3.1 Psychrometric Properties and Reading the Chart
Key Takeaways
- A psychrometric state point requires two independent properties at a known barometric-pressure basis.
- Dry-bulb temperature is read on the horizontal axis; humidity ratio is read horizontally to the moisture scale.
- Relative humidity describes proximity to saturation and is not the same quantity as humidity ratio.
- Dew point is found by moving horizontally from the state point to the saturation curve.
- Elevation changes barometric pressure, so a chart or calculation basis must match the field condition when accuracy matters.
Psychrometric Properties and Reading the Chart
Why TAB technicians use psychrometrics
The current NEBB technician course assigns two lessons to psychrometrics and says candidates spend considerable time plotting and understanding the chart. Airflow alone cannot describe an air-conditioning process. A cooling coil may reduce dry-bulb temperature and moisture; an economizer mixes outdoor and return air; a heating coil changes temperature with essentially no moisture addition; and a humidifier changes moisture content. Psychrometrics puts those measurements on one consistent thermodynamic picture.
Psychrometrics is the study of moist-air properties. For ordinary HVAC analysis, dry air and water vapor are treated as a mixture. A state point can be located when two independent properties are known at a stated barometric pressure.
The principal properties
| Property | Meaning | Typical field method |
|---|---|---|
| Dry-bulb temperature (DB) | Ordinary air temperature | Shielded temperature probe |
| Wet-bulb temperature (WB) | Temperature influenced by evaporative cooling | Properly wetted, ventilated sensor or calculated instrument |
| Relative humidity (RH) | Vapor pressure relative to saturation at the same DB | Calibrated humidity probe |
| Humidity ratio (ω) | Mass of water vapor per mass of dry air | Read or calculated from a state point |
| Dew-point temperature (DP) | Temperature at which condensation begins at constant moisture content | Read horizontally to saturation or measured |
| Enthalpy ($h$) | Total sensible-plus-latent energy per mass of dry air | Read from diagonal scale or calculated |
| Specific volume | Volume occupied per mass of dry air | Read from diagonal lines |
Relative humidity and humidity ratio are easy to confuse. If air is heated without adding moisture, its humidity ratio stays nearly constant while relative humidity falls because warmer air has a higher saturation vapor pressure. The actual moisture mass did not disappear.
Chart geometry
On a standard IP psychrometric chart:
- Dry-bulb temperature increases from left to right along the bottom axis.
- Humidity ratio is read on the vertical moisture scale using a horizontal line from the state point.
- Curved relative-humidity lines fill the field; the 100% RH boundary is the saturation curve.
- Wet-bulb and approximate enthalpy lines slope diagonally downward from left to right.
- Specific-volume lines form another diagonal family.
To plot 75°F dry bulb and 50% RH, find 75°F on the dry-bulb axis, move vertically, and stop at the 50% RH curve. From that intersection you may read humidity ratio, wet bulb, dew point, enthalpy, and specific volume. Avoid estimating from a chart whose print scale is distorted or whose pressure basis does not fit the site.
Dew point and condensation
From a state point, move horizontally left to the saturation curve. The dry-bulb temperature at that intersection is the dew point. A duct, diffuser, pipe, or access panel with a surface temperature below the surrounding air's dew point can condense moisture even when room dry-bulb temperature seems comfortable.
Example: If room air has a dew point near 55°F and an uninsulated chilled-water fitting reaches 44°F, condensation is expected unless an effective vapor barrier separates the cold surface from room air. Raising room temperature alone does not necessarily solve the moisture condition.
Measurement quality
Psychrometric results are sensitive to sensor error and sampling location. A wet-bulb wick must be clean, fully wetted with appropriate water, and exposed to adequate airflow. A humidity probe needs current calibration status and time to stabilize. Do not place a sensor against a cold casing, in direct solar radiation, immediately downstream of liquid carryover, or where two airstreams are not mixed.
Use paired readings taken close enough in time that the system has not changed. Economizer dampers, compressors, reheat, humidification, fan speed, and door position can all move the state point. Record the operating mode with the temperatures.
Barometric pressure and elevation
Most printed charts use a stated standard barometric pressure. At high elevation, lower pressure changes density, saturation relationships, and specific volume. For a quick qualitative process plot, a standard chart may still be useful. For report-grade quantities where the difference is material, use the pressure-adjusted chart, software, or calculation method required by the project and record the basis.
A plotting checklist
- Confirm the chart unit system and pressure basis.
- Verify two independent measured properties.
- Plot the intersection without mixing scale families.
- Read the requested property from the correct axis or diagonal.
- Check that dew point does not exceed dry bulb and that the point does not lie beyond saturation.
- Compare the result with sensor accuracy and field conditions.
The exam trap is usually not difficult arithmetic; it is confusing one property with another. Start by naming what remains constant in the described process.
Air is heated without adding or removing moisture. Which property remains approximately constant?
How is dew-point temperature found from a plotted state point on a psychrometric chart?
Why must the chart pressure basis be considered at high elevation?