I have long suspected that my weather station's rain sensor under reports the amount of rainfall, but in late 2008 it became clear that it was not reading at all. I confirmed this by pouring water through it - nothing was detected. I had no option but to take it apart and find the problem.
The first picture shows the underside of the rain sensor. After removing the support bracket, the upper part of the body is easily removed by pushing the clips near each mounting tab towards the centre.
The two parts of the sensor are shown in this second photograph. The "lid", which includes the rain funnel, is shown at the back. One of the clips that attaches it to the base can easily be seen. The sensor mechanism is at the front of the picture. Its operation is very simple. Rain drips from the funnel onto one side of the tipping bucket. As the bucket fills, the increase in weight causes it to tip, bringing the other side under the funnel. As it tips a magnet briefly closes a reed switch housed just behind the bucket.
The problem with my rain sensor was easy to find. A spider had taken up residence inside it and spun "silk" around the bucket, preventing it from moving. A quick wash and scrub under the tap restored normal operation. (The spider was released unharmed into the garden.)
This final picture shows my attempt to prevent any future problems. I've "spider proofed" the rain sensor by wrapping its underside in old net curtain material. This is not a pretty solution, but it should work. It wouldn't be necessary if a bit more thought had gone into the sensor's original design.
As might be expected, sunlight caused the elastic band to degrade and the net curtain material fell off in spring 2009. I've implemented a more permanent solution by cutting some stainless steel mesh to size and attaching it to the holes at each end of the base with cable ties.