Drying Gas Humidity: Relative Humidity and Humidity Ratio Have Different Bases

Define the Phase or Process Boundary

Vaisala's humidity-quantity explanation distinguishes relative humidity, a vapor-pressure ratio, from humidity mixing ratio, water-vapor mass per dry-gas mass. Record temperature and total pressure when converting the quantities.

Review a Hypothetical Comparison

Assume an invented ideal-gas comparison supplies vapor partial pressure 1.5 kPa and saturation vapor pressure 3.0 kPa at a fixed temperature. Relative humidity is 50%. At total pressure 100 kPa, the approximate humidity ratio W=0.622×1.5/(100−1.5)=0.009472 kg/kg dry air. At 200 kPa with the same assigned partial pressure and temperature, RH is still 50%, but W≈0.004700 kg/kg dry air. The exercise changes total pressure; it does not describe compressing a closed parcel without changing its partial pressures.

Keep the Evidence with the Decision

Label whether the mass basis is dry air or total moist air. Keep conversion pressure with the result and record which physical variables actually changed. For water-removal balances, use the dry-air mass basis consistently rather than interpreting a percent-RH difference as a directly removed water mass.

The pressure values and ideal-gas approximation are illustrative, not a humidity-device calibration or a compressed-air quality criterion.

Which vapor-pressure or dry-gas-mass denominator belongs to the stated drying-air condition?

This blank worksheet is for your own project. It contains no H M machine trial result.

Customer Questions

Primary References

These references support the technical principles discussed in this guide. The worked examples and review questions are educational.

• Vaisala's humidity-quantity explanation
• Reference from hmpharmamachines.com