Of the available on-line biomass assay types, radio-frequency impedance spectroscopy (RFI, often referred to as capacitance) is generally regarded as the most robust and reliable method for monitoring viable biomass during fermentation and cell culture. The first article to show that capacitance could be used to estimate microbial biomass dates back over 20 years (1). Today the technology is routinely used for monitoring and controlling mammalian cell culture processes and high-density yeast and bacterial fermentations in research, process…