01
Best input pair and density basis
Dry bulb plus humidity ratio is useful when a moisture balance already provides W; dry bulb plus RH is more common for direct probes. Enter actual station pressure when converting measured volume flow to mass flow.
Dry-air density is the reciprocal of specific volume on a dry-air mass basis. Moist-air density includes vapor mass in the same mixture volume. State which basis is used rather than labeling both simply air density.
02
Verified example
At 25 °C, 50% RH and 101.325 kPa, moist-air density is 1.1770 kg/m³ and specific volume is 0.858 m³/kg dry air. At 84.6 kPa, moist-air density falls to 0.9815 kg/m³ and specific volume rises to 1.031 m³/kg dry air.
A nominal 1 m³/s stream therefore does not carry the same mass at those pressures. Use the density corresponding to the measured state and the definition expected by the fan or balance calculation.
03
Interpretation for field work
Temperature and pressure usually dominate density; moisture has a smaller real effect because water vapor has lower molecular mass than dry air. Record whether the volume flow is actual or standardized before multiplying.
For commissioning, keep density conversion separate from sensor accuracy and duct traverse quality. A precise psychrometric calculation cannot rescue an unrepresentative velocity profile.
04
Common mistakes
Do not use gauge pressure, airport altimeter setting without understanding its reduction, or elevation as if it were measured pressure. Do not invert moist-air density and call the result dry-air-basis specific volume without converting the basis.
Avoid mixing standard cubic units with actual cubic units. Round only after completing the mass-flow calculation, especially when comparing two branches.
05
Limits and connected workflow
The perfect-gas model is intended for the documented atmospheric temperature and pressure range; non-ideal high-pressure effects are excluded. Critical metering requires the instrument and contractual density convention.
Use the altitude reference for a correction sequence, then the HVAC commissioning page to connect corrected mass flow with outdoor, return and supply states.