How to read a psychrometric chart

What this covers

• What each of the chart's axes and curved lines represents.
• Why a straight sensible cooling line runs horizontally and a dehumidifying line does not.
• How to locate a point on the chart from two measured properties.
• Why mixing two airstreams draws a straight line between their two points.

What changes the result

• Reading dry bulb temperature alone and ignoring humidity ratio, which misses half of what the chart is showing about a process.
• Assuming every cooling process is a straight horizontal line, when most real cooling coil processes remove moisture and therefore slope downward as well as leftward.
• Confusing relative humidity, a curved family of lines, with humidity ratio, the straight horizontal grid the chart is actually built on.
• Reading the chart at sea-level barometric pressure for a project at meaningfully different elevation without accounting for the pressure difference.

What the two main axes fix

The chart's horizontal axis is dry bulb temperature, the plain air temperature a standard thermometer reads. The vertical axis, on the right-hand side on most chart layouts, is humidity ratio, the actual mass of moisture carried per unit mass of dry air, read as a straight horizontal grid regardless of temperature. Locating a point from these two properties alone, if both are known, fixes every other property of the air at that point.

Relative humidity is not a straight axis. It is a family of curved lines sweeping across the chart, each one representing a constant percentage of the maximum moisture the air could hold at that temperature. Because the air's moisture-holding capacity rises with temperature, a fixed humidity ratio corresponds to a falling relative humidity as dry bulb temperature rises along that same horizontal line, which is why warming air without adding moisture lowers its relative humidity even though nothing was removed.

The other lines worth knowing before you trace a process

Wet bulb temperature lines run diagonally across the chart, and enthalpy, the total heat content of the air, tracks closely with wet bulb temperature on most chart layouts, which is why sizing and energy calculations lean on wet bulb readings rather than dry bulb alone. The saturation curve, the chart's left boundary, is the line of 100 percent relative humidity, where air can hold no additional moisture at that temperature without condensing it.

Dew point for any plotted air condition is read by moving horizontally left from that point to the saturation curve; the dry bulb temperature at that intersection is the dew point, the temperature at which the air's actual moisture content would begin to condense if cooled with no change in moisture.

Tracing the four processes

Sensible heating and sensible cooling both move a point horizontally, because neither changes moisture content, only temperature. A gas furnace heating recirculated air, or a heating coil with no moisture added or removed, draws a straight rightward line at constant humidity ratio. A sensible-only cooling process, uncommon on its own but useful as a reference case, draws the same line leftward.

Humidifying and dehumidifying both move a point vertically when they act alone, adding moisture without changing dry bulb temperature raises the point, and removing moisture without changing temperature lowers it. In practice cooling coils rarely produce a pure vertical or pure horizontal line: a real cooling coil process is a diagonal line moving down and to the left, because the coil surface runs cold enough to condense moisture from the air while also lowering its temperature, which is the combined sensible and latent cooling process most residential and commercial coils actually perform.

Mixing two airstreams, such as return air and outdoor ventilation air, draws a straight line connecting the two source points, and the mixed condition lands on that line at a position determined by the proportion of each airstream in the blend: closer to whichever source contributes the larger mass flow.

Why the chart matters for equipment selection

A coil's sensible heat ratio, discussed on its own in the article on sizing for sensible and latent load, is visible directly on the chart as the slope of the process line the coil draws between entering and leaving air conditions: a steeper, more vertical line indicates more latent removal relative to sensible cooling, and a flatter, more horizontal line indicates the reverse. Comparing a required process line against a coil's published performance at the actual entering conditions is a direct visual check that a table of numbers alone does not offer.

The chart is drawn for a specific barometric pressure, and a general-purpose chart states standard sea-level pressure unless labelled otherwise. A project at meaningfully higher elevation needs a chart printed for that elevation, or a correction applied to the standard chart's readings, because air density and the moisture-holding relationship both shift with barometric pressure.

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