Humidity Academy Theory 8 – Chilled Mirror and Aluminum Oxide Technology

Chilled Mirror Technology

How it works

A chilled mirror or condensation hygrometer is a technology used to directly measure the dew or frost point of a gas. If the temperature of a mirror is lowered to precisely the value that makes dew appear on the surface, the value of the mirror temperature is called dew point. Using the previous example, the dew point corresponding to a condition of 50 %rh and 25 °C can be found as follows:
P_s at 25°C = 3.17 kPa
p = 0.5 x 3.17 kPa = 1.585 kPa, corresponding to 50 %rh

If there is equilibrium between the dew on the mirror and the environment, it follows that ps at the temperature of the chilled mirror must be equal to the vapor pressure p. Based on a simple interpolation of the values of the saturation vapor tables, we find that a value of ps of 1.585 kPa corresponds to a temperature of 13.8°C. This temperature is the dew point. The above example shows that converting relative humidity into dew point and vice versa requires the use of a thermometer and saturation vapor tables. When the mirror is clean and dry, the intensity of the reflected light is at its maximum.

The mirror surface gradually cools until condensation occurs and water or frost forms on the mirror. When the water or frost forms, the light signal changes and the change is noted at the light receiver. As the light signal changes, the precise temperature of the mirror is recorded as the dew or frost point temperature.

If the condensate is known to be in the liquid form, even for temperature below freezing, the measured temperature is taken to be the dew point. If the condensate is known to be ice or frost, the measured temperature is taken as the frost point.

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Aluminum Oxide Technology

How it Works

An aluminum oxide sensor is a type of capacitor formed by depositing a layer of porous aluminum oxide on a conductive substrate and then coating the oxide with a thin film of gold.

The capacitance measured between the sensor’s aluminum core and the gold film fluctuates as the water vapor content in the air changes. The number of water molecules absorbed determines the electrical impedance of the capacitor, which is proportional to the water vapor pressure.

When the sensor is exposed to moisture, water vapor is rapidly transported through the exposed (positive) electrode layer where the polar water molecules form weak hydrogen bonds at the oxide surfaces. Absorption causes changes in the dielectric constant and resistivity of the oxide layers.

The measure of the sensor conductance is a measure of moisture loading on the aluminum oxide dielectric, and is proportional to the moisture concentration in the sample gas.

Aluminum oxide sensors are good for very dry and clean conditions where quick measurement is not required.

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Tip: Aluminum oxide sensors measure the absolute amount of water vapor in the gas. Capacitive sensors measure the relative humidity in the gas (the percentage of water vapor present in proportion to the maximum amount of water vapor possible at the given temperature).

Learn more about humidity in the following video: “Relative Humidity Measurement Explained”

See previous blog posts:
Theory 1 – What is Humidity?
Theory 2 – Relative Humidity, Pressure and Temperature
Theory 3 – Humidity and Vapor Pressure
Theory 4 – Definitions of Humidity: Vapor Concentration
Theory 5 – Effect of Temperature and Pressure on % rh
Theory 6 – Humidity Academy Theory 6 – The Capacitive Sensor
Theory 7 – Humidity Academy Theory 7 – The Wet Bulb/Dry Bulb Technology

Watch out for Humidity Academy Theory part 9.