Fully packaged hydrocell sensor-top down view (a), profile view (b). The piezoresistive MEMS sensor (Intersema Sensoric SA, MS767), embedded within the hydrocell, is an assembly of a micromachined silicon layer and pyrex glass mounted under vacuum. The sensor dice outer diameter is 1.82 (length) x 1.48mm (width), whilst the sensitive membrane is 0.5 x 0.5mm. Four polysilicon piezoresistances are implemented on top of the silicon, above the sensor membrane (Figure 35). The membrane is 14 micrometers thick, and deflects according to the pressure difference between its two sides. Being an absolute pressure sensor, a vacuum is sealed in the cavity underneath the sensor when the pyrex is bonded with the silicon. This sealed vacuum cavity underneath the membrane is used as a reference pressure. Peizoresistors, which are placed near the edges of the silicon membrane, and within the linear operating range of the sensor, provide an electrical output that is proportional to the deflection of the membrane and hence the pressure (i.e. the piezoresistors measure strain at the edges of the membrane). Four piezoresistors are used, arranged in a Wheatstone bridge configuration so as to maximise the signal responsivity, increase signal-to-noise ratio, and reduce effects of temperature fluctuations.