The tipping bucket has a long history. According to Wikipedia, Christopher Wren and Robert Hooke created the first tipping bucket rain gauge in 1662. This cross-section image (courtesy of Weathershack.com) shows the basic principle of the tipping bucket. We will examine the physics below. The sensor provides a single event for each bucket full of rain. This sensor scheme provides no information about partial buckets and thus can lead to under-estimates of rainfall under some conditions and over-estimates under others:

• as a rain shower ends, an almost full bucket does not tip so can give an estimate that is low by one bucket.
• when the next rain shower comes along the nearly full bucket will tip almost immediately giving the current rain shower a bonus count.

While valid, such issues are typical for any system of measurement. For the tipping bucket rain gauge, each tip is produced by 0.01 inches (0.0254 cm) of rainfall. This is the measurement least count. For most purposes, this precision is acceptable.

A more significant limitation of the tipping bucket design is due to the dynamics of its behavior: it does not tip instantaneously. It takes time for the bucket to move under the stream of rain water which keeps dripping as the full half bucket and empty half bucket trade places. During this period of transition, some water may actually land in the already filled bucket. So, it is apparent we need an estimate of the expected rainfall rates. See the discussion in How Much Rain.

How Does the Tipping Bucket Work?

The tipping bucket resembles a child’s playground teeter-totter. It is a balance-type scale and thus detects water by weight.

Since its axle is below its center of gravity the bucket is out of balance and has 2 stable positions: fallen to the left, or fallen to the right. As water fills the right half bucket, the mechanism gradually approaches the balanced condition. In the figure below, the center of gravity lies on the line of symmetry of the bucket. As water is added, the center of gravity moves to the right until it is directly above the axle. At this point it is precariously balanced: add a tiny amount of water to the bucket and it falls, rotating clockwise about its axle. This fall continues until the bucket hits a stop. And, of course, the water in the bucket spills out. Then the left half bucket is available to catch water and the next tip will be to the left.

As a design problem, the precise location of the axle below the bucket and along its line of symmetry determines the amount of water required to tip the bucket.  If the axle is placed too low, then the bucket can not hold enough water to produce a tip. As the axle is placed higher (closer to the center of gravity) less water produces a tip.

Advantages of the Tipping Bucket Design

• Self-emptying
• Records each small increment, 0.01 inches, of rainfall. If time of each tip is recorded, then rainfall rate may be estimated.
• Simple mechanism with simple transducer (reed switch or Hall effect switch).
• Wide dynamic range:
  • from 1/100 inch per hour limited by evaporation
  • to 1/100 inch per second limited by tip rate of mechanism
• Minimal maintenance.