensor Requirement 6 2.3.1 Air Temperature Sensors 6 2.3.2 Soil Temperature Sensors 6 3.1 Definition 7 3.2 Latest Developments in Rainfall Measurement Sensor Technology 7 3.3 Requirements for Precipitation Sensors 8 3.4 Non-Catching Precipitation Sensors 8 3.4.1Impact Disdrometers 8 3.4.2Optical Disdrometers 10 These sensors use one or two thin laser sheets in detecting crossing particles. Furthermore, each particle within the beam blocks transmits light intensity of certain magnitude proportional to its diameter. Then the volume of each droplet is determined from its diameter by applying its size dependent shape properties. 10 3.4.3Optical Sensors 10 3.4.4Microwave Radar Disdrometers 10 These are small radars that are used in determining spectrum of signal backscattered from falling particles. These sensors work on the principle of fall velocity of rain drops and measurement volumetric backscattering. 10 3.5 Instrument Descriptions 11 3.5.1 Rain Gauge 11 3.5.2 The Optical Rain Gauges (ORG) 12 3.5.3 Present Weather Detector 12 3.5.4 Joss–Waldvogel Disdrometer 13 3.5.5 2-D video Disdrometer 13 14 5.0 REFERENCES 14 1.0 INTRODUCTION 1.1 Requirements for Sensors The requirements for sensors are classified into in three interrelated categories: Requirements relating to maintenance of the traceable measurements over the operational cycle Requirements relating to the performance of measurements, and Requirements relating to the operational reliability. 1.1.1 Requirement in Relation to Measurement Performance This covers the sensor’s ability in providing measurements with its uncertainty within pre-determined operating range and conditions. Response time, hysteresis, long-term stability, sensitivity are some of uncertainties. 1.1.2 Requirements in Relation to the Maintenance of Traceable Measurements over the Operational Cycle Consist of equipment and recommended practices that perform regular calibration or field verification over the lifecycle of a sensor. Traceability of measurements from sensors is achieved through the implementation of regional calibration programs specific to each parameter or instrument. All sensors should have long term stability of measurements to guaranty maintenance and calibration intervals of one year or more. 1.1. 3 Requirements in Relation to Operational Reliability Sensors rely on sensor design distinctive features that enable their operation for extended periods without human intervention. Sensors should: Have reliable auxiliary equipment that mitigates the influence of environmental factors on the sensor performances (heating, cooling, venting, and decontamination.). Have minimized sensitivity to external factors (wind, solar radiation, diurnal effect). Withstand extreme conditions (hurricane force winds, ice accretion, extensive periods with extreme temperatures, and intense precipitation). Be robust in order to limit/reduce the impacts of electromagnetic discharges due to lightning, varied supplied power and other sources. Operate with low power consumption. Offer multiple communication outputs. Be Interoperable. 2.0 TEMPERATURE SENSORS Temperature is the specific degree of coldness or hotness as referenced to a specific scale. A temperature sensor is used in detecting changes in physical parameters such as output voltage or resistance that correspond to the temperature change. Two basic types of temperature sensing include: Contact temperature sensing where the sensor has to be in direct physical contact with object or media being sensed. It is used in monitoring solids, liquids or gases Non-contact measurement where interpretation of the radiant energy of a heat source is taken. Temperature sensors are in three families: electronic, electro-mechanical, and resistive. 2.1 Electronic Thermocouple is built from two electrical dissimilar metals are place at one end of a circuit. Thermocouple circuit has the most considerable temperature range when compared to other temperature sensor technologies, that is –200 to +2310°C. Their elaborate designs allow them to hold out against high levels of vibration or mechanical shock. In addition, their small size allows an immediate response to small temperature changes. Advantages of thermocouples include: Relatively inexpensive Small size provides them rapid temperature response Wide temperature range They are ANSI established calibration types More durable as compared to RTDs for use in high-shock and high-vibration applications Disadvantages of thermocouples include: Their Smaller gage wire sizes makes them less stable and shorter in operating life Have to be protected from corrosive environments Additional circuitry/components required to control application loads Special extension wires are required Less stable than RTDs in high temperatures or moderate Reference junction compensation is required They are to be tested in order to verify performance in controlled conditions for critical applications Thermocouple temperature sensors are suitable for both soil and air temperature recording due to their rapid temperature response and accuracy. In addition, they are easy to calibrate. 