This is part four of a seven-part series about measuring water vapor and hydrocarbon dew point in natural gas. Click to read parts one, two, and three.
Quartz Crystal Microbalance Sensors
Quartz Crystal Microbalance (QCM) moisture analyzers are straightforward operating devices. At the heart of the technology is a simple quartz crystal oscillator, which is coated with a hygroscopic material. When exposed to a sample gas containing water vapor, the hygroscopic layer will adsorb water from the gas phase, thereby increasing the mass loading of the quartz crystal. This increase in mass decreases the resonance frequency of the oscillator; the moisture concentration is measured as a function of the frequency change. The QCM sensor technology is incorporated into an extractive sampling system, which is used to alternately expose the sensor element to the sample and a dry reference gas. The moisture concentration is then measured as a function of the difference in oscillator frequency measured for the sample and reference streams.
Quartz Crystal Microbalance cell assembly
The measurement does not require the sensor to come to equilibrium with the moisture in the sample gas, resulting in relatively fast speed of response to both increasing and decreasing moisture concentrations. Materials that stick to, or foul, the sensor will cause a shift in the oscillator frequency but will not substantially influence the frequency difference recorded between the sample and reference gases, making these sensors more resistant to contaminants found in natural gas streams. In a typical QCM-based analyzer, a verification system using an NIST traceable moisture source is built into the sample system to allow users to test sensor performance and eliminate the need for removal and remote testing of the sensor. The moisture measurement is made under controlled operating conditions by keeping the temperature, pressure and flow of the analyzer constant.
The non-equilibrium based approach is a major advantage of QCM technology, when compared against other contact sensor approaches. In an equilibrium-based sensor, the speed of response depends on the magnitude of the change in measurement value. A large step change will result in a big difference in the partial vapor pressure of the water molecules and the sensor will respond relatively fast. If the change is small, the sensor takes much longer to equilibrate because of the small differential in the water vapor pressure. With a non-equilibrium based sensor, such as the QCM, every measurement cycle looks at a big signal even if the change in the measured value is small because a difference between a dry reference gas and wet sample gas is observed. The result is a fast response to the rate of change in the moisture concentration.
QCM devices are widely applicable in the natural gas market, due to the inherent accuracy of the measurement technology. They are extremely sensitive and capable of providing measurement accuracies of +/- 10 ppb for low concentration water vapor analysis operation in natural gas processing applications such as cryogenic turbo expansion, liquefied natural gas processing and other low-level detection applications typically requiring extremely low levels of moisture.
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.”