How to read a psychrometric chart — and a blank sheet to download

On the Carrier chart dry-bulb temperature runs along the bottom and humidity ratio up the right-hand side, in grams of water per kilogram of dry air (grains per pound in I-P). This is the ASHRAE convention and the one you will meet in North American literature and in most equipment selection software.

Enthalpy is not an axis here: isenthalps are the oblique family falling to the right, and on these sheets they double as the wet-bulb lines — see below. The saturation line closes the chart on the upper left, and the region beyond it, where no real air can be, is where the labels and the protractor live.

In this calculation core the wet-bulb temperature is defined adiabatically: it is the temperature at which saturated air would have the same enthalpy as your state. A line of constant t_w is therefore exactly a line of constant h. Drawing both families across the whole sheet would give you doubled, partly overlapping lines that add nothing and only clutter the grid.

So each sheet draws one of them in full and reduces the other to a scale along the saturation line. On the Carrier sheets the wet-bulb family is the one drawn in full — it is the family this chart is read with — and enthalpy appears as labeled stubs. On the Mollier sheets it is the other way round.

The heaviest line, 0.5 mm, is φ = 100 %. Relative-humidity curves follow every 10 % below it and are labeled on the curve itself, because on paper there is nowhere to look up a legend.

Dew point is a move horizontally to the left at constant humidity ratio until you reach the saturation line; read the temperature there. That horizontal move is the one clear advantage of the Carrier layout — on a Mollier sheet the same operation runs down a vertical.

A cooling coil rarely removes only heat. The split between sensible and total load is the sensible heat ratio, and on the chart it is a direction, not a point. The half-circle in the empty region carries the SHR scale on the inside of the arc and ε = Δh/Δx (kJ or Btu per kg or lb of water) on the outside.

Find your SHR on the arc, lay a ruler through it and the center cross, then slide the ruler parallel to itself until it passes through the room state. Where that line meets the saturation line is the apparatus dew point. SHR = 1 is the straight edge of the half-circle — purely sensible, the same direction as ε = ∞ — which is why it is not printed again on the arc.

The I-P sheets are labeled in °F, grains per pound and Btu per pound, with the enthalpy datum at 0 °F as ASHRAE defines it, not the 0 °C datum of the SI sheets. Do not mix numbers between the two systems; convert states, not enthalpies.

Altitude is printed because the chart is only valid at one barometric pressure. The set includes sea level and 5 000 ft (Denver) in I-P, and sea level, 500 m and 1 000 m in SI. Using a sea-level sheet in Denver puts the saturation line in the wrong place and every relative humidity you read off it is wrong with it.

The footer strip carries the pressure, the ranges, the scale in millimeters per unit and a 100 mm control line. Measure that line. Printer drivers shrink pages by a few percent under “fit to page”, and a chart you measure differences on has to be printed at 100 %.