Wet-Bulb Sling Psychrometer
RH, dew point, and GPP from dry-bulb and wet-bulb.
Example
You enter
- Dry-bulb (°F) 80
- Wet-bulb (°F) 67
You get
- Relative humidity 50.7 %
- Dew point 60.1 F
- GPP 77.41
Checked against published worked examples from: ASHRAE Handbook (Fundamentals).
Details, formula, and sources
Formula or tableSling psychrometer dry-bulb / wet-bulb pair → relative humidity, dew point, and grains per pound. Vapor pressure from the psychrometer relation e = e_s(Tw) - A x P x (Td - Tw) with A ≈ 0.000662 1/°C at sea level; saturation vapor pressure e_s by the August-Roche-Magnus approximation.
Source and editionAugust-Roche-Magnus formulation by name; ASHRAE Fundamentals psychrometric chart by name.
Free accessPsychrometric formulas free in published engineering texts.
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namesdry_bulb_F, wet_bulb_F, RH_percent, dew_point_F, GPP
- Atmospheric pressure 14.696 psi (sea level) unless user suppliesNIST