2.2 Resistive Devices Thermistor is temperature sensor that proportionally changes its resistance in relation to temperature changes. Thermistors are of two types: negative temperature coefficient (NTC) and positive temperature coefficient (PTC). PTCs have positive change in resistance with temperature rise, while NTCs have negative change in resistance when temperatures increase. Advantages of thermistors include: Low component cost Large change in resistance vs. temperature hence good resolution Fast thermal response Linearized resistance model types available Extremely small size leads to faster reaction to changes in temperature and also the ability to use in various assemblies Disadvantages: The have lower temperature exposures as compared to thermocouples or RTDs Limited temperature range Self- heating can affect accuracy No established resistance standards; hence it is hard to calibrate when used at weather station. Non-linear resistance change pushes for additional components in order to get accurate interpretation Additional circuitry/components required to control application loads Increased component/circuitry count decreases system reliability RTDs (Resistive Temperature Devices) embrace a change in electrical resistance in measuring or controlling temperature. A typical RTD has a sensing element, and connection wires to measurement instrument. RTDs are resistance devices hence generate their own heat that adds up to medium being measured. Advantages of RTDs: Wide temperature range –200 to +650°C Very accurate and repeatable Larger voltage output than thermocouples Extremely stable over time: >0.1°C/year drift Excellent resistance linearity Resistance might be determined in the laboratory without significantly varying over time Low variation for better interchangeability They use standard instrumentation cable for connect to its control equipment They are used for point or area sensing They have established industry accepted resistance curves Disadvantages of RTDs: Higher cost than thermocouples or thermistors Self-heating of the RTD affects overall system accuracy They are larger in size than thermocouples or thermistors Thy are not as durable as thermocouples under high-shock or high-vibration environments (Wilson, 2005) 2.3 Weather Temperature Sensor Requirement 2.3.1 Air Temperature Sensors Requirements for temperature sensors include: - Expanded operation range in cold climates (usually below -40 °C). The recommended range is between -80 °C to +60 °C. - Improved response time and sensitivity to measurement and reporting of extreme temperatures. Shields/Screens that house temperature sensors affect quality of measurement of atmospheric temperature. Hence these shields/screens should reduce the effects of atmospheric and environmental factors (such as solar radiation, wind, dew). Thermocouple temperature sensors meet the above requirements; they have rapid temperature response and accurate. In addition, they can be calibrated. 2.3.2 Soil Temperature Sensors The sensors for soil temperature are configured at depth of up to 100 cm in undisturbed soil. The quality of measurement is useful after configuring for certain period, that is, when the earth has settled for approach in consistency and density of the surrounding volume (Ojanpera, 2006). These sensors must have a long-term reliability and stability in their operations. Measuring configuration should reduce soil disturbance and allow easy access to the sensors for any maintenance or replacement with minimum disturbance (Miettinen, 2006). Soil temperature sensors should be installed in protective sleeves/ housing that tolerates both extreme temperatures and abrasive wear. RTD sensors are the best for soli temperature measurement since they exhibit stability characteristics with time. 3.0 PRECIPITATION AND MEASUREMENT 3.1 Definition Precipitation is a broad term any flux of water from the earth’s atmosphere to the land surface. This includes rain, snow, hail, drizzle, and even condensation from cloud or fog. 3.2 Latest Developments in Rainfall Measurement Sensor Technology There are many instruments used in measuring precipitation; for example conventional rain gauge is only used determination of the precipitation duration and intensity. With the development of electronic and optical techniques in the 1970s, various instruments on different principles have been developed to measure the shape, velocity and size of precipitation particles. Sensors installed at automatic weather stations must meet the following interrelated requirements: Requirement relating to the maintenance of traceable measurement over an operational cycle; Requirement in relation to measurement performance of instruments, this covers the ability of an instrument in providing readings with stated uncertainty in a specified operation range and condition; and Requirement relating to the operational reliability of the sensor, this includes feature that enable sensor’s operation for extended periods, not beyond the expected measurement performance, with least possible human intervention during the operation range (Zahumenky, 2008). 