Psychrometric Reference
The properties of moist air that every load and airflow calculation uses — dry-bulb, wet-bulb, dew point, relative humidity, humidity ratio, enthalpy, and specific volume — defined, ordered, and anchored to a couple of familiar conditions. The one relationship to memorize: dew point ≤ wet-bulb ≤ dry-bulb, converging at saturation.
Psychrometric properties
| Property | Symbol | Definition |
|---|---|---|
| Dry-bulb temperature | DB | Air temperature from an ordinary thermometer shielded from moisture and radiation — the sensible-heat indicator. |
| Wet-bulb temperature | WB | Temperature from a water-wetted wick in moving air; reflects combined sensible and latent heat (adiabatic saturation). |
| Dew point | DP | The temperature at which water vapor begins to condense as air is cooled at constant pressure (saturation). |
| Relative humidity | RH | Actual water-vapor content as a percentage of the maximum the air could hold at that dry-bulb temperature. |
| Humidity ratio | W | Mass of moisture per pound of dry air, in grains/lb (7,000 grains = 1 lb) or lb/lb. |
| Enthalpy | h | Total heat content of moist air per pound of dry air (BTU/lb) — sensible plus latent. |
| Specific volume | v | Volume per pound of dry air (ft³/lb); the reciprocal of density (~13.33 ft³/lb for standard air). |
The ordering that never changes
Dew point ≤ Wet-bulb ≤ Dry-bulb — and at saturation (100% RH) all three are equal.
Anchor points
| Condition | Wet-bulb | Dew point | RH | Grains | Enthalpy |
|---|---|---|---|---|---|
| Comfort (75°F DB / 50% RH) | 62.5°F | 55°F | 50% | 65 gr | 28.1 BTU/lb |
| AHRI rating (80°F DB / 67°F WB) | 67°F | — | 51% | — | — |
Why psychrometrics runs the whole system
Air conditioning is not just cooling — it is moving air between two states on the psychrometric chart, dropping both temperature (sensible) and moisture (latent). Every number in this reference feeds something else: the saturation temperatures behind superheat and subcooling, the enthalpy difference in the total-heat formula, and the enthalpy comparison an economizer uses to judge outdoor air. The comfort point (75°F, 50% RH) and the AHRI rating point (80°F dry-bulb, 67°F wet-bulb) are the two conditions worth carrying in your head.
Common questions
What is the difference between dry-bulb and wet-bulb temperature?
Dry-bulb is the air temperature from an ordinary thermometer — the sensible-heat reading. Wet-bulb comes from a thermometer with a water-wetted wick in moving air, so evaporation cools it; it reflects both sensible and latent heat. The drier the air, the more the wick evaporates and the further wet-bulb falls below dry-bulb. At 100% humidity, no evaporation happens and the two are equal.
How do dew point, wet-bulb, and dry-bulb relate?
They always fall in the order dew point ≤ wet-bulb ≤ dry-bulb. At saturation (100% relative humidity) all three converge to the same number. The gaps between them widen as the air gets drier, which is why a big dry-bulb-to-wet-bulb spread means low humidity.
What is enthalpy in HVAC?
Enthalpy is the total heat content of moist air per pound of dry air, in BTU/lb — sensible plus latent combined. It is what the total-heat formula (Qt = 4.5 × CFM × Δh) keys on, and it is why economizers can use an enthalpy comparison to decide whether outdoor air is actually "cooler" once its moisture is counted.