01
Pressure changes the coordinate field
Relative humidity at a given temperature establishes vapor partial pressure, but humidity ratio compares that vapor pressure with the remaining dry-air pressure. Lower total pressure therefore increases humidity ratio for the same temperature and RH. Specific volume rises and density falls.
Dew point stays tied to vapor pressure in this controlled comparison. Wet bulb changes because the mass and energy relation is pressure dependent.
02
Measured pressure before elevation
Use local station pressure when available. An elevation-derived standard atmosphere is an estimate for average conditions, not local weather. Do not use gauge pressure, and do not confuse an airport sea-level-reduced altimeter setting with station pressure.
Record the source, units and time. Pressure errors matter most for density, mass flow, humidity ratio and wet-bulb interpretation.
03
Verified sea-level and altitude comparison
At 25 °C and 50% RH, 101.325 kPa gives 9.883 g/kg humidity ratio, 0.858 m³/kg dry-air specific volume and 1.1770 kg/m³ moist-air density. At 84.6 kPa, the same temperature and RH give 11.874 g/kg, 1.031 m³/kg and 0.9815 kg/m³.
Dew point remains 13.864 °C, while wet bulb changes from 17.889 °C to 17.460 °C. The comparison shows why a single universal chart cannot preserve every coordinate.
04
Unit and workflow checks
Convert pressure explicitly: kPa, Pa, bar, inHg and psi are easy to confuse. Enter elevation only through the labeled estimator and keep its standard-atmosphere assumption visible. Do not round pressure until after the property calculation.
For volume-to-mass conversion, solve the state at site pressure, choose the declared density basis, and multiply actual volume flow. Standard flow requires its own reference state.
05
Limits and action
The current core uses perfect-gas relations and omits enhancement factors and high-pressure non-ideal behavior. Its published limits are broader than routine HVAC conditions but do not make it an atmospheric forecasting tool.
Next action: use the altitude workflow to compare measured and elevation-estimated pressure, then document which one controls the engineering record.