Principle of Operation
Sensor technology based on aluminium oxide delivers proven, reliable trace moisture measurement in gases and compressed air. Although the operating principle is similar across the industry, the precision of the manufacturing process and the quality of sensor construction play an essential role in achieving superior measurement accuracy, long‑term stability and dependable field performance.
Various manufacturers describe their process using terms such as thin film, ceramic or silicon-based technology, all of which refer to variations of the aluminium oxide sensor principle. In practice, however, the quality and uniformity of the oxide layer and the integrity of the sensor structure remain the true indicators of performance and durability.
Sensor Construction
We build each sensor element around a high purity aluminium substrate, which undergoes a controlled process to produce a porous, insulating layer of partially hydrated aluminium oxide. A thin, conductive gold film is then deposited over the oxide surface, forming the two plates of a precision capacitor: the porous gold layer and the aluminium base.
The oxide layer, which functions as the dielectric, contains a network of uniform tubular pores extending from the substrate to the surface. Careful control of the pore dimensions and structural uniformity is critical to the sensor’s performance characteristics, response rate, sensitivity and long‑term stability.
Key Performance Advantages
Selective Response to Water Vapour
The pore structure is engineered to be highly selective to water vapour molecules. During measurement, water vapour molecules diffuse into these micro‑pores, adsorbing onto the pore walls. Because water has a very high dielectric constant, its adsorption produces a measurable change in sensor capacitance, related to the moisture content of the surrounding gas.
Smaller molecules such as hydrogen may enter the pores but their low dielectric constant results in a negligible effect. Larger polar molecules, such as methanol, cannot penetrate the pores and therefore do not interfere with the measurement. This structural selectivity ensures that the sensor’s response is specific to water vapour alone.
AutoCal Feature
The same precision control of the pore formation process also determines the pore volume and adsorption capacity, which defines the sensor’s upper measurement limit. A dynamic equilibrium forms between the water vapour pressure around the sensor and the amount of water adsorbed within the pores.
For example, a sensor designed with an upper range limit of 0 °C dewpoint will saturate at this level and produce no further increase in output beyond that point. This characteristic underpins the AutoCal feature, an inherent property that allows span verification in the field without the need for reference calibration gases.
When the sensor is exposed to a source with a higher dewpoint than its range limit, the calibration curve can be verified or adjusted, effectively restoring the original accuracy. If contamination or drift is suspected, this simple procedure can be used to bring the sensor output back in line with factory calibration.
Exceptional Low Dewpoint Stability
Reliable operation at exceptionally low dew or frost points depends heavily on the structural integrity of the base insulating layer of the sensor. Imperfections such as micro‑cracks or capillary pores in this layer can absorb and release moisture slowly over time, causing drift in dry‑end readings or sluggish response to increases in moisture.
Sensors built with a uniform, fully dense insulating base layer eliminate this problem. This design prevents long term, dry end drift, avoids “flat‑spot” response behaviour and maintains calibration stability even after extended exposure to extremely low humidity conditions.
Combined with a uniform, water vapour-specific pore network, these construction techniques deliver high sensitivity, repeatable performance and long‑term stability, enabling precise and dependable trace moisture measurements across a wide range of industrial applications.
Measurement Performance
Alpha Moisture Systems’ dewpoint sensor stands out with not only exceptional performance in accuracy and speed of response to trace moisture across a wide measurement range, but also in its robust construction, with strong resistance to contamination and harsh operating conditions. It requires minimal maintenance, needing calibration and servicing only infrequently. This makes the Dewpoint Sensor suitable for a wide variety of applications spanning from extremely dry to moderately humid environments.
