This is part three of a seven-part series about measuring water vapor and hydrocarbon dew point in natural gas. Click to read parts one and two.
Capacitance Sensors
“Capacitance sensor” is the general term referring to the use of metal oxide, ceramic, or polymer films as a moisture-sensitive dielectric sandwiched between two electrodes. The most common example in industry is the aluminum oxide type. This sensor consists of an aluminum substrate upon which a thin layer of aluminum oxide is chemically formed. A thin layer of gold is then deposited on top of the aluminum oxide, which acts as the top electrode of the capacitor. The gold layer is thin enough so the water molecules can readily permeate through and enter the aluminum oxide layer below. Water molecules entering the aluminum oxide layer change the dielectric constant of the layer, and thereby change the capacitance of the sensor. The water vapor pressure is then monitored as a function of the capacitance of the sensor.
The primary advantages of capacitive sensor-based probes are low-installed cost, ability to measure in both gases and liquids, and line-pressure measurements. These sensors also have some significant weaknesses. Aluminum oxide sensors calculate dew point based on the water that has permeated through the metallic electrode and been absorbed by the dielectric material. These sensors can only provide an accurate representation of the process gas after the moisture within the sensor has reached equilibrium with the moisture in the process gas. As result, these sensors generally have a slower response time from a wet-to-dry condition due to the time required for the water molecules to desorb from the dielectric layer. Additionally, any material in the sample gas that can coat, or foul, the sensor element can prevent the sensor from contacting the sample and impact the reading. Compounds such as methanol, triethylene glycol or amine, chemically appear similar to water due to their polarity which can cause aluminum oxide sensor to read a higher (wetter) moisture content than is actually present in the process gas. These sensors typically require an annual calibration where the sensor must be removed from service and returned to the manufacturer.
Capacitance probe assembly and operation
For more detailed information about this application, refer to our White Paper, “Analytical Devices for the Measurement of Water Vapor and Hydrocarbon Dew-Point in Natural Gas.”