3.3 Requirements for Precipitation Sensors Precipitation sensor has to: Measure of traces precipitation (with an accuracy level better than 0.2 mm); Immediately correct wetting caused by wind, and evaporation losses; Eliminate all false precipitation reporting (for example allowing snow flux into the gauges); Improve on the probability of correct identifying precipitation type among mixed precipitation or at freezing points; Effectively perceive the state of ground: water/ice or wetness; Be reliable when calibrated ; 3.4 Non-Catching Precipitation Sensors Gauges for example the Tipping Bucket Rain Gauge (TBRG) collect precipitation via an orifice of well-designed to measure size or quantity of their water equivalent mass, volume or weight accumulate in a certain period of time. Nowadays catching rain gauges are only used in measuring rainfall amounts and intensities. Instruments in the second group determine quantity via contactless measurements, utilizing optical, impact measurement or radar techniques (Boyes, 2003). There are four basic types of non-catching precipitation sensors in use: 3.4.1 Impact Disdrometers These types of sensors use metal or plastic membrane as measurement surface in sensing impact of single precipitation particle. Mechanical movement of the plastic/metal membrane produce elastic waves to the sensor plate that move further onto piezoelectric sensor; which changes stresses into electrical signals that are proportional to the droplets size. Drawback that is eminent with disdrometers is that they incapable of measuring very small drop, that is, those with diameters less than 0.3mm. An example of impact disdrometer is VAISALA WXT 510-VAISALA, Figure 1. Figure 1 VAISALA Impact disdrometer Construction of the sensor: The sensor has no moving parts. The construction of the sensor is shown in below. The sensor’s cover is made of stainless steel; that is attached to the its (sensor’s) frame and a piezoelectric detector that is mounted underside. Voltage pulses delivered from piezoelectric element (transducer) are filtered, and then amplified, digitized, before being analyzed for parameter selection; in relation to the raindrop size. Computations from the sensor are performed by the micro-processor system. Figure 2 A Drawing of the Vaisala RAINCAPÒ sensor. The dimensions and material of the detector cover are selected so as the resonant vibration brought about by the impact of raindrop is attenuated rapidly. Sensor’s surface area is established by settling between two opposite specifications: Measurement principle: RAINCAPO sensor utilizes acoustic detection technology per raindrop. This drop impact creates elastic waves at the sensor plate, with further distribution to piezoelectric sensor. Mechanical stresses in the piezoelectric material result to voltage difference between the sensor electrodes. Each drop size is obtained from the equation below: U (t) ==cm= U is the voltage, while)/dt represents time varying vertical force component, mass of the drop is represented by m, is the terminal velocity of the drop, and c is the constant that is determined from piezoelectric material properties (Bentley, 2005). Sensor calibration: The sensor voltage response can be calibrated at Vaisala Rain Laboratory via dropping water drops of known quantity to the sensor surface. This is possible because the physical process behind any raindrop impact is dependent on drop shape, impacting velocity and size. Hence one requires only verification the laboratory’s functionality before calibration. This verification includes the determination of the fall shape and raindrops in the laboratory (Liu, Gao, & Liu, 2013). 3.4.2 Optical Disdrometers These sensors use one or two thin laser sheets in detecting crossing particles. Furthermore, each particle within the beam blocks transmits light intensity of certain magnitude proportional to its diameter. Then the volume of each droplet is determined from its diameter by applying its size dependent shape properties. 3.4.3 Optical Sensors These sensors are generally designed to provide meteorological visibility (MOR) via measurement of atmospheric forward scattering. When an additional rain sensor is put in place, these sensors provide precipitation intensity and amount through retrieval of the scattering from rain droplets passing through a given volume 3.4.4 Microwave Radar Disdrometers These are small radars that are used in determining spectrum of signal backscattered from falling particles. These sensors work on the principle of fall velocity of rain drops and measurement volumetric backscattering. 