Hygrometer
Modern electronic devices use the temperature of condensation (called the dew point), or they sense changes in electrical capacitance or resistance.
Later, in the year 1783, Swiss physicist and geologist Horace Bénédict de Saussure invented a hygrometer that uses a stretched human hair as its sensor.
Crude hygrometers were devised and developed during the Shang dynasty in Ancient China to study weather.
As a result of the heat of sublimation, the wet-bulb temperature will eventually be lower than the dry bulb, although this may take many minutes of continued use of the psychrometer.
Relative humidity can also be determined by locating the intersection of the wet and dry-bulb temperatures on a psychrometric chart.
A sling psychrometer, which uses thermometers attached to a handle is manually spun in free air flow until both temperatures stabilize.
This is due to their fundamental first-principle nature that refers to the core of condensation physics and measures temperature, which is one of the base quantities of the International System of Quantities (length, time, amount of substance, electric current, temperature, luminous intensity, mass).
Capacitive hygrometers measure the effect of humidity on the dielectric constant of a polymer or a metal oxide.
[8][9] This is considered the most accurate primary method of measuring absolute humidity[citation needed], and national standards based on it have been developed in US, UK, EU and Japan.
They are also used in the care of wooden musical instruments such as pianos, guitars, violins, and harps which can be damaged by improper humidity conditions.
Hygrometers play a big part in firefighting as the lower the relative humidity, the more vigorously fuels may burn.
According to the WMO Guide, "The achievable accuracies [for humidity determination] listed in the table refer to good quality instruments that are well operated and maintained.
Hygrometers must be calibrated in air, which is a much less effective heat transfer medium than is water, and many types are subject to drift[12] so need regular recalibration.
In a cold and humid environment, sublimation of ice may occur on the sensor head, whether it is a hair, dew cell, mirror, capacitance sensing element, or dry-bulb thermometer of an aspiration psychrometer.
However, a conventional hygrometer is unable to measure properly under the frost point, and the only way to go around this fundamental problem is to use a heated humidity probe.
The thermometers must be protected from radiant heat and must have a sufficiently high flow of air over the wet bulb for the most accurate results.
"[16] It is very challenging, particularly at low relative humidity, to obtain the maximal theoretical depression of the wet-bulb temperature; an Australian study in the late 1990s found that liquid-in-glass wet-bulb thermometers were warmer than theory predicted even when considerable precautions were taken;[17] these could lead to RH value readings that are 2 to 5 percent points too high.
This property may be exploited to the advantage of the psychrometer by avoiding the need to maintain an ice bulb under freezing conditions.".
[18][19] Since the humidity of the ambient air is calculated indirectly from three temperature measurements, in such a device accurate thermometer calibration is even more important than for a two-bulb configuration.
Slushy mixtures of certain pure salts and distilled water have the property that they maintain an approximately constant humidity in a closed container.
Salt solutions will vary somewhat in humidity with temperature and they can take relatively long times to come to equilibrium, but their ease of use compensates somewhat for these disadvantages in low precision applications, such as checking mechanical and electronic hygrometers.
