Introduction: Barometers are increasingly being used in smartphones, tablets, and wearable technology, opening the door to new industrial applications such as accurate height position monitoring and predictive maintenance. So how do you choose the right barometer based on your design requirements? What specific parameters should you pay attention to? What technical details should you consider when selecting a product? What are the latest barometer combinations? What new application directions are there for barometers? Through this article, I hope you can get the necessary information to match the most ideal barometer for your next design.
Learn about 3 pressure measurement methods and 4 manufacturing techniques, and choose the one that suits your design
Barometers are used to detect the air pressure of gases or liquids. As a transducer, barometers convert applied air pressure into analog or digital output signals and are usually classified according to the type of air pressure measurement and pressure-sensitive technology.
There are three ways to measure air pressure:
Absolute pressure: Absolute pressure is the pressure measured relative to a perfect vacuum. If you place an absolute barometer in air, the sensor will read the actual air pressure at that location. Donc, absolute barometers are affected by changes in altitude and weather, etc..
Differential pressure: The difference in air pressure measured between two pressure sources.
Gage pressure: When one of the pressure sources is ambient air pressure, the measured pressure difference is called gauge pressure.
After clarifying the pressure measurement method, it is also necessary to realize that the different principles used in the production of barometers will directly affect the accuracy, range, sensor size and applicable environment of the detection.
The following are the most commonly used pressure-sensitive technologies:
■ Piezoresistive barometer: Utilizes the piezoresistive effect to detect the change in resistance of one or more resistors mounted on the diaphragm when air pressure is applied. Suitable for general pressure testing required by the Internet of Things, industry and medical.
■ Piezoelectric barometer: Utilizes the characteristics of piezoelectric materials to detect the charge proportional to the air pressure applied to the surface. Suitable for high-temperature environments, such as high-dynamic pressure measurement on jet engines.
■ Capacitive barometer: Measure air pressure by detecting the change in capacitance caused by the movement of a diaphragm made of glass, ceramic or silicon. Also suitable for general pressure testing required by the Internet of Things, industry and medical
■ Fiber optic barometer: Utilizes the optical effect in optical fiber. Suitable for harsh environments such as oil and gas, aerospace, defense and medical.
Understanding the 8 Key Parameters of a Barometer
In addition to the basic principles of the barometer, you also need to understand the meaning of related parameters, which is also the main reference for your choice of barometer:
Pressure range or span: The range of pressures that a sensor can measure. The sensor’s overpressure tolerance, which is the maximum pressure that the device can withstand and still function when the barometer returns to the operating range, should also be considered.
Accuracy: Absolute accuracy indicates how close the barometer output is to the actual pressure. It is expressed as the difference between two values. Relative accuracy is the error between two measurements.
Packaging: Determined by the end application environment and size constraints. Small, waterproof packages are often preferred.
Noise: Simply put, it is the random variation of the sensor output related to changes in the sensor input.
Décalage du coefficient de température: Also known as the temperature coefficient of zero pressure. It represents the change in offset at zero pressure due to temperature, so the smaller the better.
Output data rate: The rate at which data is sampled.
Bandwidth: The highest frequency signal that can be sampled without aliasing.
Power consumption: Power consumption is extremely important for applications that run on small batteries and those that need to preserve battery life as much as possible. Power consumption is closely related to the choice of ODR and resolution. The RMS noise of the barometer is also related to bandwidth and resolution, so the power consumption and resolution should be weighed to suit the application requirements of the sensor. Of course, there are other parameters, such as power supply voltage, operating temperature, range, communication interface, etc..
Relationship between atmospheric pressure and altitude
Here are the units of measurement for atmospheric pressure:
Psi – pounds per square inch
Cm/Hg – centimeters of mercury
Cm/Hg – inches of mercury
Pa – Pascal, SI unit of pressure, 1Pa = 1 N/m2
Bar – bar, unit of air pressure, 1 bar = 105Pa
Mbar – millibar, 1mbar = 10-3 bar
We live in the lower reaches of the Earth’s atmosphere, where atmospheric pressure decreases as altitude increases. We define the standard atmospheric pressure as 29.92 in/Hg at sea level at 59°F, an average value that is not affected by time but by the geographic location of the measurement point, temperature, and air currents.
Donc, the conversion relationship between the above pressure units is:
1 standard atmosphere = 14.7 psi = 76 cm/Hg = 29.92 in/Hg = 1.01325 bar = 1013.25 mbar
The relationship between atmospheric pressure and altitude can be expressed as follows[1]:
Where:
P0 is the standard atmospheric pressure, equal to 1013.25 mbar;
Altitude is the altitude in meters.
P is the air pressure in mbar at a certain altitude
Figure 1 describes the relationship between atmospheric pressure change and altitude based on the above formula.
As shown in the figure, when the altitude rises from sea level to 11,000 meters above sea level, the atmospheric pressure drops from 1013.25 mbar to 230 mbar. It is not difficult to see from the figure that when the altitude is below 1,500 meters, the atmospheric pressure decreases almost linearly, with a decrease of about 11.2 mbar per 100 meters, c'est, about 1.1 mbar per 10 meters. In order to obtain more accurate altitude measurement data, an atmospheric pressure altitude query table can be built in the target application to determine the corresponding altitude based on the measurement results of the pressure sensor.
If an absolute MEMS pressure sensor with a full range of 300 mbar to 1100 mbar is used, the measurement altitude can reach 9,165 meters above sea level to 698 meters below sea level.
Application example: Determining floor level using MEMS sensors
The measurement resolution of 0.1 mbar (10Pa) /rms enables MEMS pressure sensors to detect height changes within 1 meter. Donc, in high-rise buildings, pressure sensors can be used to detect changes in floors.
Deuxièmement, altitude monitoring stations are deployed in multiple locations across the region to measure local ambient air pressure, correct for weather and other influencing factors, create a high-precision altitude reading, and then determine the exact floor height of the device, bringing new capabilities to geolocation.
Figure 2 shows the pressure sensor data collected in STMicroelectronics’ Castelletto office building in Italy. The sampling rate is 7Hz, and the data collection time is about 23 minutes in total. From the figure, we can clearly see the changes in atmospheric pressure on different floors. The atmospheric pressure is highest in the basement. As the floors rise, the atmospheric pressure gradually decreases.
For complex urban environments with multi-story buildings, current GPS technology cannot provide reliable three-dimensional position data. But the application of barometers has become a new solution, which is designed based on the change of air pressure – when a person moves to a certain height, the air pressure will drop.
As shown in the figure below, in this solution, first of all, the wearable device or mobile phone must have a high-quality barometer sensor, such as: WF5803F, 5803C/WF280A, etc., or industrial air pressure sensors WF5805F and 5837, with a sampling rate of 3kHz and a total data collection time of about 3ms.
WFsensors supplies a variety of piezoresistive barometer sensors with a wide range of product models, including absolute pressure non-waterproof and absolute pressure waterproof types, suitable for many terminal smart products such as mobile phones, drones, wearables, watches/bracelets, montres de sport, etc.. It is used as an altitude meter, as well as an indicator for weather forecasting and environmental humidity and temperature monitoring.
Capteur de pression absolueWF5803F 10Bar 1000kPa IIC/SPI modèle numérique capteur de pression absolue paquet LGA8
- Capteur de pression absolue
WF5803F 7Bar 700kPa IIC/SPI modèle numérique capteur de pression absolue paquet LGA8
- Capteur de pression absolue
WF5803F 3Bar 300kPa IIC/SPI modèle numérique capteur de pression absolue paquet LGA8
