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Measuring Relative Humidity

Ambient humidity is an important atmospheric parameter. Humidity measurement and control is critical in wide ranging industrial and medical applications. Let’s begin with a review of some basic concepts to learn about humidity and how it changes.

Humidity refers to the amount of water vapor present in air. When water evaporates, it turns to an invisible gaseous state, called water vapor. Absolute humidity is measured in grams per cubic meter. Typically, the amount of water vapor present in air is dependent on the temperature of the air. Higher the temperature, more water can evaporate into it. The pressure exerted by the water vapor is called vapor pressure . When no more water can be evaporated into air, then the air is called saturated.

Relative Humidity (RH) is a more useful metric for describing the vapor content of air. It indicates how close to saturation the air is. RH is given as a percent. So, 100% means that the air is saturated and 0% means that the air is completely dry. When we think of controlling moisture or vapor, it is RH that we should be measuring and controlling. For instance, for building climate control, RH values of 50-60% are considered to be ideal for human comfort.

How is Relative Humidity Measured?

There are many ways to measure the RH of air. Two commonly used techniques in commercial RH sensors are:

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1. Capacitive Sensor: This sensor is based on the principle of change in capacitance with change in relative humidity. In these sensors, the capacitance increases when RH increases.  Typically, the capacitance increases linearly with relative humidity, making it a useful principle to deduce the humidity. The change in capacitance is due to the change in the dielectric constant of the material used in the sensor. The main limitation of these type of sensors is that they lose accuracy at very low (less than 5%) or very high (greater than 90%) RH values. Also, the range of capacitance between 0 and 100% RH may be so high that circuit designs could be complicated.

2. Resistive Sensor: This sensor is based on the principle of change in impedance with change in relative humidity. The material used is hygroscopic (one that absorbs water) whose impedance varies with RH. Typically, the relationship is inverse. i.e., impedance drops as RH increases.

Image source: http://machinedesign.com/news/sensor-sense-humidity-sensors

References:

http://www.sensorsmag.com/sensors/humidity-moisture/choosing-a-humidity-sensor-a-review-three-technologies-840

http://www.tdk.co.jp/tfl_e/sensor_actuator/CHS/