Humidity Institute Theory 8 - Cold Mirror and Aluminum Oxide Technologies

Cold Mirror Technology

How it Works

The condensing mirror or condensation hygrometer is a technique for directly measuring the dew point or frost point of a gas. If the temperature of the mirror is lowered to the exact value at which the dew point appears on the surface, the value of the mirror temperature is called the dew point. Using the previous example, the dew point corresponding to the conditions of 50% rh and 25°C can be found as follows:

Ps at 25°C = 3.17 kPa

p = 0.5 x 3.17 kPa = 1.585 kPa, corresponding to 50% rh

If there is an equilibrium between the dew on the mirror and the environment, then ps must be equal to the vapor pressure p at the temperature of the refrigerated mirror. Based on a simple interpolation of the values in the saturated vapor table, we find that the ps value of 1.855 kPa corresponds to a temperature of 13.8°C. This temperature is the dew point. The above example shows that converting relative humidity to dew point and vice versa requires the use of a thermometer and a saturated vapor table. When the mirror is clean and dry, the intensity of the reflected light reaches its maximum.

The mirror is gradually cooled until condensation occurs, and water or frost forms on the mirror surface. When water or frost forms, the light signal changes, and this change is recorded on the light receiver. Along with the change in the light signal, the exact temperature of the mirror is recorded as the dew point temperature or frost point temperature.

If it is known that the condensate is liquid, the measured temperature is considered the dew point even if the temperature is below freezing. If it is known that the condensate is ice or frost, the measured air temperature is considered the frost point.

Advantages:

· Very low uncertainty (low at ±0.1 Cdp at 95% confidence level, k = 2)

· Wide measurement range

· Ability to measure very low frost point temperatures (as low as - 95°Cfp)

Disadvantages:

· Much more expensive than other technologies

· Requires training for operation and maintenance

· High - level maintenance

· Requires a clean environment

· Not suitable for process measurements

Aluminum Oxide Technology

Working Principle

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

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

When the sensor is exposed to moisture, water vapor quickly passes through the exposed (positive) electrode layer, and polar water molecules form weak hydrogen bonds on the oxide surface. Absorption causes changes in the dielectric constant and resistivity of the oxide layer.

The measurement of the sensor's conductivity is a measurement of the water load on the aluminum oxide medium and is proportional to the moisture concentration in the sample gas.

The aluminum oxide sensor is suitable for very dry and clean conditions where rapid measurement is not required.

Advantages:

· Small - sized sensor

· Suitable for in - line use

· Wide measurement range

· Suitable for measuring very dry environments

· (Dew point temperatures as low as - 75°C to - 100°C)

Disadvantages:

· The sensor is vulnerable to contamination, and condensate deposited on the sensor can cause significant calibration offsets.

· Slow response time (up to 24 hours in the worst - case scenario)

· Long stabilization time

· Requires frequent calibration

· May experience significant drift over time

· Considerable hysteresis

Tip: The aluminum oxide sensor measures the absolute amount of water vapor in a gas. The capacitive sensor measures the relative humidity in a gas (the ratio of the percentage of water vapor to the maximum possible amount of water vapor at a given temperature).