PFP

Pressure sensors can be classified in terms of the type of pressure measurements they gauge as well as the pressuring-sensing technology the transducer operates.

Differential Pressure Sensor: Differential pressure is a measurement of the pressure difference between two pressure values or two pressure points in the system, thus measuring by how much the two points differ from each other, not their magnitude relative to atmospheric pressure or to another reference pressure such as absolute vacuum. This is different from a static or absolute pressure sensor that would measure pressure using just one port and typically differential pressure sensors are packaged with two ports to which pipes can be attached and connected to the system in two distinct pressure points from where the differential pressure can be measured and calculated.

This pressure measurement approach is typically used to measure the flow of a liquid or a gas in pipes or ducts.

Absolute or Vacuum Pressure Sensor: This sensor measures the absolute pressure, which is defined as the pressure measured relative to a perfect sealed vacuum. Absolute pressure sensors are used in applications where a constant reference is required. These applications require reference to a fixed pressure as they cannot be simply referenced to the surrounding ambient pressure. For example, high-performance industrial applications such as monitoring vacuum pumps, liquid pressure measurement, industrial packaging, industrial process control and aerospace and aviation inspection use this technique. When it comes to measuring air pressure, specifically for applications such as barometric measurements for weather or in altimeters, an absolute pressure sensor is the device of choice.

Gauge or Relative Pressure Sensor: Gauge pressure is simply a special case of differential pressure with pressures measured differentially but always relative to the local ambient pressure. In the same respect, absolute pressure can also be considered a differential pressure where the measured pressure is compared to a perfect vacuum. Changes of the atmospheric pressure due to weather conditions or altitude directly influence the output of a gage pressure sensor. A gauge pressure higher than ambient pressure is referred to as positive pressure. If the measured pressure is below atmospheric pressure it is called negative or vacuum gage pressure.

There are a variety of pressure-sensing technologies or sensing principles capable of transducing pressure into a measurable and standardized electrical signal. This article will focus on the force collector types, which are the ones that use a force gauge (i.e. diaphragm) to measure strain (or deflection) due to applied force over an area (pressure).

Resistive or piezoresistive effect: Resistive pressure sensors utilize the change in electrical resistance of a strain gauge bonded to the diaphragm (also known as a flexure element) that is exposed to the pressure medium.

The strain gauges often comprise of a metal resistive element on a flexible backing bonded to the diaphragm (i.e. metal foil strain gage), or deposited directly using thin-film processes.

Normally, the strain gauges are connected to form a Wheatstone bridge circuit to maximize the output of the sensor and to reduce sensitivity to errors. This is the most commonly employed sensing technology for general-purpose pressure measurement and uses the same principle of how a load cell works.

Capacitive: Capacitive pressure sensors use a diaphragm that is deflected by the applied pressure to create a variable capacitor to detect strain due to applied pressure. As pressure is applied, the external force compresses the diaphragm, and the capacitance value decreases. As the pressure is released, the diaphragm returns to its original shape and capacitance follows. Common technologies use metal, ceramic, and silicon diaphragms. The capacitance can be calibrated to provide accurate pressure reading.

Capacitive sensors, which display a capacitance change as one plate deflects under applied pressure, can be highly sensitive and withstand large overloads. Constraints on materials, and joining and sealing requirements, however, can restrict applications.

Piezoelectric effect: Piezoelectric pressure sensors utilize the property of piezoelectric materials like ceramic or metallized quartz, to generate an electrical potential on the surface when the material is subjected to mechanical stress and strain is generated. The charge magnitude is proportional to the load applied, and the polarity is defined by the force direction. The electrical potential accumulates and dissipates quickly as pressure changes, allowing measurement of fast-changing dynamic pressures.

Please note that piezoelectric effect is different from piezoresistive effect. Although both terminologies are associated to the effects of pressure applied on materials (piezo is derived from the Greek word for physical pressure), piezoresistive effect is related to the change in the electrical resistance while piezoelectric effect is related to the production of electrical potential (voltage charge) in the material.