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Salt Humidity

Reference data and engineering information about salt humidity for air psychrometrics applications.

salthumidity

Overview

Engineering reference data for Salt Humidity in air psychrometrics.

Key Formulas

Humidity Ratio

ω=0.622PvPa\omega = 0.622 \frac{P_v}{P_a}

Mass of water vapor per mass of dry air.

Relative Humidity

ϕ=PvPvs×100%\phi = \frac{P_v}{P_{vs}} \times 100\%

Ratio of actual to saturation vapor pressure.

Wet Bulb Temperature

Twb=Tdb−Pvs−PvγT_{wb} = T_{db} - \frac{P_{vs} - P_v}{\gamma}

Temperature measured by wet-bulb thermometer.

Enthalpy of Moist Air

h=cpT+ωhgh = c_p T + \omega h_g

Sensible + latent heat per unit mass of dry air.

Variables

Symbol Description Unit
ω\omega Humidity ratio kg/kg
ϕ\phi Relative humidity %
PvP_v Vapor pressure Pa
PvsP_{vs} Saturation vapor pressure Pa
TdbT_{db} Dry bulb temperature °C
TwbT_{wb} Wet bulb temperature °C

Data Table: Saturated Salt Solutions for Humidity Control

Properties and Calibration Notes

  • Calibration Use: Saturated aqueous solutions of inorganic salts provide a convenient and reliable method for calibrating instruments that measure relative humidity. Each salt establishes a specific, stable humidity level at a given temperature within a sealed container.

  • Temperature Dependence: The equilibrium relative humidity is highly sensitive to temperature. Precise temperature control (±0.5°C or better) is critical for accurate calibration, especially for salts with steep humidity-temperature curves.

  • Preparation: A saturated solution is a slushy mixture of distilled water and a chemically pure salt. The solution must be saturated (excess solid salt present) and given sufficient time (often 24+ hours) to reach equilibrium before use.

  • Important Considerations:

    • Accuracy: The tabulated values are theoretical and can vary by several percent in practice due to factors like incomplete saturation, salt impurities, or dissolved gases.
    • Common Salts: Sodium chloride (NaCl) is one of the most common references, maintaining ~75% RH near room temperature. Lithium chloride (LiCl) is used for very low humidity (~11% RH).
    • Stability: The generated humidity is stable as long as temperature is constant and excess solid salt is present.

Interactive Charts

References