11.1 Reading the Psychrometric Chart: Properties and Plotting
Key Takeaways
- Any two independent air properties fix a point on the psychrometric chart, and every other property can then be read from that point.
- Dry-bulb temperature lines run vertically, wet-bulb and enthalpy lines slope down to the right, and relative humidity curves arc upward with the saturation curve at 100% RH.
- Wet-bulb depression is dry-bulb minus wet-bulb temperature, and it is zero at saturation, which is why the wet-bulb, dew point, and dry-bulb temperatures are all equal on the saturation curve.
- Standard air is defined as 0.075 pounds per cubic foot at 70 degrees Fahrenheit and 29.92 inches of mercury, and every airside constant such as 1.08 and 4.5 is derived from that density.
- Humidity ratio is measured in grains of moisture per pound of dry air, and 7,000 grains equal one pound of water.
11.1 Reading the Psychrometric Chart: Properties and Plotting
The HVAC Excellence Competency and Task List asks candidates to "identify what property each line on a psychrometric chart represents," to demonstrate "the ability to plot any two basic points on the psychrometric chart and evaluate the data," and to understand "the thermodynamics of air and water vapor." The requirement appears on five separate competency sheets. No other single skill is repeated as often.
A psychrometric chart is a graphical solution to a set of equations relating the properties of moist air at one barometric pressure — normally sea level, 29.92 in. Hg. Air conditioning is not temperature control; it is the simultaneous control of temperature and moisture, and the chart is where those two are handled together.
1. Air and Water Vapor
Atmospheric air is a mixture of dry air (78% nitrogen, 21% oxygen, 1% other) and water vapor. The two behave independently: Dalton's Law of partial pressures states that each gas in a mixture exerts pressure as though it alone occupied the volume, and total pressure is the sum. The vapor's partial pressure is what drives condensation and evaporation.
The water vapor cycle — a named task-list item — is the same process at atmospheric scale: solar energy evaporates surface water, moist air rises and cools, vapor condenses into clouds at the dew point, and precipitation returns it. An air conditioner runs one leg of that cycle deliberately, cooling air below its dew point to force condensation on the coil.
Standard air is the reference condition for every airside calculation:
- Density: 0.075 lb/ft³
- Temperature: 70°F, barometric pressure 29.92 in. Hg
- Specific volume: 13.33 ft³/lb (the reciprocal of density)
Density falls with altitude and with temperature. At 5,000 feet, air is roughly 83% as dense as at sea level, so all the standard constants must be corrected — a fact that matters for both duct sizing and gas furnace derating.
2. The Seven Properties on the Chart
Learn the direction of each line; the numbers follow.
| Property | Line direction | Units | What it measures |
|---|---|---|---|
| Dry-bulb temperature (DB) | Vertical lines; scale along the bottom | °F | Sensible heat — what an ordinary thermometer reads |
| Wet-bulb temperature (WB) | Sloping down to the right; scale along the saturation curve | °F | Total heat content — what a wetted-wick thermometer reads |
| Relative humidity (RH) | Curved lines arcing up to the right | % | Ratio of actual vapor pressure to saturation vapor pressure at that dry bulb |
| Humidity ratio (W) | Horizontal lines; scale on the right edge | grains/lb or lb/lb | Actual mass of moisture per pound of dry air |
| Dew point (DP) | Read horizontally left to the saturation curve | °F | Temperature at which the air becomes saturated |
| Enthalpy (h) | Sloping down to the right, nearly parallel to WB; scale on the upper-left diagonal | BTU/lb of dry air | Total heat, sensible plus latent |
| Specific volume (v) | Steeply sloping, nearly vertical, down to the right | ft³/lb | Volume occupied by one pound of dry air |
The saturation curve is the left/upper boundary of the chart — 100% RH. On it, dry-bulb, wet-bulb, and dew point are all the same temperature. Air cannot exist to the left of the curve; moisture would condense out.
Understanding wet bulb
A wet-bulb thermometer has a water-soaked wick. Water evaporates from the wick, and evaporation absorbs latent heat from the thermometer, cooling it. The drier the air, the faster the evaporation and the lower the wet-bulb reading.
Wet-bulb depression = DB − WB. It is the direct indicator of how much moisture the air can still absorb:
- Large depression (say 78°F DB / 62°F WB, 16°F depression) → dry air.
