Rain gauges come in all shapes and sizes; they can be inexpensive plastic ones which cost less than ten dollars all the way up to extrmely accurate (and pricey) versions in the many hundreds of dollars.
The most popular design for a residential-quality device is a wireless “tipping bucket” type. These can usually be located as much as 100 feet away from the base unit; this is more than sufficient for most homes.
Inside the collection unti housing is a very small “see-saw” style pivot. This balances a plastic or metal beam composed of two scoop-like ends, equally spaced apart and of the same size. Each scoop, or “bucket”, has a capacity of about .01 inch of rain. This amount may vary, but the more accurate models promise to provide readings down to one-hundredth of an inch.
As the precipitation falls into the gauge housing, it is funneled into a hole where the water dips through into the collector cups which alternately fill with water. As they fill they become heavier, and “tip” downward to empty their contents. This is where the name “tipping bucket” comes from. When one side lowers to empty, the other side begins filling so that that water doesn’t “escape” detection.
Attached to the central pivot is an optical or mechanical/electrical sensor which records each swing of the lever. The signal is sent to the home unit which interprets this as .01″ of rainfall.
This type of rainfall collector can be quite inaccurate during heavier rain. As the rainfall rate increases, splashing begins to affect the accuracy of the device. Furthermore, a heavy rain will prevent the device from emptying fast enough to “keep up” with the rapid cup refill rates, resulting in lower than actual totals indicated on your display.
Comparative studies have shown the device to be within a few percent of a manual gauge during light to moderate rain, but the error rate increases to as much as 15% during heavy rain episodes. Of course the advantage to the wireless gauge is that you don’t have to go outside in the rain, or when it’s dark, cold, etc. to check it.
However, the manual cylinder-and-funnel style gauge is far more accurate, has no moving parts, and deosn’t need batteries. A simple graduated (plastic or metal) cylinder with markings from .01″ to 1.00″ (some models go up to 2″) is placed inside a larger smooth cylinder.
A round groove in the bottom of the larger tube keeps the smaller one centered inside. A funnel is inserted into the device which fits down into the inner graduated tube. Notches at the top of the funnel-tube interface allow water to overflow into the outer cylinder when the measured rainfall exceeds an inch.
Which type you choose might depend on your budget, accuracy requirement,and convenience considerations.