How to read a Mollier h-x chart — and a blank sheet to download

The h-x chart is named after its two coordinates: enthalpy h and humidity ratio x. Humidity ratio runs along the horizontal axis in grams of water per kilogram of dry air — not per kilogram of the mixture, which is what makes the whole chart work: the dry air is the part that does not change as you humidify or dehumidify.

Enthalpy is the oblique axis. Lines of constant h fall from upper left to lower right, and their labels sit outside the saturation line, in the region no real air can occupy. Dry-bulb temperature is then not an axis in the strict sense: the isotherms are the near-horizontal family that fans out slightly as humidity rises.

Mollier tilted the coordinate system on purpose. If enthalpy were drawn horizontally, the whole usable region of moist air would be squeezed into a narrow wedge and the interesting part of the chart would be unreadable. Skewing the h axis opens that wedge up into the roughly rectangular sheet you are holding.

The practical consequence: an adiabatic process is a straight line parallel to the isenthalps. Adiabatic humidification, evaporative cooling and — to within the accuracy of the drawing — the wet-bulb temperature all follow that direction. A ruler laid parallel to an h line is a valid construction, which is exactly why the line weights on this sheet are absolute in millimetres.

The heaviest line on the sheet, 0.5 mm, is φ = 100 %: the saturation line. Air cannot exist to the other side of it; that region is where the enthalpy labels live, and on most sheets the protractor and the mark as well. Every other curve is clipped against it.

Relative-humidity curves are drawn every 10 %. They are labelled on the curve itself, not in a legend — on paper there is nowhere to look things up. All labels of one family sit on a common isotherm so they read as a row.

Dew point is the temperature at which the air would saturate if you cooled it without changing its humidity ratio. On the h-x chart that is a move straight down the x axis — humidity ratio constant — until you hit the saturation line; read the temperature there.

Sheets that do not carry the wet-bulb family have short dew-point ticks set perpendicular to the saturation line, so you do not have to interpolate between isotherms in the crowded corner. Where both families are present the ticks stay but lose their numbers — the wet-bulb labels are already there and the two would overprint.

The half-circle in the empty region is the one part of the sheet that depends on the scale of the axes, and it is the reason a chart cannot simply be scaled on a photocopier. It gives the direction of a process, not a point.

Two scales share one arc: ε = Δh/Δx in kJ per kg of water on the outside, SHR (sensible heat ratio) on the inside. Take the value your load calculation gives, find it on the arc, lay a ruler through it and the centre cross, then slide the ruler parallel to itself until it passes through your starting state. That line is the process. ε = 2501 kJ/kg is pure evaporation; the straight edge of the half-circle is ±∞, i.e. purely sensible heating or cooling.

The stamp is not decoration. It carries the barometric pressure and altitude the sheet was drawn for, the axis ranges, the scale in millimetres per unit, the chart type and the unit system. A psychrometric chart is only valid at one pressure, so a sheet without that number is a sheet you cannot check.

Next to it is the 100 mm control line. Measure it with a ruler after printing. If it is short, the printer scaled the page and the millimetre figures no longer apply — reprint at 100 % with “fit to page” off.