Specific humidity vs humidity ratio
If you work with psychrometrics, you’ll see several “absolute moisture” variables. Two of the most common are specific humidity (often q) and humidity ratio (often W, sometimes called mixing ratio). They are closely related, but they are not the same.
Definitions (what the denominator is)
Mass of water vapor per mass of dry air.
Common in HVAC and psychrometric charts because many energy/moisture balances are written on a dry‑air basis.
Mass of water vapor per mass of moist air (dry air + water vapor).
Common in meteorology and atmospheric science because it’s bounded between 0 and 1.
Conversion (the key relationship)
Since m_moist = m_da + mᵥ, you can convert between them:
In typical comfort/HVAC conditions, W is small (a few g/kg), so q ≈ W numerically — but the distinction still matters when comparing formulas, units, or software outputs.
Humidity ratio vs relative humidity (don’t mix them)
Relative humidity (RH) is temperature‑dependent and describes “how close to saturation” the air is. W/q are closer to “absolute moisture content.” If you want the short version:
Using FluidTool
FluidTool’s humid air calculator uses humidity ratio W as an input option (dry‑air basis), which is standard for psychrometric work. If you have specific humidity q from another source, convert it to W first using the formulas above, then enter Tdb + W (or W + h) depending on what you know.
Common pitfalls
- Units mismatch: W is often shown as g/kg or kg/kg; q may be kg/kg. Always confirm units before comparing numbers.
- Naming differences: “mixing ratio” and “humidity ratio” are sometimes used interchangeably in literature, but the denominator matters. Check the definition in the source you’re using.
- Pressure/altitude: derived moisture properties depend on pressure. Use the correct site pressure (or altitude) for best accuracy.