Barometric pressure sensors
The technology and applications of barometric pressure measurement have come a long way since the old barometer you remember on your grandparents’ wall.
Today’s compact electronic barometric pressure sensors can be found fulfilling functions in your smartphone and your car engine, for example, along with their traditional weather-forecasting role.
Since atmospheric pressure decreases with increasing height, a barometer can also serve as an altimeter if appropriately calibrated. Many of the most exciting recent and ongoing developments in applying barometric pressure measurement relate to this capability.
Applications
To this day, barometric pressure sensors are contributing to the field of weather forecasting, but now they allow weather stations to be miniaturised. In fact, you can even forecast the weather with the help of tiny sensors in your mobile phone or tablet.
Other environmental applications include evapotranspiration calculations, whereby scientists monitor the transfer of water into the atmosphere via evaporation from surfaces and transpiration from plants. Pressure sensors also provide supporting data to correct the output of instruments such as oxygen sensors which are affected by fluctuating pressure.
For both indoor and outdoor navigation, the altimeter function of barometric pressure sensors enables accurate vertical positioning. This is important when, for example, you’re moving between floors in a building or levels in a car park. In some cases, the accuracy is sufficient to distinguish between points separated by less than the height of one step.
Barometric measurements can help with dead reckoning, in which a device calls on other sensors to calculate its current position when GPS signals are temporarily difficult or impossible to receive.
Systems like those outlined above are now small enough to fit into your smartphone or tablet. You might already have a barometer in your phone without even knowing it.
Wearable devices for monitoring leisure, sport and fitness activities are certain to benefit increasingly from the expanding capabilities of barometric pressure sensors. For instance, instead of relying on accelerometers to count steps, the new sensors will do this by monitoring air turbulence as the body moves. Even gesture recognition is possible.
Another application is in engine management. Changes in atmospheric pressure, when driving between different altitudes, in particular, have a bearing on performance. Accurate monitoring of pressure enables computation of the ideal air-fuel mixture and control of spark advance for optimum efficiency.
Unmanned aerial vehicles, or drones, are fast becoming a realistic answer to certain industrial challenges. Precise height monitoring through barometric pressure sensors will have a part to play in their continued development.
In warehouses, for instance, the ability to rise to an exact specified shelf height will be a great advantage in stocktaking and retrieving stored items. For delivery of goods in built-up areas, the height as well as the geographical location of addresses will need to be specified.
Looking more widely, there are multiple opportunities within the Internet of Things for objects and equipment to be monitored remotely in relation to issues affected by pressure. Meanwhile, virtual reality, gaming equipment and toy developers are sure to be looking for ways of exploiting advances in positional sensing.
Measurement Options
When measuring pressurised air in other contexts (e.g. the pressure of air within a sealed system), the choices include gauge pressure (air pressure compared to atmospheric pressure) and differential air pressure (pressure difference between points). Barometric pressure sensors instead measure absolute air pressure. This is the air’s pressure in relation to a perfect vacuum.
A classic aneroid barometer |
Technology
For much of our history, barometers depended on the behaviour of mercury or some other liquid in response to changing air pressure. The aneroid barometer, whose name refers to the absence of liquid, was invented in 1844. It uses deformation of metal instead.
In the aneroid barometer, a partially evacuated metal cell is subjected to pressure from the atmosphere. As the pressure increases or decreases, the cell contracts or expands. This movement is translated and amplified, via an opposing spring, a system of levers and a pointer, to register a reading on the barometer’s dial.
Modern barometric pressure sensors are, in a sense, aneroid barometers, as their method of operation does not involve liquid. In construction and appearance, though, they are very different from their predecessors – often using the latest microelectromechanical system (MEMS) technology.
Modern MEMS sensors are so small they can be integrated into almost anything
In common with the original aneroid barometers, they detect atmospheric pressure via its effect on a flexible structure – in this case a membrane or diaphragm. The degree of deformation in the membrane is proportional to the pressure and is translated into an electrical signal – hence the sensors are sometimes referred to as pressure transducers.
Size comparison for a modern MEMS sensor |
These small pressure transducers are built around one of two main measurement approaches: resistive or capacitive.
A resistive barometric pressure sensor is also known as a piezoresistive sensor or a strain gauge. One face of its diaphragm is in contact with the atmosphere. The other face has strain gauges attached to it.
Increasing pressure deforms both the diaphragm and the strain gauges. Deformation of the strain gauge material alters its resistance, due to the piezoresistive effect, and the sensor reflects this change in its electrical signal.
A capacitive barometric pressure sensor’s technology is based on two capacitive plates with a small gap between them. The plate in contact with the atmosphere is flexible and forms a diaphragm which deforms under pressure. The other is stiff.
Deformation of the diaphragm alters the distance between the plates and changes the system’s capacitance. The sensor’s electrical signal reflects this proportional variation.
Options and Specifications
Here are some of the variables you might want to consider when trying to match your application with the right sensor:
- Precision. There are resistive pressure sensors which offer thermal compensation and calibration to produce a linear, stable, accurate output. Those who favour the capacitive approach stress that their sensors are naturally less susceptible to temperature-induced variation and are simpler to calibrate.
- Sensitivity. In addition to being reliably accurate, you also need to consider whether your sensor needs to be able to distinguish between very small pressure differences. In positioning or navigation devices, for instance, will it be able to tell between one step and the next on a staircase?
- Pressure and temperature limits. As well as being able to function at each extreme, make sure the sensor can deliver the accuracy you need throughout the specified range.
- Energy consumption. If the pressure sensor is part of a compact device with space for only a small battery, low power needs will be a big advantage. Resistive pressure measurement tends to add significantly to energy demand compared to capacitive. A sleep mode, where appropriate, is one aid to conserving power.
- Operating environment. If the sensor is to be deployed in harsh conditions, is it robust enough? Does it need a waterproof and impact-resistant housing?
- Size. Many of the trending applications, such as wearable devices, require miniaturisation. The tiny footprint of some pressure sensors on the market is very welcome in those cases.
Limitations
As the above criteria suggest, measurement accuracy and other performance criteria vary greatly amongst products. This is true both within and between the two main categories (resistive and capacitive). Some argue, however, that capacitive pressure sensing technology has major inherent advantages over resistive – especially in relation to temperature stability.
If you want to learn more about the different types of media that pressure sensors can measure, the applications of each type, and the different sensor options for your design, click the links below to jump to the section you're interested in.
Looking for more on pressure sensor technology? Check out the further chapters of this guide below, or if you're pressed for time you can download it in a PDF format here.
