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ARG100 Raingauge
The ARG100 is a well-designed tipping bucket raingauge which combines durable construc-
tion with very reasonable cost. The gauge offers less resistance to air flow than most previous
designs, which helps to reduce the sampling errors that inevitably occur during wind-driven
rain. The gauge is manufactured for Campbell Scientific by Environmental Measurements
Ltd. (under licence from the Institute of Hydrology), and will provide many years of reliable
operation when carefully sited and installed.
1. Description
The ARG100 raingauge is constructed from UV-resistant, vacuum-moulded plas-
tic and consists of a base and an upper collecting funnel. The base splits into two
parts, the inner section supporting the tipping-bucket mechanism and the outer
providing protection and allowing the unit to be bolted firmly to a suitable
mounting plinth or concrete slab.
The tipping bucket arrangement is similar to most other gauges of this type;
precipitation is collected by the funnel and is passed to one of the two buckets
situated at either end of a short balance arm. The balance arm tips when the first
bucket is full, emptying this bucket and positioning the second under the funnel.
The tipping process repeats indefinitely as long as rain continues to fall, with each
tip corresponding to a fixed quantity of rainfall. At each tip, the moving balance
arm forces a magnet to pass a reed switch, causing contact to be made for a few
milliseconds. A two-core cable is used to connect the gauge to the datalogger
where the switch closures are counted.
The ARG100 is adjusted at manufacture to tip once for each 0.2 mm of rain (a
sensitivity of 0.25 mm/tip is optional). More information on gauge calibration is
given in Section 6.
2. Installation and Siting
2.1 Choice of Site
Site the gauge carefully, avoiding obvious sources of error such as nearby trees or
buildings. A useful ‘rule of thumb’ is that the distance between the gauge and any
obstruction should be at least as great as twice the height of the obstruction above
the ground. For standard meteorological sites in the UK, the Meteorological
Office specify the height at which the rim of a raingauge should be above a short
grass surface, and the ARG100 should be exposed similarly if measurements are
required for comparison with those from agrometeorological or synoptic sites.
No two raingauge designs are ever likely to produce identical
results, and identical raingauges can give slightly different catches
even when sited within a metre of each other.
Research has shown that a raingauge obstructs the flow of air and that the flow
accelerates and turbulence increases over the top of the funnel. This can cause less
rain to be collected in the funnel than otherwise would have fallen on the ground.
In most cases, this phenomenon is ignored but it may be corrected for
arithmetically or overcome physically by placing the gauge in a pit so that the rim
of the funnel is level with the ground. The pit is covered by a grating to simulate
NOTE
