Psychrometrics and Drying Science
What this domain tests
Psychrometrics is the physics behind every drying decision. Expect to read a psychrometric chart in English and metric units, convert between dry-bulb, wet-bulb, dew point, relative humidity (RH), humidity ratio (grains per pound), specific volume and enthalpy, and use equilibrium moisture content (EMC) to set a defensible drying goal.
Core relationships
- RH is actual vapor pressure divided by saturation vapor pressure at the same dry-bulb temperature - a ratio, not a quantity of moisture.
- Humidity ratio (grains per lb of dry air) is the absolute measure; dew point is the temperature at which that air saturates when cooled at constant humidity ratio.
- Vapor pressure deficit - saturation minus actual vapor pressure - drives evaporation. Air movement strips the saturated boundary layer; dehumidification lowers the vapor pressure the air returns to.
- Heating air lowers RH but removes no water; it raises moisture-holding capacity and speeds evaporation until the space re-equilibrates.
- Wood EMC at 70°F runs roughly 6% at 30% RH, 9% at 50% RH and 16% at 80% RH. The material's dry standard, not a chart value alone, sets the goal.
Worked example
Air at 80°F dry bulb and 60% RH has a saturation vapor pressure near 1.03 inHg, so actual vapor pressure is about 0.62 inHg. Humidity ratio is roughly 92 grains/lb and dew point is about 65°F. A slab or supply pipe at 60°F sits below that dew point, so condensation forms no matter how many air movers run. The correction is to warm the air, dehumidify until dew point drops below surface temperature, or both.
Traps
Raising temperature and calling the space drier; confusing RH with humidity ratio; targeting a fixed 40% RH instead of a material dry standard.