
Humidity Sensors

Humidity sensors are electronic devices that detect and measure the humidity in the environment where it is deployed and converts the findings into a corresponding electrical signal. Humidity sensors can be divided into two groups: relative humidity (RH) sensors and absolute humidity (AH) sensors. Relative humidity is calculated by comparing the current humidity reading, at a given temperature, to the maximum amount of humidity for air at the same temperature. RH sensors measure temperature in order to determine relative humidity. Absolute humidity is measured without reference to temperature.
There are three basic types of humidity sensors: capacitive, resistive and thermal. Capacitive humidity sensors measure relative humidity by placing a thin strip of metal oxide between two electrodes. The metal oxide’s electrical capacity changes with the atmosphere’s relative humidity. Capacitive type sensors are linear and can measure relative humidity from 0% to 100%. Resistive humidity sensors utilize ions in salts to measure the electrical impedance of atoms. As humidity changes, so does the resistance of the electrodes on either side of the salt medium. Thermal humidity sensors utilize two thermal sensors to conduct electricity based on the humidity of the surrounding air. One sensor is encased in dry nitrogen while the other measures ambient air. The difference between the two measures the humidity.
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Digital signals are represented in either a true or false. There is no gray area with digital signals. An example of this might be a light switch. A light switch is either on or off. Another example of this might be a motor that is running or not running. Digital signals can be generated with both AC and DC circuits with varying voltages, currents and resistance. Some practical examples of using digital signals in an industrial environment might be if a pump is running or not running or a whether a valve is open or closed.
Analog signals convey information in the form of a range. A light switch might be on or off as a digital signal, but a dimmer switch would be an analog signal. It can be on or off, but it can also be somewhere in between. A practical example of using analog signals in an industrial environment would be if there is a need to measure the level of a tank; whether it's full, empty or somewhere in between. Analog signals can take many different forms with some of the more common being a 4 to 20 milliamp signal or a 0 to 5 or 0 to 10 volt signal.
Communication in a device can either be sent or received. Whether that data is sent or received depends on the type of information. Is there a need to monitor the status of something? If so, an input needs to be received about that information. Is there a need to control something? If so, an output needs to be sent about what needs to occur. Receiving inputs and setting outputs are both things that can be accomplished by using both digital and analog signal types. Therefore, the signals are referred to as analog outputs (AO), analog inputs (AI), digital inputs (DI) or digital outputs (DO).