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A hygrometer is a device that senses relative humidity. This technology, sometimes coupled with thermometers, allows monitoring of humidity conditions for numerous industrial, commercial, and residential applications. An outdoor hygrometer differs from the indoor type in several ways: it is typically weatherproof, and provides readings over a greater range with less precision than indoor types. Determine whether you need to spot check or monitor humidity, the intended placement of the unit, and if you need to use a datalogger for computer tracking.
To ensure the most accurate readings, you can use a psychrometer, or wet-and-dry bulb thermometer, to calibrate a hygrometer. Devices are sometimes protected under solar radiation shields or ventilated housing. They should not be placed in direct sunlight or precipitation.
Humidity is the degree of moisture present in the atmosphere. To best place an outdoor hygrometer, you must consider qualities of ventilation, heat absorption of the ground, and air and water table flow, all of which can skew readings. In other words, a hygrometer should be placed about 5 ft (about 1.5 m) above ground in a level, open space. It should be placed ideally above trimmed grass, which permits air between the sensor and the ground to mix. Hard ground radiates temperatures more intensely; slopes divert water table and air flows, thus distorting readings.
Other considerations for placement of an outdoor hygrometer include nearby objects and environs. It should not be placed near any heating or cooling systems or equipment. Avoid areas that collect water or snow, as well as the shade of trees. A rule of thumb is to place the unit at a distance four times the height of an obstruction, and over 100 feet (about 30 m) from parking lots or streets.
Match the unit to its environmental conditions. An outdoor hygrometer may detect, monitor, and log specified ranges of high or low humidity. The conditions of humidity to be measured should match the unit's design specifications; units may have a margin of error from one to five percent. Typical devices can measure humidity levels from 20% to 90%.
Readouts can be either analog or digital, like a clock. These faces come in all sizes, so knowing your intended placement will allow you to choose a readout that you can see from a given vantage point. If the weather station is a hygrometer-thermometer couple, make sure it can provide readings in the unit you prefer, that is, Celsius or Fahrenheit.
Different models of outdoor hygrometer have added uses. They may work as a standalone gauge or with a remote sensor via cable or wireless link. This permits more convenient access to the control unit in a separate location from the sensor. Some units permit daily log graphs or tallies, while others set alerts if humidity falls outside a preset range. By assessing placements, distorting factors, and a unit's tolerance levels, you can ensure more accurate data collection, environmental monitoring, and analysis insights.