Pressure is the force per unit area exerted by a fluid or gas. The recognized International System of Units (SI) for pressure measurement is the Pascal (Pa); however, pounds per square inch (psi), inches f water (in-H2O), Newtons per millimeter squared (N/mm2) and Bar are also common. The most critical mechanical component in any pressure transducer is generally the pressure sensing structure (spring element). The pressure of the fluid or gas is a force on the pressure sensing structure. The function of the structure is to serve as the reaction for this applied force; and, in doing so, to focus the effect of the force into an isolated uniform strain field where strain gages can be placed for pressure measurement. While there are various types of pressure sensing technologies, two will be discussed in this paper: Piezoresistive-Type Pressure and Foil-Based Pressure.
Piezoresistive-Type Pressure Sensors
In piezoresistive-type pressure sensors, the transduction elements, which convert the stress from the diaphragm deflection into an electrical signal, are called piezoresistors. Piezoresistance equals changing electrical resistance due to mechanical stress. The pressure sensing element is a diaphragm which is made from silicon. This silicon diaphragm is attached to a glass substructure (i.e., that acts as a constraint/mounting structure for the silicon). This silicon diaphragm structure performs in a predictable and repeatable manner as the pressure is applied (i.e., a very slight deflection in the structure). This pressure is translated into a signal voltage by the resistance change of the strain gages which are doped (i.e., implanted) onto the silicon diaphragm surface, then organized in an electrical circuit.
The silicon diaphragm, with the exposed doped Wheatstone Bridge, in test and measurement pressure sensors, is isolated from the pressure media being measured (i.e., media isolated pressure sensors). This is achieved by creating a cavity between the media being measured and the silicon diaphragm, then filling it with oil that does not attack the silicon or electrical circuit. On the opposite side of the cavity is a metal/steel diaphragm that is flexible to transmit the pressure being measured to the oil in the cavity, and the silicon diaphragm. This metal/steel diaphragm is called the isolating diaphragm.
At a very top level, this technology can be described as a pressure sensor consisting of a micro-machined silicon diaphragm with piezoresistive strain gages diffused into it, fused to a silicon or glass back plate. Pressure induced strain increases or decreases the value of the resistors (i.e., strain gages). This resistance change can be as high as 30 %, that typically yields one of the higher outputs from a pressure sensing technology. The resistors are connected as a Wheatstone Bridge, and the output of which is directly proportional to the pressure.
Foil-Based Pressure Sensors
Another common type of pressure sensor utilizes a bonded foil strain gage to measure an applied pressure in one of two ways. In some models, such as miniature pressure sensors, foil strain gages are bonded to the back of a steel diaphragm that is exposed to the media being measured. The diaphragm structure performs in a predictable and repeatable manner as the pressure is applied (i.e., a very slight deflection in the structure). This pressure is translated into a signal voltage by the resistance change of the strain gages, arranged strategically around the diaphragm surface, and is organized in an electrical circuit.
However, in many other models, the foil strain gages are bonded to an element that is mechanically connected to a diaphragm, then exposed to the media being measured. The strain gaged element is measuring the force transmitted from the diaphragm by the mechanical linkage. This element acts as a load cell (i.e., designed to measure force that is directly proportional to the load applied to the diaphragm).
To learn more about Piezoresistive and Foil-Based Pressure Sensors, please download the attached document by Honeywell Sensing and Control called "Effectively Using Pressure, Load, and Torque Sensors with Today’s Data Acquisition Systems," which was the source of information for this document.
 | White_Paper_EffectivelyUsingPressureLoadandTorqueSensorswithTodaysDataAcqusitionSystems_008883-2-EN.pdf |