1. Moisture Measurement

2. Sensible heat and Latent heat

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

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

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

6. 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. 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

8. Moisture measurement
Dry bulb temperature
Wet bulb temperature

9. 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

10. Wet bulb temperature
Since

12. Other Humidity Measurements
Physical principles: condensation

13. 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

14. Hair tension hygrometer

15. 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

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