In order to operate a demand driven anaerobic digester, improvements to monitoring of process parameters are necessary. Therefore a mid infrared spectroscopic gas sensor utilizing a MEMS (Microelectromechanical systems) based Fabry‐Perot interferometer was tested for its suitability to measure the biogas concentration online. The sensor measures from 3,000nm – 3,700nm. The spectral data is analyzed using principle component regression to make predictions of methane and carbon dioxide. RMSEP values were 0.95% and 2.75% respectively. Testing was performed in a laboratory, at a 1000L pilot scale digester, and at a 1.2MW landfill site. Scans took under 8 minutes during testing and it is expected that this can be reduced further to give a shorter delay from the online measurements.