Reading a Psychrometric Chart
Dry bulb, wet bulb, humidity, and enthalpy all live on one dense graph. Once you can plot a single point, the chart hands you every property of the air you are conditioning — and tells you exactly what your equipment is doing to it.
Skip the chart lookup?
Turn enthalpy and airflow into total, sensible, and latent load with the Ventora BTU load calculator.
In This Guide
What a Psychrometric Chart Really Shows
A psychrometric chart is a graphical map of the physical properties of moist air at a fixed barometric pressure. Air is a mixture of dry gases and water vapor, and how much water it holds changes everything about how it behaves in a duct, across a coil, and inside a conditioned space. The chart ties all of those moisture-and-temperature relationships together so you do not have to solve them by hand.
The single most important idea is this: any two independent properties fix a point, and that point gives you all the others. Measure dry-bulb and wet-bulb temperature in the field, land on the chart, and you can immediately read relative humidity, dew point, moisture content, and total heat. No property stands alone — they are all connected.
Watch the pressure. A standard chart is drawn for sea level (14.696 psia). Above roughly 1,000 ft, air density drops and the chart's density-dependent values (humidity ratio, enthalpy, specific volume) shift. In Denver or the mountain West, use an altitude-corrected chart or software, or your load numbers will read high.
The Seven Properties of Air
Every psychrometric chart plots the same seven properties. Learn where each one lives and the chart stops looking like a plate of spaghetti.
- Dry-bulb temperature (DB) — the horizontal axis along the bottom. This is straight sensible temperature, what a standard thermometer reads. Lines run vertically.
- Wet-bulb temperature (WB) — read on the saturation curve; lines slope diagonally down to the right. WB captures the cooling effect of evaporation and reflects both sensible heat and moisture.
- Relative humidity (RH) — the family of curved lines sweeping up from left to right. The outermost curve is 100% RH (saturation); interior curves mark 10%, 20%, and so on.
- Dew point (DP) — read horizontally to the saturation curve, then down. It is the temperature at which the air becomes saturated and moisture starts to condense.
- Humidity ratio (W) — the vertical axis on the right, in grains of moisture per pound of dry air (7,000 grains = 1 lb). This is the actual water content, and it does not change when you only heat or cool the air sensibly.
- Enthalpy (h) — total heat content in BTU per pound of dry air, read on the diagonal scale off the upper-left edge. Enthalpy lines run nearly parallel to wet-bulb lines.
- Specific volume — steep diagonal lines in cubic feet per pound of dry air. It matters for airflow and fan work, and it is the value most affected by altitude.
Wet-bulb depression
Dry-bulb minus wet-bulb is the wet-bulb depression. A big spread means dry air (lots of room for evaporation); a small spread means humid air. When DB and WB read the same, the air is saturated at 100% RH. It is the fastest gut check you can make with two thermometers.
How to Plot and Read a Point
In the field you almost always start with dry-bulb and wet-bulb, because both come straight off a sling psychrometer or a decent digital probe. Here is the routine:
Step 1: Confirm the chart matches your altitude (sea level for most work).
Step 2: Measure two independent properties — dry-bulb and wet-bulb are the easiest pair.
Step 3: Find your dry-bulb value on the bottom axis and trace that vertical line up.
Step 4: Find your wet-bulb value on the saturation curve and follow its diagonal line down and to the right.
Step 5: Mark the intersection — that is your state point.
Step 6: Read the rest: RH from the nearest curve, humidity ratio straight across to the right axis, dew point straight left to the saturation curve, enthalpy along the diagonal scale.
Why any two properties are enough
Because pressure is already fixed by the chart, the state of moist air only has two degrees of freedom left. Lock down any two independent values — DB and WB, DB and RH, DB and dew point — and the fifth, sixth, and seventh properties are no longer free to move. That is the whole reason the chart works.
Ventora — The AI HVAC Assistant in Your Pocket
Psychrometrics, charging targets, code answers, and load math without the paper chart. Free on iOS.
Worked Example: Coil Performance
Here is where the chart earns its keep. Plot the air entering and leaving a cooling coil, read the enthalpy at each point, and you can size the total, sensible, and latent load the coil is actually delivering — including the moisture removal a temperature-only check completely misses.
Setup:
A system moves 1,200 CFM. Return (entering) air measures 75°F DB / 63°F WB. Supply (leaving) air off the coil measures 55°F DB / 52°F WB. What is the coil doing?
