Specific humidity (SH) is the mass of water vapor present in a given mass of gas, e.g., kg water vapor / kg dry gas.
Relative humidity (RH) is the amount of water vapor present in the gas compared to the amount that could be present in the gas at the same temperature. Thus, RH = SH / saturation SH x 100%. Alternatively, relative humidity is calculated as the fraction of water vapor pressure in the gas (pv) relative to the saturated water vapor pressure (pv,sat) at that temperature: RH = pv / pv,sat x 100%.
Water vapor pressure is the partial pressure due to the water vapor in the humid gas mixture. Dalton’s Law of partial pressures states that the total pressure in a gas is the sum of all the partial pressures of the constituents. Ideal or near ideal gases occupy the same volume for the same number of molecules (at the same temperature and pressure). So, the fraction of water vapor pressure relative to the total pressure is the same as the fraction of water molecules relative to the total number of molecules. Multiplying the amount of water and other (carrier) gasses by their respective molecular weights bring us back to specific humidity.
Conceptually, relative humidity is an indication of how close a gas is to being saturated; a gas with 100% RH is saturated in water vapor. Note that specific humidity is unaffected by temperature whereas relative humidity can be changed by changing the temperature of the gas and/or quantity of water vapor present in the gas. Relative humidity is empirically useful because most materials respond, absorb or adsorb in proportion to relative humidity rather than specific humidity. Specific humidity is useful when considering chemical equilibrium because it is related to the absolute amount of water vapor in a gaseous mixture.
Dew point (sometimes spelled dewpoint) is the temperature at which the gas will become saturated. Dew point is a direct measure of vapor pressure (pv) expressed as a temperature. The dew point temperature is always less than or equal to the temperature of the gas. The closer the dew point is to the temperature of the gas, the closer the gas is to saturation and the higher the relative humidity. If the gas cools to the dew point temperature it is saturated in water vapor and the RH is 100 %. Condensation will occur on any surface cooled to or below the dew point of the surrounding gas.
Dry bulb temperature is the commonly measured temperature from a thermometer. It is called “dry bulb” since the sensing tip of the thermometer is dry (see “wet bulb temperature” for comparison). Since this temperature is so commonly used, it can be assumed that temperatures are dry bulb temperatures unless otherwise designated.
Wet bulb temperature is roughly determined when air is circulated past a wetted thermometer tip. It represents the equilibrium temperature at which water evaporates and brings the air to saturation. Inherent in this definition is an assumption that no heat is lost or gained (i.e., adiabatic system) and the heat loss due to evaporation is balanced by thermal conduction from the air. In practice only carefully constructed systems approach this ideal condition. Wet-bulb temperature differs from dew point. The latter is the balance point where the temperature of liquid or solid water generates a vapor pressure (a tendency to evaporate) equal to the vapor pressure of water in the gas so that no net evaporation occurs. Therefore the dew point is always lower than the wet bulb temperature because at the surface temperature of the wet bulb the water must evaporate to maintain a cooling rate whereas at the dew point temperature the water must be so cold that it will not evaporate (but not so cold that condensation occurs).
Methods for measuring humidity – Common approaches employed to measure humidity and dew point temperature are described below; pros and cons of each are summarized in the table below.
• Wet bulb: In the wet bulb method, water is allowed to evaporate and so cool itself to the point where the heat loss through evaporation equals the heat gain through thermal conduction. This method usually involves a wicking material to bring replacement water to the wet bulb, a sufficient wicking distance (with evaporation) to achieve temperature equilibrium for the replacement water, sufficient gas flow rate and precise temperature measurement.
• Polymer humidity sensor: The operating principle of solid state humidity probes is measurement of some material property of a water-sensitive material. Polymeric materials are generally used for this type of moisture sensor. Water vapor permeates the plastic and changes its electrical properties such as dielectric constant or conductivity. Sorption or desorption of water from the polymeric material occurs as the humidity of the surrounding environment changes. The change in the materials property are measured and converted to various humidity-related values using established calibration data.
• Chilled mirror: In this approach, a sensor head is heated to a temperature well above the expected dew point and a mirror within the sensor is cooled until dew just begins to form on its reflective surface. An optically-controlled servo loop controls the mirror temperature so that the dew neither evaporates nor continues to condense (i.e., the definition of dew point). The temperature at which this equilibrium occurs is measured as the dew point. The chilled mirror technique is a first principles method meaning that the dew point is measured directly as opposed to via correlation of some other measured parameter to a response (calibration) curve.
• Optical: In this method, light of specific frequencies is passed through a cavity of known dimensions. Water vapor absorbs some of the light and the decrease in transmitted light is measured. The reduction in transmitted light is then correlated to the amount of water in the path of the light, and from this the various parameters related to water vapor content of the gas can be calculated.
