Introduction

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Swimming with the fishes

I once remarked, “The humidity must be about 100%,” to which someone scoffed, “If it were, we would all drown.”  Humidity is the amount of water vapor in the air.  Absolute humidity is the amount of water content in a given volume.  It is expressed in grams per cubic meter. Relative humidity on the other hand, is expressed as a percentage.  It expresses the absolute humidity relative to the maximum absolute humidity for the current temperature.  That sounds complicated until you realize that the capacity of air to hold moisture changes with the temperature. Hot air can contain more water per cubic meter than cold air.  As the temperature of a gas containing moisture decreases, the relative humidity will increase until the dew point is reached.
The dew point is the temperature where the air is saturated with water vapor.  That means it cannot hold any more water. So, what happens if the temperature continues to fall?  You guessed it, it rains.  That does not mean you are experiencing 100% humidity every time it rains, but in the cloud above your head, it got to 100% humidity.  I am sure you have experience 100% humidity at least once in your life.  Fog occurs when the air temperature and the dew point is less than 2.5 degrees Celsius (4 degrees Fahrenheit).  As the temperature drops and the relative humidity raises to 100% a light drizzle begins.  And no one had to drown.

A Humidity sensor

In order to measure humidity, we will need a sensor.  There are three types of sensors that can measure humidity, they are capacitive, resistive and thermal.
A capacitive sensor is constructed with a hygroscopic dielectric material.  The moisture content of the dielectric will change with the water content of the air.  The change in moisture content of the dielectric will affect the its value.  Since the value of a capacitor is dependent on geometry and the value of the dialectic, you now have a capacitor that changes value with changes in humidity.
A resistive sensor works in a similar way, by measuring the resistance of a substance that changes resistance with humidity.  Such a resistor can be constructed of a noble metal electrode on a substrate that is coated with either a salt or a conductive polymer.  The substance can be suspended in a liquid binder and evenly coated on the electrode.  The substrate may also be treated with an activating chemical such as an acid.  These types of devices are excited with an AC signal to prevent polarizing the sensor.
A thermal senor compares the thermal conductivity of dry air and the ambient air sample to determine the amount of water in the ambient air.  This is done by heating two elements one in dry nitrogen the other in ambient air and measuring the heat dissipation.  The element in dry nitrogen will dissipate more heat than the exposed element due to the difference in thermal conductivity of water vapor.

Six of one, half dozen of the other

While all three will measure humidity, they are not created equal and the use case needs to be examined to determine which device is best for a given situation.  Resistive sensors are interchangeable. They can be used in remote locations and are cheap. Capacitive sensors can measure a wide range of relative humidity, they are tolerant of condensation and can be made to be interchangeable.  Thermal sensors perform well in corrosive environments and at high temperatures.
You can purchase sensors from our friends at Digi-Key for between $2 and $80.  The output can be an analog voltage, I2C, Resistive, Capacitive, PWM, SDM, SPI or digital.  Raw sensors will require more signal processing than a sensor that contains conditioning electronics.  The choice is up to you.

Final thoughts

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