Hygrometers have many applications: in museums, at home, monitoring weather, in cigar humidors. A hygrometer is an instrument that records relative humidity, and digital hygrometers provide this information in easy-to-read digital format. For many applications, temperature is also needed, and most hygrometers are thermohygrometers that measure both relative humidity and temperature. Choosing a hygrometer involves a detailed knowledge of where it will be placed, degree of accuracy needed and if it will be used for spot-checking or constant monitoring.
Decide if the digital hygrometer will be for indoor or outdoor use. Outdoor hygrometers are waterproof, rugged and designed for extreme swings in humidity. Indoor hygrometers are often (but not always) designed for greater accuracy.
Decide what level of accuracy is required. The accuracy of hygrometers varies by model and price, and margin of error varies from 1 to 5 per cent. For most applications a high degree of accuracy is not needed, but monitoring of highly sensitive objects (such as many museum artefacts) will require great accuracy. To improve accuracy, use a psychrometer to calibrate hygrometers.
Decide if the hygrometer will be for spot-checking conditions or constant monitoring. Hygrometers used for spot-checking are very different than models designed for constant monitoring. Dataloggers are special types of digital hygrometers that record constant data (at user-specified intervals), which can be downloaded into a computer.
Choose a stand-alone digital hygrometer if constant monitoring is not needed. Standard models also provide temperature information and the maximum and minimum readings over the last 24 hours. Accuracy depends on price, and if accuracy is not necessary (5 per cent margin of error), this type of digital hygrometer is inexpensive.
Choose a model with a sensor. If the readout of the hygrometer will be in a separate space from where measurements will be taken, you need a sensor. The sensor can be attached with a cable or wireless (cables are usually more accurate). This type of digital hygrometer is useful when the sensor will be outside or in an exhibit case, but the user wants to record the measurements in an area with easier access.
Choose a portable, digital psychrometer if you need mobility and quick readings. For some applications, a user will need to move the digital hygrometer from location to location. A handheld, digital psychrometer with built-in sensor is useful for mobility. This instrument will adjust more rapidly to changing environments when moved to provide faster readings.
Choose a datalogger when constant monitoring is necessary. Dataloggers record constant data in user-adjusted intervals that can range from seconds to hours. This is critical in certain situations (such as in humidors or museum exhibit areas) when it is necessary to provide a constant, unchanging (or little changing) humidity level for preservation purposes. Data can be downloaded into a computer to view graphs and tables and alert users to humidity problems.
When a high degree of accuracy is needed, it will be necessary to occasionally calibrate digital hygrometers by either shipping the unit to the manufacturer or doing it yourself with a psychrometer.