Humidity Sensor
Humidity is the
amount of water vapor in car. It could affect human comfort and some industrial
systems as well. So humidity sensor is needed in order to control systems for
industrial process and human comfort. Humidity controls are important for
semiconductor industry, medical applications and biological products. So humidity
sensors are employed to provide an indication of moisture levels in the
environment.
Common units for
humidity measurement are relative humidity, Dew/Frost point and parts per
million. Relative humidity indicates temperature; Dew/Frost indicates pressure
of gas and parts per million is an absolute measurement. Dew point is used as
an indicator of water vapor in high temperature processes.
Humidity sensors
are divided into two types, the relative humidity sensors and absolute humidity
sensors. But most of the humidity sensors are relative humidity sensors. Humidity
sensors are consists of a hygroscopic dielectric material in the small
capacitor. At normal room temperature, the dielectric constant of
water vapor has a value of about 80, a value much larger than the constant of
the sensor dielectric material. Therefore, absorption of water vapor by the
sensor results in an increase in sensor capacitance.
Relative humidity is a function of both the ambient
temperature and water vapor pressure. The relationship between relative
humidity, the amount of moisture and the sensor capacitance can governs the
operation of a capacitive humidity instrument.
Basic structure of capacitive type
humidity sensor is shown below:
Lower electrode is formed using gold, a polymer layer such as PVA is
deposited on the electrode and this layer senses humidity. On top of this
polymer film, gold layer is deposited which acts as top electrode. The top
electrode also allows water vapor to pass through it, into the sensing
layer. The vapors enter or leave the hygroscopic sensing layer until the vapor
content is in equilibrium with the ambient air or gas. Thus capacitive type sensor is basically
a capacitor with humidity sensitive polymer film as the dielectric.
Basic structure of resistive type
humidity sensor is shown below
Resistive type humidity sensors pick up
changes in the resistance value of the sensor element in response to the change
in the humidity. A polymeric film is applied on the electrode; the film acts as
a humidity sensing film due to the existence of movable ions. Change in
impedance occurs due to the change in the number of movable ions.
http://www.engineersgarage.com/articles/humidity-sensor
To Qian: I found that the movement sensor
is useful from your post. I learnt the difference between radar sensor and
infrared sensor, but they are both movement sensor. They are used in automatic
doors, automatic lights and building security. I agree with you that the sensor
technology will continue to grow and contribute to modern age.
To Qi: I just figure out that the flow
sensor can be used in the automobile industry. It’s interesting that the
airflow can tell the computer to adjust how much fuel need to be added into the
motor. And I agree with you that the accurate flow sensors ensure the safety of
the process.
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