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In this type of instrument, the rainfall is fed into a float chamber containing a light float. As the level of the water rises, the vertical movement of the float is transmitted, by a suitable mechanism, into the movement of the pen on the chart.

By suitably adjusting the dimensions of the receiving funnel and the float chamber, any desired scale on the chart can be obtained.


To provide a record over a useful period (at least 24 hours is normally required) the float chamber has either to be very large, or some automatic means has to be provided for emptying the float chamber quickly whenever it becomes full. The pen then returns to the bottom of the chart. This is usually done with some sort of siphoning arrangement. The siphoning process should start fully at a definite time with no tendency for the water to dribble over, either at the beginning or at the end of the siphoning, which should not take longer than 15 seconds.


All gauges in any area or country should have comparable exposures, and the same siting criteria should be applied to all. The gauge should be exposed with its orifice being horizontal over ground level. The height of the gauge orifice above the ground should be as low as possible because the wind velocity increases with height, but it should be high enough to prevent splash from the ground. A standard height of one metre is recommended.

When adequate conditions from the wind is not possible, individual objects should not be closer to the gauge than a distance equal to four times their height.

It can be noted that, in sand wind or dusty areas, combined with humidity during the night for example, the rod of the float can be sometimes stuck at the entrance of the float chamber. To prevent this phenomenon, a regular cleaning of the apparatus has to be done.


Ring collector with a sharp edge linked to a cone-shaped funnel with a sufficient slope to avoid splashing effects.
Opening area : 400cm² to 1000 cm²
Range : 0 to 50 mm
Emptying time of float chamber : 30 seconds
Accuracy ? 1%
Frequency of chart record (indicate on order) :
- drum recorder : daily, weekly,
- strip chart : 2 weeks to monthly or more (paper speed selectable from 10 to 20mm/h)
Stand housing