Relative and absolute humidity: understanding the difference

What is relative humidity?

‍

Relative humidity (RH) is the Percentage ratio between the pressure of water vapor in the air and the pressure of saturated vapor at a given temperature.
In other words, it is the Humidity level present in the air compared to the maximum quantity that the air can contain before condensation.

This measure varies according to the Room temperature : the hotter the air, the more water vapor it can contain without reaching the saturation point. That's why we talk about rates relative : at 20°C, 50% relative humidity does not represent the same quantity of water as 50% at 30°C.

🌡️ Example: at 25°C, a 60% relative humidity level means that the air contains 60% of the water vapor that it could potentially retain before saturation.

‍

Why measure relative humidity?

‍

It's the most used value in the areas of building, of the logistics, or even Indoor humidity control, because it provides simple and directly usable information. In particular, it makes it possible to:

• Prevent the condensation problems (e.g.: Ceiling humidity, wall condensation)
  
  
• Maintain a good thermal comfort
  
  
• Preserve materials sensitive to humidity (wood, cardboard, textiles, etc.)
  
  

What is absolute humidity?

‍

Absolute humidity (HA) corresponds to the Mass of water vapor contained in a given volume of air, expressed in g/m³. Contrary to relative humidity, It does not depend on the temperature.

For example, 10 g/m³ means that every cubic meter of air contains 10 grams of water vapor, regardless of thermal conditions.

Absolute humidity is less commonly used on a daily basis, but very useful for:

• The calculations Physics or Climatic
  
  
• The analyses techniques (in industry or laboratories)
  
  
• The measures of dew point And the dehumidifier design
  
  

‍

Relative and absolute humidity: what are the key differences?

‍

‍

How do you measure relative and absolute humidity?

‍

Measuring tools:

• Humidity sensor or professional humidity tester
  
  
• Hygrometer (show theRelative humidity)
  
  
• Wall humidity meter (useful in case ofHumidity in the walls)
  
  
• Some connected sensors make it possible to calculate the two values from the Room temperature and pressure.
  
  

‍

In summary

• Relative humidity varies with temperature and is used in everyday life.
  
  
• Absolute humidity remains constant for a volume of air and is used for technical purposes.
  
  

The two are complementary for a good humidity control in a space.