01
Use the chart when relationships matter
The chart is most useful when one number is not enough: comparing outdoor, return and supply air; checking whether a cooling path crosses saturation; or explaining why heating changes relative humidity without removing moisture. Select dry bulb plus relative humidity for a direct field-reading workflow, then enter measured station pressure.
PsychroField draws saturation, relative-humidity families and state markers from the same calculation engine used by the result table. It does not use scanned chart artwork or third-party coordinate data. The accessible table is the numeric record; the graphic explains the relationships.
02
Worked state: 25 °C and 50% RH
At 101.325 kPa, 25 °C and 50% RH solve to 9.883 g water/kg dry air, 13.864 °C dew point, 17.889 °C thermodynamic wet bulb and 50.326 kJ/kg dry-air enthalpy. The chart point sits above 25 °C on the x-axis and below the 10 g/kg horizontal level.
Move horizontally left from that point to the saturation boundary to understand dew point: moisture content is unchanged while temperature falls. A horizontal move right represents sensible heating at constant humidity ratio; its relative humidity decreases because saturation pressure rises.
03
Plot comparison states deliberately
Give states operational labels such as Outdoor, Return, Mixed and Supply rather than Point 1. Keep pressure common when interpreting a process or mixture on one chart. If pressure changes, make a separate comparison because the chart coordinate field itself changes.
A line between two points is evidence of direction, not proof of equipment performance. Real coils can have bypass, fan heat, condensate carryover and control cycling. Use the process workbench for ideal balances and retain measured conditions separately.
04
Common chart-reading errors
Do not read relative humidity as moisture mass, treat a sea-level chart as universal, or infer dew point by moving vertically. Do not compare values expressed per kg of dry air with values per kg of moist air. On a small display, use the table rather than estimating from pixels.
A point above saturation is rejected instead of being clipped to the boundary. A calculated wet bulb can differ from a sling or aspirated psychrometer because airflow, wick wetting, radiation and sensor uncertainty are measurement effects outside the equilibrium model.
05
Limits, source context and next step
The plotted field uses the documented perfect-gas moist-air model and retained saturation equations. It is suitable for learning, preliminary analysis and independent checking, not final equipment selection, compliance or safety-critical control. The active pressure and units belong with every exported record.
For a first workflow, plot outdoor and return readings, inspect their humidity ratios and dew points, then open the air-mixing calculator. The methods page explains equation ranges, fixture tolerances and independent comparison evidence.