A psychrometric chart plots the properties of moist air on one sheet, so you can read any air condition from two known values and see what a coil, humidifier, or mixing box does to it. This guide shows how to read each line, find a state point, and follow the common HVAC processes, with a worked mixed-air and coil example in °F.
The two axes
- Dry-bulb temperature runs along the bottom. It's what a normal thermometer reads. Vertical lines are constant dry-bulb.
- Humidity ratio runs up the right side. It's the mass of water vapor per mass of dry air, in lb/lb or grains per pound (7,000 grains = 1 lb). Horizontal lines are constant humidity ratio. Moving straight up means adding moisture without changing temperature.
The other lines
- Relative humidity curves sweep up and to the right. The top curve is 100% RH, the saturation line. Air can't sit to the left of it.
- Wet-bulb temperature lines slope down to the right. Where a wet-bulb line meets the saturation curve, read the wet-bulb value off the curve.
- Dew point is the temperature where a horizontal line from your point hits the saturation curve. Same humidity ratio, cooled until it's saturated.
- Enthalpy lines run nearly parallel to wet-bulb lines, read on a diagonal scale outside the saturation curve, in Btu per pound of dry air. They're close to wet-bulb lines but not identical, so use the enthalpy scale for energy math.
- Specific volume lines are the steep diagonals, in cubic feet per pound of dry air. Use them to convert CFM to mass flow. Standard air (0.075 lb/ft³) is about 13.3 ft³/lb.
How to find a state point
- Pick any two independent properties you know. On a job that's usually dry-bulb plus wet-bulb (from design conditions) or dry-bulb plus relative humidity (from a space setpoint).
- Find the dry-bulb line on the bottom axis and follow it up.
- Find the second property's line (the wet-bulb line, or the RH curve) and mark where they cross. That's your state point.
- Read everything else from that point: go right for humidity ratio, follow the wet-bulb line up to the saturation curve, go left horizontally to the saturation curve for dew point, and read the enthalpy and specific volume scales.
Dew point and humidity ratio aren't independent of each other (they're the same moisture content), so that pair won't fix a point.
Worked example: outdoor air, return air, mixed air, supply air
This is the example HVAKR's psychrometric chart opens with. All values are at sea level, computed with the ASHRAE Handbook—Fundamentals psychrometric relations and rounded.
- Outdoor air (OA): 100°F dry-bulb, 78°F wet-bulb. Reads as about 109 grains/lb, 38% RH, dew point 70°F, enthalpy 41.2 Btu/lb, specific volume 14.5 ft³/lb.
- Return air (RA): 75°F dry-bulb, 50% RH. Reads as about 65 grains/lb, wet-bulb 62.6°F, dew point 55°F, enthalpy 28.1 Btu/lb.
- Mixed air (MA): 300 CFM of OA mixed with 1,000 CFM of RA. The mixed point sits on the straight line between OA and RA, closer to RA because there's more of it. By mass of dry air it lands at about 80.6°F dry-bulb, 66.5°F wet-bulb, 74.5 grains/lb, 31.0 Btu/lb. The quick CFM-weighted average of dry-bulb, (300 × 100 + 1,000 × 75) ÷ 1,300, gives 80.8°F, close enough for a check.
- Supply air (SA): 55°F, saturated. About 64 grains/lb, enthalpy 23.2 Btu/lb.
The MA to SA line is the cooling coil. It moves left (sensible cooling) and down (moisture removed).
Reading coil load off the chart
For 1,300 CFM through the coil, at sea level, with the common standard-air factors:
- Total: 4.5 × CFM × Δh = 4.5 × 1,300 × (31.0 − 23.2) ≈ 45,800 Btu/h, about 3.8 tons.
- Sensible: 1.08 × CFM × ΔT = 1.08 × 1,300 × (80.6 − 55) ≈ 35,900 Btu/h.
- Latent: 0.68 × CFM × Δgrains = 0.68 × 1,300 × (74.5 − 64.4) ≈ 9,000 Btu/h.
- Sensible heat ratio: 35,900 ÷ (35,900 + 9,000) ≈ 0.80.
A mass-flow calculation using the actual specific volume of each stream gives about 44,100 Btu/h total. The 4.5, 1.08, and 0.68 factors assume standard air, so expect a few percent difference.
