This is part five of a seven-part series about measuring water vapor and hydrocarbon dew point in natural gas. Click to read parts one, two, three, and four.
Electrolytic Sensors
Electrolytic moisture sensors have been in use since the late 1950’s and were among the first online water vapor analyzers available to industry. They continue to be in service today due primarily to their simplicity and selectivity for detecting water vapor. Electrolytic sensors, also known as phosphorous pentoxide (P2O5) sensors, consist of two wire electrodes wound around insulating glass tube core. A thin film of hygroscopic material (phosphorous pentoxide, or P2O5) covers the insulating core. Water vapor enters the cell and is absorbed by the P2O5. Voltage, applied to the electrodes, electrolyzes the water molecules into their hydrogen and oxygen elements. The current required to electrolyze the absorbed water molecules is equal to the water concentration in the sample, based on Faraday’s Law of Electrolysis.
Basic operation of an electrolytic moisture analyzer
The advantages of electrolytic sensors are their simplicity, speed of response to changes in the moisture concentration, and accuracy. Additionally, this measurement technology is considered a primary or first principles technique. Moisture analyzers that use electrolytic sensors are typically small and light enough to be portable. Some commercially available instruments are designed to be battery powered.
The primary weakness of the electrolytic sensors is their erroneous performance on sample streams containing hydrogen or oxygen. The presence of high concentrations of these gases in the sample can lead to recombination. Specifically, water molecules formed from recombination are then electrolyzed in the sensor. Because of the recombination effect, it is possible for a water molecule entering the sensor to be “counted” more than once. A second weakness of the electrolytic sensor is that the presence of liquid water in the sample will deteriorate the phosphorous pentoxide coating requiring cell replacement. And finally, the presence of background current in the sensor can add bias to low-level measurements.
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.”