  Squeeze film pressure sensors, exploiting the dynamical modification of the
mechanical properties of oscillating elements due to the compression of a fluid
in a small gap region, allow for direct and absolute pressure measurements.
This tutorial article discusses the working principles of membrane sandwich
squeeze film pressure sensors, i.e. sensors consisting in a parallel
arrangement of two large area, ultrathin suspended films forming a few-micron
gap and immersed in a fluid, and focuses on their operation in the free
molecular flow regime. The effects of gas pressure on the vibrations of the
membrane resonators and their coupled dynamics are discussed in general terms
before recent experimental implementations using high tensile stress silicon
nitride membranes are presented.
