Specifications

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efficient or cost - effective the sensor is. If all these conditions are met, then the
customer is on the right track in purchasing a sensor.
Another most commonly used sensor is the humidity sensor. In most electronic
control systems, there are three different varieties of humidity sensors: the
Dunmore element, Pope Cell, and finally the Polymer sensor. The Dunmore
element also called resistive humidity sensors was built using a dual winding
palladium wire on a metal substrate that is covered with lithium bromide solution.
Altering the concentration of the solution will result in having high resolution
sensors that cover a wide humidity range. This type of humidity sensor is
normally used in HVAC system controls in residential and commercial units. The
conductivity which is 1 / resistivity is calculated using an AC Wheatstone Bridge.
The Dunmore element is characterized by low temperature coefficient, can work
with relatively high temperatures, and able to recovery from moisture residue.
Figure 29 illustrates the exponential relationship in voltage / current change
between resistance (kΩ) and relative humidity (%).
Figure 29 Resistive Humidity sensor graph with Resistance vs. Relative
Humidity variables (Reprinting Permission Requested from Questex Media
Group LLC)
The next type of humidity sensor is the Pope Cell which was designed using
polystyrene coated with sulfuric acid on a metal base material. The Pope Cell
has a wider humidity range than the Dunmore element. Thermal conductivity
humidity sensors measure the absolute humidity from the conductivity of dry air.
Figure 30 is a Thermal Conductivity Humidity Sensor illustration that depicts
temperatures in degrees Celsius with respect to their output signal in mV.
Ranging from 40 degrees Celsius to 200 degrees Celsius, the output signal in
mV decreases in higher temperature but as a result the absolute humidity
generally increases in g/M
3
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