Air has the capacity to absorb water vapor. This vapor is typically formed when evaporation occurs or when hot water gives off steam. The amount of water vapor that can be absorbed depends mainly on temperature. Humidity is a measure of the presence of water vapor in air or a gas. Because humidity levels vary with temperature changes, a humidity probe, or gauge, can be used to measure and record temperature and the amount of moisture in air or a gas.
A humidity probe measures relative humidity. Relative humidity is a measure of the amount of water vapor in an air or gas compared to the maximum amount of vapor that can be absorbed at a given temperature. Air that contains the maximum amount is often referred to as saturated. Relative humidity is expressed as a percent of saturation.
Humidity measurement and control is important in many manufacturing processes. Examples include pharmaceuticals, microelectronics, and medical devices and equipment. Humidity can affect the expansion and contraction of a material, liquid flow properties, growth of bacteria, and the rate of corrosion and rust.
Humidity probes can be purchased as portable handheld units or as permanent monitors for applications requiring continuous moisture detection. These meters come with a wide range of features and typically include some analysis capabilities. After a pre-determined period of use, a humidity probe should be removed from service and returned to a lab for routine testing and recalibration. Humidity probe calibration is usually conducted in an environmental chamber under tightly controlled temperature conditions.
A frequently used type of humidity probe is the capacitive sensor. Changes in electrical capacitance, which is the ability of a material to hold an electrical charge, will occur as water vapor diffuses between metallic-sensing elements. The change in capacitance corresponds to a change in humidity. Capacitive sensors are generally accurate and require recalibration every two to three years.
Too much humidity can negatively impact the performance of a material or product. Most materials contain some water or moisture content and will either release water vapor or soak up moisture from the surroundings. Detecting and controlling the level of moisture in a material or product is important in many applications. A moisture analyzer may be used to determine the moisture content in a material or a solution. These types of analyzers are often used in pharmaceutical manufacturing, chemical preparation, and food preparation to ensure the level of moisture in the final product meets specifications.