Also Known As Dew Point, Moist Air, Hygrometer, Psychrometric Chart
What is Relative Humidity?
Relative humidity (RH) is a measure of how much water vapor is in the air, usually expressed as a percentage.
It is the ratio of a mass of water vapor present in a given volume of air to the mass which would be present if the air were saturated assuming that the temperature and total pressure remain the same.
How does it work?
For conditions where perfect gas laws apply [i.e. within the temperature range of –40°F to +150°F (–40°C to +65°C)], relative humidity is expressed as the ratio of the vapor pressure of water present in moist air (mixture of air and water vapor) to the water vapor pressure if the air were saturated at the same temperature and pressure:1,2
When working with gases, pressure is an indication of quantity and can be converted into moles or mass by using the perfect gas law.
- A 100% RH reading means that at a given temperature, a given volume of moist air is saturated with water vapor so it can no longer hold any more water. It also means that the moist air holds the maximum possible number of water vapor molecules which creates the possibility for water vapor to condense if temperature decreases.
- Since the maximum amount of moisture that air can carry increases with increased temperature, it is important to specify air temperature whenever RH is expressed.2 Air and other gases can double the amount of water vapor they can hold with every 18°F (10°C) rise in temperature.
- Use a psychrometric chart to determine your Relative Humidity.
- Find the correct zone of relative humidity with this chart
Relationship between relative humidity and water activity of food materials
Shelf-life and chemical stability (rate of deteriorative reactions) as well as microbial stability of bakery products are controlled by intrinsic (formulation) and extrinsic factors (processing parameters and environmental conditions to which the product is exposed during processing, distribution and storage).
Examples of intrinsic factors that can be controlled by selection of raw materials and ingredients:
- Water activity
- Microorganisms and concentration of reactive compounds
Examples of extrinsic factors: