The air in the environment that surrounds us always contains a certain amount of water vapor. The total amount of water vapor is the humidity we feel in the air. The temperature of the environment determines the maximum amount of water vapor that the air can hold. That is why the air feels significantly different in the summer months versus the winter months.
The measurement of the humidity in the air plays a large role in a wide variety of industrial applications. Typical applications for humidity measurement instruments include various systems for measuring moisture in gases, monitoring of laboratory environments and in many drying processes such as compressed air drying.
Each application has a different set of requirements for humidity instruments depending on the amount of water vapor being measured. Humidity can either be measured through the direct measurement of the moisture in the air; or indirectly, by measuring other parameters such as dew-point and temperature, and then calculating the humidity value.
Humidity measurement systems are able to measure the amount of water vapor in a number of different ways.
Absolute Humidity is the measurement of the actual amount of water vapor or moisture in the air, regardless of the air’s temperature.
Relative Humidity (RH) also measures the amount of water vapor, but the measurement is dependent on the temperature of the air. Relative humidity is expressed as a percentage between 0% and 100%. Lower numbers equate to dry conditions and higher numbers represent saturated and humid air conditions.
Dew Point is a measurement of the temperature at which the water vapor in the air condenses into a liquid form. A sample of a gas or the environment is cooled until the air is saturated and dew begins to form. That temperature is defined as the Dew Point temperature. The Dew Point temperature is often used to determine more precise humidity levels for dryer air conditions.
Humidity Measurement Instruments
There are many different types of instruments used in the measurement of humidity. Due to the wide range of humidity values and applications, there is not a single device that is suitable for all measurement types. The types of humidity measurement equipment available are quite diverse and vary in the technology used, price and accuracy.
There are both mechanical and electronic types of humidity measurement equipment, called hygrometers. Most of the electronic hygrometers are also equipped with additional temperature and pressure sensors due to the influence of those parameters in determining highly accurate humidity readings.
Mechanical Hygrometers use an absorbent material, such as human or animal hair or a special paper that is put under some tension and attached to a pointer mechanism at one end. As the material absorbs the water vapor it will change its dimensions and move the pointer on a dial scale to indicate the humidity.
Psychrometers are simple devices that consist of a pair of thermometers. One of the thermometers has a fabric sheath (wick) around it that is dampened with water prior to its use. A sample of the air is moved over the thermometers usually by a small fan or by the sling method. The evaporation of the water in the wick causes the wetted thermometer to read a lower temperature than the other. By using a chart or reference tables, the measurement values of each of the thermometers can determine the relative humidity of the air.
Capacitive or Resistive instruments contain a hygroscopic material that changes its capacitive or resistive properties as the material either absorbs or desorbs water. Electronic sensors in the units measure the changes in the electrical properties. These sensors are usually linear and can measure relative humidity from 0% to 100%. The capacitive hygrometers are able to be used in areas with high condensation, while the resistive ones usually cannot be used in those environments.
Chilled Dewpoint Hygrometers measure the dewpoint and temperature of the sample air and the humidity is calculated from those temperatures. These instruments measure the dewpoint by reducing the temperature of a mirror, or other highly polished surface, to the point that condensation begins to form on it. The temperature of the mirror is measured by using highly accurate RTD’s and results in a highly accurate dewpoint and humidity measurement. These instruments are the most accurate, and most expensive of the devices used to measure humidity.