This instrument uses a tipping-bucket mechanism to produce a
contact closure every time it receives a predetermined small
quantity of rainfall (0.2 mm). The body and funnel of the gauge are
of aluminium alloy. An accurately machined septum-ring at the top
gives an aperture of exactly 400 cm 2 . The tipping-bucket
mechanism is mounted inside the body on a cast aluminium-alloy base
equipped with fixing slots, three levelling screws and a spirit
The mechanism consists of a divided bucket pivoted at its centre.
Rain collects in the upper half. When this is full, the mechanism
tilts and discharges the collected water, allowing the other half
of the bucket to begin filling. A siphon device is fitted to the
base of the funnel to control the rate of flow into the buckets.
By ensuring a constant flow rate into the tipping bucket,
calibration is made easier and accuracy improved. The alternate
filling and discharging continue as long as rain is falling, and at
each tilt, magnet momentarily closes the contacts of a reed switch.
In the rain gauge of type RG13H, a heater element is provided
inside the body in order to melt snow.
The heater switches on at temperatures below +4 °C.