Introduction:
The technology for Oxide Humidity Sensors work well for process gas streams that have dew points from 0°C down to extremely dry process with dew point of -90°C. Dew point is the typical measurement used for dry gas, moisture content metrics. Dew point is a convenient measurement to use in this range, as is the volume measurement, parts per million (ppm). Water condenses in crags: Capacitance is the electronic property that exists whenever an insulating material separates two conductors.
The material may be air, paper, mica ceramic, glass, a polyester film, or as in this case air, water, and an oxide. Different materials are capable of storing different amounts of electrostatic energy. Each material’s dielectric constant (κ) reflects that material’s ability to store energy. Fig. A capacitor as a whole stores electric energy, blocks the flow of direct current, and permits alternating current to flow per the unit’s capacitance value (C). C equals the area (A) of the electrodes times the dielectric constant (κ) divided by the distance between the two electrodes (d). The porous oxide sensor is a capacitive sensor. The porous oxide surface is craggy and has many apertures. The area in between these peaks contains air. If we place an electrode on either side of the surface, a capacitance measurement can find the combination of the porous oxide and the air gap. As moisture enters the system, water condenses to equilibrium in the sensor’s gaps and the capacitance changes based on the volume relationship of air and water. The capacitance relates to the humidity in the measured gas. This is indirect measurement but it is sufficiently accurate and fast for many process applications. Because the dielectric constants do not change significantly and the total volume does not change, the dielectric constant of the sensor relates to the volume of air in the porous structure and the volume of condensed water in the pores. Fig. The combination of the vapor/substrate equilibrium and the two- component dielectric constant model creates a direct relationship between the gaseous concentration of the water in the air and the capacitance of the porous oxide sensor. Configuration of a transmitter: Oxide sensors usually divide into a sensor head and a monitor to display & transmit the data. Generally, combining the sensor head with a limited electronics set makes an inexpensive transmitter product. Usually, users can draw a sample stream from a line carrying the gas. The pressure is dropped and regulated through the sensor and either vented to atmosphere or returned to the gas stream. Iron drifts across oxide: Aluminum oxide demonstrates logarithmic oxidation - a process that slows as the oxide film becomes thick and protective. Aluminum has a limited oxide thickness and can go above this thickness only with an electrical field applied to the layer. When used as a sensor, a 1- volt excitation cuts across the sensor, which causes the oxide layer to change over time due to the electrical field that drives ions across the oxide. This causes the sensor to drift over time. Silicon has parabolic oxidation kinetics, and the oxidation is diffusion limited. This can become an extremely slow process once a thin oxide layer goes on the device. An electrical field impacts the device only slightly. Silicon oxide sensors need calibration or replacements only once a year while aluminum oxide sensors need replacements every six months. Temperature Compensation: The intrinsic accuracy of various oxide sensors is roughly equal. The difference among various sensors is due to the liberalization & calibration methods. Data sheet accuracy ranges from +2°C to +4°C. in practice, the accuracy of these sensors is optimized around one dew point and temperature value. Additionally, all forms of oxide sensors have a significant temperature effect, and unless the system compensates this factor, inaccuracies can be as large as 10°C. Fig (graph) Response to changes is a critical factor for most process applications, as the sensor monitoring upsets in the process. The porous oxide films used for both aluminum & silicon oxides are thin & have rapid response to changes because the mass transport distance is low. Fig (graph) Response to absorption of water is fast because these structures are good receptors of water. Desorption, however, has two regimes in porous structures: the desorption of physically absorbed water and the desorption of chemically absorbed water. The second process is much slower than the first.
Applications:
1. Power generators need to quench the arc generated in their high power switchgear. Sulfur Hexafluoride is excellent for this application. The humidity of SF6 is critical, as the quenching is not effective when the gas is wet.
2. Water content in natural gas pipelines is important. Water can cause undesired reactant products to form in the pipe, the water can freeze & block the pipe & water content can negatively impact the gas heating value.
3. Many polymers are hygroscopic and need drying before molding to prevent reductions in properties.
4. Bright annealing of furnace requires a very dry gas stream to prevent surface corrosion
5. Air dryers provide compressed air for industrial plants, medical applications, and instruments requiring low moisture.
Conclusion: These types of humidity sensors are found to be very efficient, accurate and give longer response. They are sophisticated, can work in hazardous areas and are inexpensive.
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