Pumps & Systems, Feb. 2017, by Dick Laan and Rick Lowrie.
http://www.pumpsandsystems.com/instrumentationcontrols/february-2017-ultrasonic-flow-meters-improve-biogas-measurements

"As interest in biogas grows, industry is paying more attention to measuring biogas flow, which has long been a challenge in process measuring technology. While high water and carbon dioxide (CO2) content makes biogas a demanding medium and a difficult measuring task, a new generation of ultrasonic flow meters is helping to make biogas measurements stable and manageable. One type uses the time transit differential method to guarantee flow measurement with a high degree of long-term stability regardless of the gas composition."