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The relative humidity can directly indicate the relative degree of air distance saturation.
The relative humidity is a direct measure of how close the air is to being fully saturated with moisture. When the air is completely dry, the relative humidity is 0%, and as the humidity increases, the air becomes more moist. When the relative humidity approaches 100%, it means the air is nearly saturated and can hold no more moisture.
One important concept related to humidity is the dew point temperature (or frost point in cold conditions). This is the temperature at which air must be cooled to reach saturation without changing its water vapor content or pressure. In simpler terms, it's the temperature at which moisture in the air condenses into dew. The dew point is a key indicator of humidity because when the air reaches saturation, the actual temperature equals the dew point. If the air isn’t saturated, the temperature will always be higher than the dew point. Therefore, the difference between the air temperature and the dew point reflects how far the air is from being fully saturated.
Common instruments for measuring humidity include wet and dry bulb thermometers, hair hygrometers, resistive humidity sensors, thin-film capacitors, and dew point meters.
A wet and dry bulb thermometer consists of two identical thermometers: one measures the ambient air temperature (dry bulb), while the other has a wick soaked in distilled water (wet bulb). As water evaporates from the wet wick, it cools the wet bulb, causing its temperature to drop. The greater the evaporation, the larger the temperature difference between the two bulbs. This difference is used to calculate the relative humidity, though factors like airflow, pressure, and wick size can affect accuracy. These thermometers are widely used but not suitable for temperatures below -10°C.
Hair hygrometers use human or animal hair that expands when humid and contracts when dry. They are less accurate than wet and dry bulb methods but are useful in low-temperature environments.
Resistive humidity sheets work by detecting changes in electrical resistance caused by moisture absorption. Carbon film resistors and lithium chloride-based sensors are common examples. While these are convenient, they are less precise than wet and dry bulb measurements and are often used in weather balloons and remote sensing equipment.
Thin-film capacitors use a polymer dielectric that changes capacitance based on moisture levels. These are compact and reliable, making them popular in telemetry systems.
Finally, dew point meters directly measure the dew point temperature by cooling a mirror until condensation forms. This method is highly accurate but requires clean sampling lines, precise temperature control, and sensitive optical detection to avoid errors from contamination.