- Small depression (78°F DB / 76°F WB, 2°F depression) → very humid air.
- Zero depression → saturated air at 100% RH.
A sling psychrometer or digital psychrometer takes both readings at once; the task list names it as a required instrument. Sling for 30–60 seconds until the wet-bulb reading stops falling, and read the wet bulb first because it rises as soon as slinging stops.
Humidity ratio and grains
Humidity ratio is expressed in grains of moisture per pound of dry air. There are 7,000 grains in one pound of water. Typical values: comfortable indoor air at 75°F/50% RH holds about 65 grains/lb; humid outdoor air at 90°F/70% RH holds about 155 grains/lb. The difference between entering and leaving grains is exactly what a cooling coil removes.
Relative humidity is a ratio, not a quantity
Air at 40°F and 90% RH holds far less moisture than air at 90°F and 40% RH, because warm air can hold much more vapor. This is why winter air is dry indoors: cold outdoor air at high RH, heated to 70°F without adding moisture, arrives at a very low RH. Plot it — the point moves horizontally right along a constant humidity ratio line, crossing RH curves downward.
3. Plotting a Point
Any two independent properties fix a point. Once plotted, every other property is read from that location. In the field the two you measure are almost always dry bulb and wet bulb.
Worked example. Return air measures 75°F DB / 63°F WB.
- Find 75°F on the bottom scale and follow the vertical dry-bulb line up.
- Find 63°F on the saturation curve and follow the wet-bulb line down and to the right.
- Mark the intersection. That is the state point.
- Read the rest:
- RH ≈ 51% (interpolate between the 50% and 60% curves)
- Humidity ratio ≈ 65 grains/lb (horizontal, right-hand scale)
- Dew point ≈ 55°F (horizontal left to the saturation curve)
- Enthalpy ≈ 28.6 BTU/lb (diagonal scale, upper left)
- Specific volume ≈ 13.68 ft³/lb
Second worked example — supply air. Measure 55°F DB / 54°F WB off the coil. Plot it: RH ≈ 94%, humidity ratio ≈ 60 grains/lb, dew point ≈ 53°F, enthalpy ≈ 22.6 BTU/lb. Comparing the two points tells you the coil removed $28.6 - 22.6 = 6.0$ BTU per pound of air, and $65 - 60 = 5$ grains per pound of moisture.
4. What the Chart Tells You in the Field
Comfort zone
ASHRAE Standard 55 defines an acceptable comfort region on the chart — roughly 68–78°F DB and 30–60% RH for typical clothing and activity. The task list requires explaining "the comfort zone and the different temperatures and relative humidity's effect on human comfort."
- Above 60% RH: evaporation from skin slows, the space feels warmer than the thermostat says, and mold and dust-mite growth accelerate.
- Below 30% RH: rapid evaporation makes the space feel cold at the same dry bulb, and occupants report static shock, dry skin, and respiratory irritation.
- Raising RH from 30% to 50% lets many buildings drop the dry-bulb setpoint 2–3°F with no loss of comfort — a real energy saving.
Diagnosing a coil from two plotted points
| Observation on the chart | Meaning |
|---|---|
| Supply point nearly vertically below return (little moisture removed) | Airflow too high, coil too warm, or the load is nearly all sensible |
| Supply point far down and left with a large grain drop | Low airflow, or a very high latent load |
| Supply air at high RH but only a small DB drop | Low airflow across a cold coil — check filter and static pressure |
| Return air at 80°F DB / 72°F WB in a residence | Very high latent load: infiltration, unbalanced ventilation, or duct leakage from a crawlspace |
Apparatus dew point (ADP) is where the coil's process line, extended, crosses the saturation curve — the effective surface temperature of the coil. Comparing the actual leaving-air point to the ADP gives the coil bypass factor, the fraction of air that slips through without touching a fin. A dirty or badly configured coil has a high bypass factor and poor dehumidification even when the total capacity looks acceptable.
A technician measures return air at 78 degrees Fahrenheit dry bulb and 65 degrees Fahrenheit wet bulb. What is the wet-bulb depression, and what does it indicate?
Outdoor air at 35 degrees Fahrenheit and 80% relative humidity is heated to 70 degrees Fahrenheit with no moisture added. What happens on the psychrometric chart, and what is the practical result?
On the psychrometric chart, which set of line directions is correct?