Air Humidity Measurement

Author: Hans-Jürgen Schwarz

English Translation by Sandra Leithäuser
back to Climatic measurements

Measuring humidity overview

• Physical Principles
  • Dalton´s Law
  • Water Vapor Partial Pressure in the air
• Measured Variables
  • Dewpoint
  • Saturation humidity
  • Relative humidity
  • Absolute humidity
  • Mixing ratio
• Moisture measurement methods
  • Chilled Mirror Hygrometer
  • Psychrometric Measuring Methods
  • Capacitive Measuring Methods
  • Hygrometric Measuring Methods
    • Hair Measuring Element
    • Plastic Measuring Element
  • LiCl Condensation Hygrometer
  • Electrolytic Hygrometer
  • Infrared Hygrometer
  • Bi-Strip Hygrometer (Feather Hygrometer)
  • Aluminium Oxide Sensor
  • Resistive Sensors
• Comparison of Measuring Methods
• Measuring Humidity in Practice

Introduction

Air is a mixture of gases. The main ones are summarized in Table 1. In addition to these, air always contains a small amount of water, which exists in three states of matter:

• solid: ice, hail, snow
• liquid: rain, fog, dew
• gas: water vapor

Table 1: Composition of air

Gas	Symbol	Content in Vol. %
Nitrogen Oxigen Argon	N2 O2 Ar	78 21 0,8
Krypton Xenon Neon Helium Hydrogen Carbon dioxide Methane Ozone	Kr Xe Ne He H2 CO2 CH4 O3	Trace gases

Water is the only gas in the atmosphere that is present in its three states, gas, liquid and solid. For the building trade, this leads to some undesirable phenomena such as condensation, corrosion and frost. Both temperature and humidity are the main parameters needed to describe ambient conditions and both influence our well-being significantly.