Step 1: Plot both points and read enthalpy
Entering air h₁ ≈ 28.2 BTU/lb (also ~51% RH, ~66 gr/lb, ~55°F dew point). Leaving air h₂ ≈ 21.4 BTU/lb.
Step 2: Total heat (enthalpy method)
Qt = 4.5 × CFM × Δh = 4.5 × 1200 × (28.2 − 21.4) = 4.5 × 1200 × 6.8 = 36,720 BTU/hr
Step 3: Sensible heat
Qs = 1.08 × CFM × ΔT = 1.08 × 1200 × (75 − 55) = 25,920 BTU/hr
Step 4: Latent heat (the moisture work)
Ql = Qt − Qs = 36,720 − 25,920 = 10,800 BTU/hr
Step 5: Sensible heat ratio
SHR = Qs / Qt = 25,920 / 36,720 = 0.71
What the numbers tell you
This coil is delivering about 36,700 BTU/hr total (roughly 3 tons) with an SHR of 0.71 — about 71% sensible, 29% latent. That is healthy dehumidification for a humid climate. A tech who only checked the 20°F temperature split would have logged the sensible side and never seen the 10,800 BTU/hr of moisture the coil is pulling out.
Remember the constants: 1.08 = 0.075 lb/ft³ × 60 min × 0.24 BTU/lb·°F for sensible, and 4.5 = 0.075 × 60 for the total-heat form. Both assume standard air density, which is exactly why altitude correction matters.
Where Each Property Lives (Quick Reference)
Tape this to the inside of your chart until the layout is muscle memory:
| Property | Where to read it | Units |
|---|---|---|
| Dry-bulb temp | Bottom axis, vertical lines | °F |
| Wet-bulb temp | Saturation curve, diagonal lines | °F |
| Relative humidity | Curved sweeping lines (100% = saturation) | % |
| Dew point | Horizontal left to the saturation curve | °F |
| Humidity ratio | Right vertical axis, horizontal lines | gr/lb (or lb/lb) |
| Enthalpy | Diagonal scale, upper-left edge | BTU/lb dry air |
| Specific volume | Steep diagonal lines | ft³/lb dry air |
Note the two ways moisture shows up: humidity ratio is an absolute amount (grains of water per pound of dry air), while relative humidity is a percentage of how much the air could hold at that temperature. Heat air up without adding water and RH falls even though the humidity ratio never moved — a horizontal slide straight across the chart.
What Techs Actually Use It For
The chart is not classroom theory. These are the everyday calls where it pays off:
- Verifying dehumidification. A 20°F temperature split can look fine while the coil removes almost no moisture. Plotting entering and leaving air exposes the latent side and the true SHR.
- Coil sizing and diagnosis. Enthalpy in minus enthalpy out, times 4.5 times CFM, gives real total capacity to compare against nameplate tonnage.
- Comfort complaints. "It's 72 but it feels clammy" is a humidity-ratio problem, not a thermostat problem. The chart shows why.
- Condensation and sweating ducts. Read the dew point of the space; any surface below it will sweat. Great for chasing attic and basement moisture.
- Economizer and ventilation decisions. Comparing the enthalpy of outdoor air to return air tells you whether bringing in outside air actually saves energy.
- Evaporative cooling. Adiabatic saturation follows a constant wet-bulb line — the chart shows exactly how much sensible cooling you can wring out in a dry climate.
Common Mistakes and Pro Tips
Mistakes that wreck your reading
- Using a sea-level chart at 5,000 ft — humidity ratio and enthalpy will read high and your load math drifts.
- A dry or dirty wet-bulb wick, or no airflow across it. Without real evaporation the wet-bulb reading is meaningless.
- Confusing humidity ratio with relative humidity. One is an absolute amount, the other is a percentage — they move differently.
- Sampling supply air where it has already mixed with room air. Read it in the plenum or duct, not at the register.
Pro tips
- Pure sensible heating or cooling is a horizontal move; pure humidification or dehumidification is a vertical move. Real coil processes are a diagonal blend of both.
- Sling your psychrometer for a solid 60–90 seconds and take the lowest stable wet-bulb reading — that is the true value.
- Target-space design conditions of 75°F DB / 50% RH sit around 55°F dew point. Keep indoor surfaces above that and you avoid condensation.
- A digital psychrometer or a chart app removes the parallax and interpolation errors that creep into paper-chart readings.
Psychrometrics Without the Paper Chart
Ventora is the AI HVAC assistant in your pocket — plot air states, run enthalpy and load math, pull charging targets, and get instant code answers from any job site.