Moisture Measurement

Sensible heat and Latent heat

Water vapor in the Air Equation of state, gas law of dry air 1. Water vapor pressure

2. Mixing ratio, r 3. Specific humidity, q

Relationship between Vapor Pressure and Saturation Saturated water vapor pressure, E

4. Clausius-Clapeyron equation 5. Relative humidity, h 6. Dew-point, the temperature to which air is cooled to become saturated at constant pressure.

7. Moist virtual effect T v : the temperature that dry air would need to have at the given pressure in order to have the same density as moist air, assuming idea gas behavior. r l is the liquid water mixing ratio If it’s saturated,

Moisture measurement Wet bulb temperature Dry bulb temperature

The change of enthalpy is more useful in thermodynamics The first law of thermodynamics Work may be represented by Thus, For isobaric process Enthalpy U: internal energy (in joules), p: pressure of the system, (in pascals) V : volume, (in cubic meters)

Wet bulb temperature Since

Other Humidity Measurements Physical principles: condensation

Sling psychrometer heating How to measure moisture below freezing ? 1.A fan draws outside air past a thermometer to measure the ambient dry-bulb temperature, 2.A heating device, 3.A second thermometer to measure the dry-bulb temperature of the heated air, 4. A wet-bulb thermometer

Hair tension hygrometer

Electronic hygrometers Capacitive sensor Resistive sensor Polymer membrane Capacitive sensors sense water vapor by measuring the change in capacitance caused by the amount of water vapor present Resistive sensors use a thin film that changes conductivity according to absorbed water.

Extinction of light Lyman alpha hydrometer At certain wavelengths the absorption and scattering of light can be a strong function of water vapor density.