Moisture, Humidity, Dew Point

The sensing elements of these designs include (a) wet-dry bulb psychrom-eters, (b) hair or fiber element, (c) cellulose element, (d) thin-film capacitance, (e) Dunmore type, (f) lithium chloride for dew point, (g) surface resistivity sensors (Pope cell), and (h) condensation on chilled surface. The inaccuracies of these sensors are (a) 2% relative humidity (RH), (b) 3-5% RH, (c) 3-5% RH, (d) 2-3% RH standard, (e) Dunmore cell under 1% RH, (f) 1°C (2°F), (g) 3-5% RH, and (h) 0.2-0.4°C (0.3-0.8°F). 

The most frequently used unit in expressing the amount of water vapor in air is relative humidity (0-100% RH). It is the ratio of the mol fraction (or amount) of moisture in a gas mixture to the mol fraction (or amount) of moisture in a saturated mixture at the same temperature and pressure. 

The operating ranges of different humidity sensors. Most humidity/dew point detectors can make measurements at higher values of humidity and temperature, but are limited at low temperatures and at low concentrations. Most are also limited to a maximum operatng temperature of about 200°F (95°C). 

Another frequently used unit is dew point (saturation) or wet-bulb temperature. Dew point is the saturation temperature of a gas-water vapor mixture (the temperature at which water condenses as the gas is cooled). Finally, moisture is also expressed in volume or mass ratio as parts per million (ppm) volume or weight. 

The operation of RH detectors can be based on the dry- and wet-bulb reading, temperature difference, elongation of cellulose, resistance of lithium chloride (Dunmore), surface resistivity of polystyrene (Pope), and change in capacitance. The capacitors can be formed from aluminum or hygroscopic polymers. 

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