Moisture Measurement

Sensible heat and Latent heat

Equation of state, gas law of dry air Water vapor in the 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 Tv: 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. If it’s saturated, rl is the liquid water mixing ratio

Moisture measurement Dry bulb temperature Wet bulb temperature

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

Wet bulb temperature Since

Other Humidity Measurements Physical principles: condensation

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

Hair tension hygrometer

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

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