Sensors 2008. 8 2319 Figure 1. Bulk-metal/elastomer capacitive pressure sensor in the form of the L-C tank for frequency-based pressure monitoring: (a: left) Cross sectional view of the sensor oupled with a separate inductor, and(b: right)a device with an inductor wound on the sensor Insulated wire 1000 Capacitive pressure sensor Inductor Metal Elastomer layer plates Elastomer (unit: um) Inductor Metal plates 2400 Figure 2. Electrical representation of the wireless measurement set-up with the L-C tank device Magnetic coupling L-C tank Spectrum External analyzer antenna cO Sensing capacitor 2. Device Principle and design The capacitance of the device is determined by the thickness of the intermediate elastomer that is varied with the ambient pressure. An elastomer layer sandwiched between two rigid plates exhibits higher compression stiffness than the same layer without the plates in the direction perpendicular to the layer plane. For a rectangular layer of an incompressible, homogeneous elastomer that is bonded with rigid plates on both sides, the relationship between an applied pressure, P, on each of the plates and the resultant strain, e, can be expressed as [ 18] EA P 3(y2+W2Sensors 2008, 8 2319 Figure 1. Bulk-metal/elastomer capacitive pressure sensor in the form of the L-C tank for frequency-based pressure monitoring: (a: left) Cross sectional view of the sensor coupled with a separate inductor, and (b: right) a device with an inductor wound on the sensor. Figure 2. Electrical representation of the wireless measurement set-up with the L-C tank device. 2. Device Principle and Design The capacitance of the device is determined by the thickness of the intermediate elastomer that is varied with the ambient pressure. An elastomer layer sandwiched between two rigid plates exhibits higher compression stiffness than the same layer without the plates in the direction perpendicular to the layer plane. For a rectangular layer of an incompressible, homogeneous elastomer that is bonded with rigid plates on both sides, the relationship between an applied pressure, P, on each of the plates and the resultant strain, e, can be expressed as [18]: ( ) ( ) e Y W Y W S S E EA P − ⎥ ⎥ ⎦ ⎤ ⎢ ⎢ ⎣ ⎡ ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ + − = − − + log 1 3 1 1 2 2 2 2 2 2 2 0 2 (1)