How to use the psychrometric calculator
Dry-bulb and wet-bulb temperature together fix the complete state of moist air, so those two readings plus barometric pressure are all this calculator needs. It applies the standard psychrometric relationships from ASHRAE Fundamentals in inch-pound units and reports relative humidity, humidity ratio in grains per pound, enthalpy in BTU per pound of dry air, dew point, specific volume and vapor pressure.
The sequence is the same one used to draw a psychrometric chart. Saturation pressure at the wet-bulb temperature gives the saturation humidity ratio, the adiabatic saturation relationship converts that into the actual humidity ratio W, and W is turned back into a vapor pressure and compared with saturation pressure at the dry-bulb to give relative humidity. Enthalpy follows from h = 0.240t + W(1061 + 0.444t), the equation that sits behind every cooling coil total-load calculation.
Barometric pressure defaults to the standard sea-level value of 14.696 psia. At altitude the real station pressure matters: Denver at roughly 12.2 psia carries noticeably more moisture at the same wet-bulb reading, which changes both the humidity ratio and the specific volume used to convert CFM into pounds per hour. As a sanity check, 86 °F dry-bulb with a 68 °F wet-bulb at sea level returns about 40% relative humidity and roughly 32.2 BTU/lb of enthalpy, matching the printed chart.
Frequently asked questions
The air is saturated, so relative humidity is 100% and the dew point equals the dry-bulb temperature. That is the point where fog or condensation begins.
Total coil load equals airflow times air density times the enthalpy difference across the coil. Because enthalpy combines sensible and latent heat, it is the basis for sizing chillers and DX equipment.