CastorPrep

Psychrometric calculator

Every moist-air property from dry bulb plus one more, at any altitude, from the ASHRAE equations.

Free, right in your browser. Every result shows its work.

°F
%
ft

Sets the pressure from the standard atmosphere. 0 = sea level.

Result

Humidity ratio
0.01413lb/lb (98.92 gr/lb)
Dew point
66.89°F
Wet bulb
75.08°F
Relative humidity
40%
Enthalpy
38.39Btu/lb dry air
Specific volume
14.3ft³/lb dry air
Vapor pressure
0.3265psia
Pressure
14.696psia
  1. 1.Pressure at 0 ft: p = 14.696 psia
  2. 2.Saturation pressure at 95 °F (Hyland–Wexler): p_ws = 0.8162 psia
  3. 3.p_w = RH × p_ws = 0.4 × 0.8162 = 0.3265 psia
  4. 4.W = 0.621945 × p_w / (p − p_w) = 0.01413 lb/lb
  5. 5.RH = p_w / p_ws = 40%
  6. 6.Dew point: the temperature where p_ws(t) = p_w → 66.89 °F
  7. 7.Wet bulb: solve the ASHRAE wet-bulb relation for this W → 75.08 °F
  8. 8.h = 0.240(95) + 0.01413 × [1061 + 0.444(95)] = 38.39 Btu/lb dry air
  9. 9.v = 0.370486 × (95 + 459.67)(1 + 1.607858W) / p = 14.3 ft³/lb dry air

How it's tested on the exam

Shows up on: PE Mechanical (HVAC & Refrigeration) · PE Mechanical (Thermal & Fluid Systems)

  • Using the sea-level chart at altitude. At 5,000 ft the same dry bulb and wet bulb give a noticeably higher humidity ratio. Set the altitude or pressure first.
  • Humidity ratio units. Charts and load equations often use grains per pound (7,000 gr = 1 lb); the enthalpy equation uses lb of water per lb of dry air.
  • Enthalpy per what? Moist-air enthalpy here is per pound of DRY air — multiply by the dry-air mass flow, not the total airflow mass.
  • Reading a chart to three figures it can't give. The equations and the chart agree to the chart's resolution; when choices are close, compute.

Worked example

Outdoor air is 95 °F dry bulb and 40% relative humidity at sea level. Find the humidity ratio, wet bulb, dew point and enthalpy.

  1. 1.Saturation pressure at 95 °F (Hyland–Wexler): p_ws = 0.8162 psia; p_w = 0.40 × 0.8162 = 0.3265 psia
  2. 2.W = 0.621945 × p_w / (14.696 − p_w) = 0.01413 lb/lb (98.9 gr/lb)
  3. 3.Dew point is where p_ws(t) = p_w: 66.9 °F
  4. 4.Wet bulb solves the ASHRAE wet-bulb relation for the same W: 75.1 °F
  5. 5.h = 0.240(95) + 0.01413 × [1061 + 0.444(95)] = 38.39 Btu/lb dry air

W ≈ 0.0141 lb/lb · twb ≈ 75.1 °F · tdp ≈ 66.9 °F · h ≈ 38.4 Btu/lb

Where it is in the Handbook

  • Psychrometrics — Psychrometric Properties — PE Mechanical Reference Handbook
  • Psychrometrics — Psychrometric Charts — PE Mechanical Reference Handbook

Section names as printed in the current NCEES PE Reference Handbooks (Mechanical 2.1, Civil 2.2); numbering can shift between versions. On exam day you'll use your own approved calculator — here's which models NCEES allows.