Absolute vs gauge vs differential pressure sensors
What's the difference between absolute, gauge and differential pressure senors?
Different applications require different ways of measuring things. In electronic systems, as an example, sometimes we want to know the voltage across a specific component. Other times we’re only interested in the difference in voltage between two points in the circuit.
It’s a similar situation when we’re measuring the pressure of liquids and gases. And for each situation there’s a pressure sensing method that best matches the application.
An absolute pressure sensor provides a pressure measurement relative to a reference of zero pressure. This reference pressure is as close as possible to a vacuum (as shown in the diagram top right). This can be compared to measuring temperature in Kelvin, a measurement unit that uses the coldest possible temperature, 0°K, as its reference point. A pressure measurement of 1 bar will be the same, regardless of where in the world, or at what altitude, it’s measured.
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Gauge pressure sensors provide a pressure measurement relative to the local atmospheric pressure. This is comparable to measuring a DC voltage with a voltmeter, where the voltage at the red probe is either positive or negative with respect to the point to which the black probe is connected.
If the gauge pressure sensor measures a pressure of 1 bar in a vessel, this is 1 bar more than the atmospheric pressure. A 1 bar reading at high altitude (where air pressure is lower) would mean the pressure in the vessel has a lower absolute pressure than a 1 bar reading at sea-level.
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Finally, differential pressure sensors measure the difference in pressure between two points in a system. Typically, this is because this difference can be used to measure the flow of a liquid or a gas in pipes or ducts. Alternatively, it may simply be used to detect a blockage or seized valve. If the pressure before a valve is higher than after it (in the direction of flow), there must be something impeding the progress of the media between the two measurement points.
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How do I know which pressure sensor to use?
In order to select the appropriate pressure sensor for an application, you’ll need to consider the purpose of the measurement you’re making.
If the measurement should not be influenced by local atmospheric pressure changes, you most probably need an absolute pressure sensor. When the application is using air pressure to determine elevation, such as in an altimeter, an absolute pressure sensor is needed. These sensors are also used in weather stations to measure atmospheric pressure changes.
Sometimes only a small pressure or partial vacuum is all that’s required. This is often the case in medical applications, where partial vacuums are used to remove fluid from wounds. In such situations, the amount of vacuum or pressure needs to be generated with reference to the local atmospheric pressure. This is where the gauge pressure sensor would find a home.
Gauge pressure sensors are also used in industrial applications to determine the fill level of open tanks. The level of liquid can be calculated using the hydrostatic method, which leverages knowledge of the liquid’s specific gravity.
If the exact pressure measurement is of less importance, and you only need to determine the pressure difference between two points in the system, a differential pressure sensor is required. Many systems, such as HVAC, employ filters to clean the air passing through their ducts. You could use a differential pressure sensor here to determine whether or not the filter needs replacement. The sensor would measure the air pressure both before and after the filter. Once the pressure difference rises above a predefined threshold, it is time to replace the filter (see diagram below).
Do absolute, gauge and differential pressure sensors measure pressure differently?
The sensing element of a pressure sensor, the part that turns the pressure into an electrical value, is independent of the type of pressure sensor and its sensing method.
Environmental conditions where the sensor will be used and the media being measured will influence which sensing element should be used. This will have been considered by the pressure sensor manufacturer during the sensor’s development.[MJ1]
When you research board-level pressure sensors for example, you’ll most likely find there’s a single datasheet covering all three sensors. The absolute, gauge and differential sensors share the same type of element and are simply provided in packages that differ in the number of ports provided for attaching hoses and pipes.
Are absolute, gauge and differential pressure sensors connected to my circuit differently?
As with the sensor technology discussed above, the sensing method won’t determine how the pressure measurement is presented to the circuit or system.
Pressure sensors can be broadly split into devices that are board-level, and those that are industrialised.
Board-level sensors are typically designed to connect to other electronic circuitry, commonly in association with a microcontroller that’s capable of evaluating its output. Such sensors range from the simple, requiring signal conditioning and amplification, through to the intelligent, which combine signal conditioning and deliver the measurement via a digital output. Most digital output pressure sensors support I2C and SPI.
Industrial pressure sensors are designed to be integrated into industrial automation systems. Such systems utilise a programmable logic controller (PLC). These are designed to support the various analogue and digital interfaces supported today.
On the analogue side, outputs range from a voltage signal to 4 – 20 mA current loops. The system will need to be programmed to understand how the pressure measured relates to the voltage or current the PLC sees.
On the digital side, there are a wide range of bus networks available. These include CANopen, IO-Link, Fieldbus and PROFIBUS. With such sensors, the PLC receives the pressure measurement encapsulated in a packet of data.
For more information on the individual types of pressure measurement and key design considerations for each, follow the links below:
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.
Discover the keys to designing pressure sensor applications with this 30-minute technical presentation and Q&A with Nicholas Argyle, Applications Engineer EMEA, TE Connectivity.Watch On Demand
Need a more digestible introduction to pressure sensors? Download the white paper, 'Pressure sensors: Design considerations and technology options'.Download
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