How2Day: Sensors Used for a Wireless Weather Station

An anemometer: Measuring wind speed for a wireless weather station. Hemera Technologies/AbleStock.com/Getty Images
Weather sensors were once tethered to a base station in a wired network. Shortcomings included shorter range, routing and concealing the wires, protecting them from lawnmowers, animals and weather elements.

Wireless weather sensors remove all of these obstacles. Most consumer-grade systems offered today package the sensors with an indoor base unit. The sensors may incorporate solar power with battery backup; multi-frequency radio bands (on some models) reduce potential interference with other devices. The majority have an advertised range of 100 to 400 feet. The addition of small radio amplifiers increases the claimed range in higher-end wireless units to 1,000 feet. It's safe to say the home weather enthusiast has many choices available depending on needs and budget.

A temperature-sensitive resistor, called a thermistor, controls the thermometer. Its resistance varies even with minute changes. For example, the Oregon Scientific WMR968 and the Honeywell TM005X weather station use one, and both claim an accuracy of plus or minus 0.2 degrees F.

The humidity sensor, or hygrometer, measures change in an electrical circuit as it's impacted by moisture. Most now combine temperature and moisture monitoring into a thermo-hygrometer. It uses a thermistor for temperature and an electrostatic capacitor for humidity. This detects the change in electric storage potential (capacitance) in a very thin piece of plastic film and converts it to a relative humidity value.

An anemometer measures wind speed based on the principle that a changing magnetic field produces electricity in nearby wires. Three equally spaced cups are attached to a vertically mounted post. A small magnet generates electricity as the shaft turns; this voltage is converted to a value of wind speed by the circuitry. A variation is a propeller mounted on a swiveling horizontal shaft (to keep it facing the wind), but the operating principle is the same. It's essentially the same way a speedometer works.

A downside: these units are subject to ice accumulation, bearing wear, and corrosion. More expensive designs which use sound or lasers are not offered in consumer grade kits due to their expense.

Wind direction is determined with a wind vane. It utilizes a either a simple variable resistance device or magnet attached to some style of pointer. It may be integrated into the anemometer, or be mounted as a separate unit.The voltage or magnetic field varies depending on the direction indicated.

The electronic barometer uses a piezoelectric device, which in this case is a very thin silicon chip. Small pressure changes create a variation in the current flow through the chip, which is interpreted and sent to the display. These are quite accurate; the Davis Weather Pro 2 manual claims accuracy to 0.03 inch with its version.

The rain gauge will be a "tipping bucket" type. A small piece of plastic or metal is mounted on a central swivel. Each end contains a tiny cup which is designed to hold 0.01 inch of rainfall equivalent. A funnel mounted on the housing directs the rain inward.

Each side alternately fills as it rains. It tips downward from the water's weight and empties. The other side swings upward, fills, and the process repeats. It's basically a miniature "see-saw." Electronics within log changes every 0.01 or 0.02 inch depending on design. The data is sent at frequent intervals to the base receiver inside. These are maintenance free as they do not have to be emptied.

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