3.5 Instrument Descriptions 3.5.1 Rain Gauge Tipping bucket rain gauge (TBRG): Is siphon controlled. The quantity of rain falling on the funnel moves through the siphon control unit eventually being discharged as steady stream into a bucket with two compartment mounted in an unstable equilibrium. Volume of one compartment bucket is equal to a rainfall of 0.2 mm. Hence, resolution and precision is one full bucket (that is 0.2 mm), while accuracy of ±1% at rainfall intensities up to 250mmh−1 or ±3% at 500mmh−1. The major source of TBRG’s sampling error is its inability to capture the small temporal variations during the rainfall time series. Substantial errors especially in the 1 min estimates at low rain rate are evident; this error decreases substantially if timescale of the rainfall increases. Figure 3 Tipping bucket rain gauge Weighing rain gauge (WRG) has the rainwater collected by the buckets then weighed. Rain rate is determined from different rainwater accumulation over interval of time. Accuracy of the rain rate is proportional to the precision of the water accumulation measurement, and the sampling interval. Figure 4 Weighing bucket rain gauge 3.5.2 The Optical Rain Gauges (ORG) Measure the sparkles of infrared light from liquid water drops that fall between receiver and light source. Variable light intensity brought about by a given drop is a proportional to fall velocity, drop size, coherence, and shape of the light source. Figure 5 Optical rain gauge 3.5.3 Present Weather Detector The present weather detectors (PWD are multivariable sensors for automatic weather observing systems. Model Vaisala PWD22 has capacitive component known as Vaisala RAINCAP® rain sensor that is double plated, an optical sensor and a Pt100 thermistor. This optical sensor utilizes the principle of forward scattering in measuring precipitation amount and type or visibility; in addition the sensor has both optical receiver and transmitter. Transmitter discharges pulses with wavelength of 875 nm at 2 kHz (frequency). Sensor’s receiver measures the intensity of light scattered in the a volume of approximately 100 cm3 ; at an angle of 45. Signals produced through forward scattering from suspended particles and precipitation hydrometeors are analyzed to estimate precipitation rate, type, and visibility. RAINCAP® capacitive sensor extracts information on the amount or presence of water on its surface. Platinum Pt100 thermistor is used in monitor the temperature; this thermistor is used as an adjustable parameter in classifying precipitation types. Figure 6 Present weather detector 3.5.4 Joss–Waldvogel Disdrometer The Joss–Waldvogel disdrometer (JWD) is an impact type device that measures the drop sizes using Styrofoam® cone in sampling cross-sectional area of about 50 cm2 l (1967) via measurement of radar reflectivity. JWD transforms energy of falling drops into an electric current. The JWD deduces size an individual drop from the determined impact velocity of the drops via an experimental nonlinear relationship between drop diameter and fall velocity in still air. JWD’s accuracy is susceptible to background noise; this can lead to underestimation of the small raindrops in a heavy rain especially when the Styrofoam cone is hit by many drops. Figure 7 Joss-Waldvogel disdrometer 3.5.5 2-D video Disdrometer A 2-D video disdrometer (2DVD) has two light sources and two perpendicular CCD line-scan at 34.1 kHz; a two-light sheet with spacing of 6.2mm vertically from a virtual area measuring 10 cm by 10 cm. Three-dimensional shape information of drop particles is recorded when passing through the sampling area; regarding the shape, volume, size, oblateness and equivalent drop diameter are calculated. Vertical velocity per particle is determined relative to the distance between the two-light sheets and the traveling time. Hence drop size distribution, precipitation intensity, and velocity distribution is obtained through time integration. The precipitated drops are sorted into 50 sample size at intervals ranging from 0.1mm to 9.9 mm. The resolution of 2DVD is 1 pixel of linear CCD (finer than 0.2 mm) hence cannot measure the drops that are smaller than 0.2 mm. An enclosure of 2DVD might result in errors when detecting small drops; some might be counted more than once or are not counted at all. In addition, the spatial distribution of raindrops that cross the measurement area might be distorted by the wind. Drops seen by light sheet on the upper side might be matched with different drop by the lower side light sheet. Mismatching causes errors in velocities and shapes of particles (Liu, Gao, & Liu, 2013). Figure 8 2D video disdrometer 5.0 REFERENCES Bentley, J. (2005). Principle of Measurement Systems. Essex: Pearson Education. Boyes, W. (Ed.). (2003). Instrumentation Reference Book (3rd ed.). Boston: Butterworth Heinemann. Liu, X., Gao, T. C., & Liu, L. (2013). ATMTO. Retrieved March 31, 2014, from PLA University of Science and Technology: www.atmos-meas-tech-discuss.net/6/519/2013/ Miettinen, V. (2006). Viasala Weather Transmitter WXT510. Retrieved March 31, 2014, from Vaisala: http://www.vaisala.com/wxt50 Ojanpera, O. (2006). Weather Precipitation Gauge VRG101. Retrieved March 31, 2014, from Vaisala: http:www.vaisala.com/vrg101 Wilson, J. (2005). Sensor Technology Handbook. New York: Elsevier. Zahumenky, I. (2008). Surface Measurement Techniques. Commision for Instruments and Methods of Observation. II, pp. 2-9. Geneva: World Meteorogical Organization. Read More