Rain Monitoring - Be Prepared for Torrential Rain


In 1859 Wilhelm Lambrecht started the production of meteorological


instruments. Today Wilh. Lambrecht GmbH is the most experienced company in the branch. LAMBRECHT products which are characterized by excellent quality, performance and longevity, are supplied to 20.000 customers in more than 150 countries all over the world. The product range was extended by sensors and system solutions for medium budget. The company’s ambition was to reduce the price but to hold the high quality. The result is a sensor portfolio of very best price- performance ratio.


Lambrecht’s long term experience on various markets world wide as well as the successful combination of sales and service network together with highly motivated employees and reliable suppliers are the key and solid foundation for our continuous success and growth.


Further peripheral instruments and accessories can also be provided. The wide range of meteorological sensors and systems completes the product portfolio. The data loggers and software packages allow a comfortable and easy use. Substantial service packages in combination with an effective customer support and a long-term availability of spare parts distinguish Wilh. Lambrecht GmbH as a reliable supplier and partner.


LAMBRECHT supplies all these components and facilitates a simple design of a precipitation network. All components are approved high- quality measuring and evaluation systems for a most reliable and cost- efficient flood warning system.


The Precipitation Sensor (Rain Sensor)


A precipitation sensor is a measuring device used in meteorology and hydrology to measure and record precipitations over a defined period of time.


In this regard sometimes differences are made between rain gauges just for liquid precipitations and snow gauges designed for frozen precipitations only. Precipitation sensors with controlled heating can be used to measure liquid precipitations as well as frozen prec- ipitations (past molten).


Rain gauges exist for many years. Around 500 B.C. the ancient Greeks already used to record precipitations. The first standardised rain gauge was esta-


Precipitation sensor 15189


blished during the mid of the 15th century by king Sejong and crown prince Munjong in the present Korea. These standardised containers were used in the whole country as official gauges.


In 1662 the Briton Sir Christopher Wren invented the first rain gauge based on a mechanical self-emptying tipping bucket. Most of the currently used tipping buckets operate with two buckets which are designed like a seesaw, mirror-symmetrically arranged around a central tipping axis. The “tipping bucket” is designed in such a way that always one bucket is placed under an above fitted funnel. The collected precipitation runs through the funnel into the bucket. If a certain volume is exceeded the tipping bucket moves and the other bucket will be placed below the funnel. Typically at the tipping movement a magnet will be passed over a reed switch and cause a pulse. The ratio of collecting area of the funnel and the filing volume of the bucket results in metric areas for example in 0.1 millimetres precipitation per square metres (mm/m²) or 0.2 mm/m² per pulse. In countries where the inch system is used this ratio is usually 0.01 inch per square metres per tipping.


Example of rain alarm network Rain monitoring - alarm network


The change of global weather is an important matter and a daily discussed issue everywhere. Water and rain are the most important thing on earth and crucial for everybody’s survival. The world is increasingly faced with dangerous and life-threatening heavy rain disasters.


The present flood cases and the related disasters all over the world draw the attention of the responsibles. More and more rain measuring systems are installed to realise an early flood warning.


AUTHOR DETAILS L. Wilhelm


Sales Department Lambrecht Friedländer Weg 65-67, 37085 Goettingen, Germany Tel: 0049 551 4958 0 Fax: 0049 551 4958 312 Email: info@lambrecht.net Web: www.lambrecht.net


Depending on the topographical position of valuable goods or human life, rain must be measured at different points within the catchment area of the rain water. Valley rivers’ volumes increase dramatically as a result of water flow from the mountains. Therefore the rain must be measured at different levels on the mountain and also in the valley. By knowing the topographical situation, the mass of water can be calculated based on continuous data support from automatic rain gauges with online data transfer. This is the only way to warn the population and protect goods against the flood in time.


A precipitation alarm station basically consists of a precipitation sensor (a), a data logger with capable intelligence (b), a communication unit (c), a suitable message receiver (d) and a corresponding software (f). (The commonly needed power supply will not be discussed in more detail at this point.)


The generated pulses are collected, recorded and summed up to a total quantity of precipitation per time period. Established units of precipitation intensities are millimetres per minute (mm/min.), per hour (mm/h) or per day (mm/d), in which the amount of precipitation always refers to one square metre (1m²). Therefore the term “per square metre (/m²)“ generally will be ignored.


In general tipping buckets show an intensity- dependant error of measurement. This error can be minimised by mechanical techniques, but not totally eliminated. Such intensity- dependant deviations of precipitation amount can be electronically compensated directly by the sensor or by a following data logger.


A different method of measuring precipitation weighs the precipitations collected in a storage vessel. Normally such systems measure very precisely and with comparatively high resolution, but they require intensive maintenance and are mostly extremely expensive. (Furthermore non-precipitation events like bugs, leafs or small branches are measured by this sensor as precipitation too. Within the winter period these sensors need a special kind of antifreeze fluid.)


Other measuring systems work optically or even acoustically. The optical measurement often uses a light band and count the drops fallen through. The sensor simultaneously measures the drop diameter to compute the corresponding amount of precipitation. Since these optical precipitation sensors also detect the mode of precipitation and other parameters, they are also named disdrometers. This type of sensor is typically very high in price and suboptimal for quantity measurements, but they give a good response of current intensity of precipitation.


IET


May/June 2010


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