Gas pressure sensors

Applications

Gas pressure sensors can be used to gauge altitude in aircraft, rockets or balloons. They’re frequently used in automotive design, from optimising engine function and controlling emissions to monitoring pressures in tyres and airbags, and even controlling inflatable air bolsters in dynamic seats.

In industrial settings, they can be used to measure the speed with which gas is flowing (sometimes known as ‘impact pressure’), to confirm that suction is present, to manage source pressures or to test for leaks.
 

Measurement options

Gas pressure sensors are designed (or can be configured) to measure gas pressure in different ways.

• Gauge pressure is measured in relation to the surrounding atmospheric pressure. Atmospheric pressure is around 100kPa (14.7 PSI) at sea level. The sensor built into air pumps for tyres measures pressure in this way, showing the air pressure inside the tyre in relation to the local atmospheric pressure. A reading of zero indicates the pressures are equal inside and out.
• A sealed gas pressure sensor is similar to a gauge gas pressure sensor but has been pre-calibrated to measure gas pressure in relation to sea-level atmospheric pressure. So its readings won’t change if the unit is taken to a different altitude or location.
• Vacuum pressure is the measure of the negative difference between the gas pressure at a given location and atmospheric pressure.
• Absolute gas pressure is measured from zero, or a perfect vacuum (0 PSI). Again, unlike gauge pressure, this isn’t affected by the conditions around the unit, which can vary with changes in altitude and other factors.
• Differential pressure is the difference between two gas pressures – for example, those in two gas hoses connected to the sensor. As with gauge pressure, the sensor may be able to measure changes of gas pressure in either direction (that is, positive or negative differences).

Beyond the different types of measurement, some gas pressure sensors are also designed to measure rapid pressure changes in dynamic environments, such as combustion pressure in an engine cylinder or a gas turbine.
 

Technology

Gas pressure sensors are transducers: they generate an electrical signal in proportion to the pressure they measure. This allows pressure to be monitored by microprocessors, programmable controllers, computers and other electronic devices connected to the sensor.

Some gas pressure sensors are analogue, providing pressure feedback in the form of an electrical current. There are also digital sensors, which provide a digital value for gas pressure, and sensors that provide other types of feedback, such as optic, visual or auditory signals. The most commonly used technology in analogue gas pressure sensors is the piezoresistive strain gauge, which uses the principle of piezoresistance.

A cross-section of a semiconductor distortion gauge, as used in many gas pressure sensors

The sensor is based around a diaphragm made from monocrystalline silicon, polysilicon thin film, bonded metal foil, thick film or sputtered thin film. The diaphragm acts as a semiconductor distortion gauge: when gas presses on it, it is bent out of shape, which distorts the crystalline structure of the material. This, in turn, changes the electrical resistance of the diaphragm, allowing the sensor to reflect changes in pressure in the form of a change in current (see diagram below).

Other, less commonly used, technologies for gas pressure sensors include capacitance (similar to piezoresistance, but the capacitance of the material changes), electromagnetic, piezoelectric (for changes in pressure only), optical and potentiometric.

Some electronic sensors use other properties, such as density, ionisation or thermal conductivity, to infer the pressure of a gas rather than measuring it directly.

A resonant sensor uses changes in resonant frequency (the frequency at which a gas vibrates most readily) to measure changes in gas density caused by pressure. The sensing element can be made from vibrating wire, a vibrating cylinder, quartz or silicon.

Ionisation sensors measure gas pressure by monitoring the flow of charged gas particles (ions), as it varies as a result of density changes. Examples of ionisation sensors include hot-cathode and cold-cathode gauges.

Thermal sensors use changes in the thermal conductivity of a gas (how readily it conducts heat) to measure pressure. An example is the Pirani gauge, which features a heated metal filament suspended in a tube and measures the heat lost from the filament to the surrounding gas.

In digital gas pressure sensors, a silicon chip converts the current through the semiconductor distortion gauge into a numerical reading, and the data is then passed out of the unit via a process connector. This can then be monitored and/or stored by a computer or other electronic monitoring device.

In recent years, wireless pressure sensors have been introduced. These advanced sensors  can be controlled remotely, which allows them to be used for applications where wired connections wouldn’t be possible. They are usually battery-powered, making them completely self-contained and self-sufficient until the battery needs replacing. They typically offer more customisation and control options than standard sensors, and some allow settings such as high and low limits to be altered while the unit is in operation.

Some wireless sensors can connect to mobile devices such as smartphones, which can monitor, collect and store data from the sensor, carrying out functions which previously required a computer.
 

Options and specifications

A wide range of gas pressure sensors is available. They vary in terms of application suitability, cost, technology used, physical dimensions, fittings, process connectors and manufacturing materials used.

Gas pressure sensors normally have a working range defined in kilopascal (kPa), atmospheres (atm) or millimetres of mercury (Hg). They’ll also have an accuracy rating. For example, a sensor might have a working range of 0–210kPa, with accuracy of ±4kPa.

They may come with a stated response time, which reflects how long it takes them to provide a pressure reading – for example, 10ms.

And they typically have a temperature range of operation, since the sensitivity of a pressure gauge can be affected by temperature.