Advances in Precipitation Measurement Technology


OTT MESSTECHNIK has developed,what the company believes is the most advanced precipitation measurement technology available. The initial version of the instrument offered users much greater resolution and accuracy across a broader range of weather conditions in comparison with older techniques, and the latest version provides even better performance at a significantly lower cost. This article will illustrate the advantages of the new technology over techniques that have existed for centuries.


Automatic Raingauges Sir Christopher Wren, the 17th


History


The first known records of rainfall were kept by the Ancient Greeks about 500 B.C. This was followed 100 years later by people in India using bowls to record the rainfall. The readings from these were correlated against expected growth, and used as a basis for land taxes. Subsequently, graduated cylinders were employed to collect precipitation that could be inspected daily for


the manual collation of precipitation records. century English designer, astronomer,


geometer, and one of the greatest English architects of his time; was best known for the design of 53 London churches, including St Paul's Cathedral. However, one of his lesser known achievements was the creation in 1662, of a rain gauge that did not require daily visits - the world's first tipping bucket raingauge. The real advantage of this technique was that it enabled a clockwork chart to record rainfall so that Sir Christopher did not need to read and record every single day's data.


Astonishingly, the same basic principle survives to this day in raingauges currently operating in many countries world-wide.


The Tipping Bucket Raingauge (TBR)


The TBR consists of a funnel which is mounted in the top of a cylinder set into the ground or standing upon it. The funnel collects precipitation and passes it on to one of two small buckets which are balanced upon a pivot.


After a specific amount of precipitation (typically 0.2 or 0.5 mm) falls, the bucket tips and an electronic signal is sent to a recorder or datalogger. This process tips (almost all of) the water from the bucket so that it is ready to repeat the process.


Anyone that has been responsible for the collection of data from a tipping bucket raingauge (TBR) will be well aware of its shortcomings:


1. The bucket within a TBR will only tip once it has collected the requisite amount of precipitation. This means that a small amount of, say, light rain will not be recorded, but when at a later stage further rain falls, a tip is induced which would show in the record (incorrectly) that the rain fell at that moment.


2. When a bucket tips, not all of the precipitation is removed; some water adheres to the bucket as a result of surface tension which means that less rain is required for a further tip.


3. Particles of dust or dirt collect in the buckets and affect the surface tension of the drips as well as the tipping volume, until the buckets are cleaned.


4. A filter is often placed in the funnel to limit the entry of detritus to the buckets, but these can become blocked and create significant errors in the data.


5.Water will also adhere to the funnel of a TBR and may be lost through evaporation.


6. In periods of intense rain, TBRs tend to under-read because rain is splashed away by the rapid movement of the pivoting bucket. Precipitation may also be lost under such circumstances as the collection moves from one bucket to the other.


7. A further limitation of TBRs is their inability to collect snow or hail accurately, or to function at low temperatures. This can be assisted by the addition of heating, but remote locations may lack the power requirement that this necessitates.


In summary, whilst the TBR was a brilliant invention in 1662, meteorologists have been looking for something more accurate and less labour-intensive for over 300 years!


October/November 2008 AET OTT Pluvio2 The key advantages of OTT Pluvio2 accurately and reliably in all weather conditions. OTT Pluvio2


are that it can measure precipitation is able to


record the smallest amount of precipitation at exactly the moment that it takes place.


The instrument's accuracy is not diminished by the intensity of precipitation - the measurement range extends from 0.1 to an impressive 30 mm/min.


This technology is also well suited to freezing conditions. The collection chamber contains antifreeze liquid which prevents the accumulation of hail and snow. As a result, the OTT Pluvio2


is able to offer an accurate


measurement range of minus 40 to +60 °C. OTT Pluvio2


cm2 and a 400 cm2 is available with a choice of two collection orifices; a 200 orifice offers a collection capacity up to 1500 mm of precipitation


orifice offers 750 mm capacity. This is a greater capacity than was offered by the original Pluvio which extends the period of operation even further.


An optional heating ring is available for the smaller of the two orifices to prevent snow capping.


During the summer months, most of the collected water evaporates which extends the measurement period further.


Older technologies such as the TBR occasionally


include thermostatically controlled internal heaters to address the problem of snow/ice accumulation, but this can incur a delay in the measurement and may be impractical in remote locations for reasons of power consumption. In contrast, the OTT Pluvio2


operates on very low


power – typically 12 mA at 12V DC and the power requirement is 9.6 to 28 V DC. A sophisticated software filter eliminates the potential effects of wind and each OTT Pluvio2


is individually temperature


calibrated. In contrast with TBRs, this is a life-time calibration so users can expect long-term accurate data throughout all weather conditions. The OTT Pluvio2


weighing measuring principle is based upon


an edge compensated and sealed single load cell, overload protection, direct mounting on the weighing platform and highly sophisticated electronics and algorithm features. The individual temperature characteristics of the load cell are memorised in the


New Technology


OTT MESSTECHNIK has developed a raingauge ('Pluvio') that addresses the weaknesses of the TBR by collecting precipitation and weighing the collected liquid with a highly accurate load cell.


The Pluvio was first launched in 1994 and there are now more than 5000 units in operation all over the world. However, the first version of the OTT Pluvio was expensive. So the company has developed a successor, 'OTT Pluvio2


', which provides even better precipitation


measurement but at significantly lower cost, which makes it accessible to a much larger range of applications.


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