The HVAC processes, as lines on the chart
- Sensible heating: horizontal line to the right. Temperature rises, moisture stays the same, RH drops. A heating coil or reheat.
- Sensible cooling: horizontal line to the left. Happens when the coil surface is above the air's dew point, so nothing condenses.
- Cooling and dehumidifying: line down and to the left toward the saturation curve. The coil is below the dew point, so water condenses. Where the line points on the saturation curve is the apparatus dew point.
- Evaporative (adiabatic) cooling: line up and to the left along a constant wet-bulb line. Temperature drops, moisture rises, wet-bulb stays about the same. That's why direct evaporative cooling works in dry climates and barely works in humid ones: the gap between dry-bulb and wet-bulb is the cooling you can get.
- Steam humidification: nearly vertical line up. Moisture rises with almost no temperature change.
- Mixing: straight line between two air streams. The mix point divides the line in proportion to the mass flows, closer to the bigger stream.
- Economizer: the same mixing line, but you're choosing the OA fraction to land the mixed air as close to the supply condition as possible.
Altitude changes the chart
Charts are drawn for one barometric pressure. A standard chart is sea level. At 5,000 ft the standard-atmosphere pressure is about 83% of sea level, so the air is about 17% less dense at the same temperature. The same humidity ratio and dry-bulb give a different wet-bulb and enthalpy, and the 1.08 factor drops to about 0.90. Use a chart or tool set to your site elevation. HVAKR's chart has an Elevation control that redraws every line and recalculates every point.
Common reading mistakes
- Reading wet-bulb off the enthalpy scale (they're close, not the same).
- Using a sea-level chart in Denver.
- Averaging temperatures by CFM for a mix and treating it as exact. Fine for a check, but mix by mass for coil selection.
- Picking dew point and humidity ratio as your two inputs. They're the same information.
- Forgetting that RH changes when temperature changes, even with no moisture added or removed.
Use a digital chart
Paper charts are good for learning and bad for revisions. HVAKR's psychrometric chart lets you drop state points by clicking or by typing two values, draws process lines with airflow, shows ASHRAE 55 comfort zones, adjusts for elevation, and copies the chart as a PNG for your report. For the background theory, see psychrometrics background, and for the outdoor points you start from, see design conditions.
FAQ
How do you read a psychrometric chart?
Find two known properties, usually dry-bulb and wet-bulb temperature, and mark where their lines cross. From that state point, read humidity ratio on the right axis, relative humidity from the curved lines, dew point by moving left to the saturation curve, and enthalpy and specific volume from their diagonal scales.
What is humidity ratio on a psychrometric chart?
Humidity ratio is the vertical axis: pounds (or grains) of water vapor per pound of dry air. It only changes when moisture is added or removed, so heating and sensible cooling are horizontal lines.
How do you find dew point on a psychrometric chart?
Move horizontally left from the state point (constant humidity ratio) until you hit the 100% RH saturation curve. The dry-bulb temperature there is the dew point.
What does specific volume mean on a psychrometric chart?
Specific volume is the volume one pound of dry air (plus its moisture) takes up, in ft³/lb. It's read from the steep diagonal lines. Divide CFM by specific volume to get pounds of dry air per minute.
How is evaporative cooling shown on a psychrometric chart?
As a line moving up and to the left along a constant wet-bulb line. Dry-bulb drops, humidity ratio rises, and wet-bulb stays about the same. The lowest temperature you can reach is the entering wet-bulb.
How do you find mixed air on a psychrometric chart?
Draw a straight line between the two air states. The mixed point divides the line in proportion to the mass flow of each stream and sits closer to the larger one.
Does altitude change the psychrometric chart?
Yes. Charts are drawn for one barometric pressure. At higher elevations, wet-bulb, enthalpy, and specific volume differ for the same dry-bulb and humidity ratio, so use a chart for your site elevation.
Sources
- ASHRAE Handbook—Fundamentals, Chapter 1, Psychrometrics.
- PsychroLib, an implementation of the ASHRAE Fundamentals psychrometric equations, used to compute the example state points: github.com/psychrometrics/psychrolib
- HVAKR docs, Psychrometric Chart.
