While printed charts are useful for quick look‑ups, they suffer from low precision, interpolation errors, and inconvenience in iterative calculations. An solves these problems by embedding the governing equations directly into a spreadsheet, allowing rapid, accurate, and repeatable analysis.
[ \ln(p_ws) = \fracC_8T + C_9 + C_10 T + C_11 T^2 + C_12 T^3 + C_13 \ln(T) ]
=B5 * (B4/100) Cell B7:
(SI, kJ/kg dry air )
[ p_ws = 0.61094 \cdot \exp\left( \frac17.625 \cdot T_dbT_db + 243.04 \right) ] where ( T_db ) is in °C, result in kPa. 1. Humidity ratio from vapor pressure
[ v = \frac0.2871 \cdot (T_db + 273.15)P \cdot (1 + 1.6078 \cdot W) ] where 0.2871 = gas constant for dry air (kJ/kg·K), ( P ) in kPa.
– requires iterative solution of the carrier equation: psychrometric chart calculator excel
| Property | Symbol | Typical Units | Description | |----------|--------|---------------|-------------| | Dry‑bulb temperature | T | °C or °F | Ordinary air temperature measured by a standard thermometer | | Wet‑bulb temperature | T w | °C or °F | Temperature recorded by a thermometer with a wet wick; indicates cooling by evaporation | | Dew‑point temperature | T dp | °C or °F | Temperature at which condensation begins for a given moisture content | | Relative humidity | RH | % | Ratio of actual water vapor pressure to saturation pressure at same dry‑bulb | | Humidity ratio (mixing ratio) | W | kg water /kg dry air | Mass of water vapor per mass of dry air | | Enthalpy | h | kJ/kg dry air or Btu/lb dry air | Total heat content (sensible + latent) | | Specific volume | v | m³/kg dry air | Volume per unit mass of dry air | | Vapor pressure | p w | kPa or psi | Partial pressure exerted by water vapor in the mixture | Excel does not have built‑in psychrometric functions. Instead, we must implement empirical correlations from ASHRAE Handbook—Fundamentals. The most important is the saturation vapor pressure over liquid water (Hyland‑Wexler formulation, valid 0–200°C):
– solved iteratively from ( p_ws(T_dp) = p_w ).
For (pressure in kPa, temperature in K): While printed charts are useful for quick look‑ups,
[ W = 0.62198 \cdot \fracp_wP - p_w ] where ( P ) is total atmospheric pressure (typically 101.325 kPa at sea level). The factor 0.62198 is the ratio of molecular weights of water (18.01528) to dry air (28.9645).
=0.62198 * B6 / (B2 - B6) Cell B8:
