Despite the wide range of pressure sensors, knowing which one is right for a particular job can be challenging. When it comes to measuring air pressure, especially for applications such as barometric measurements for the weather or in the altimeter, the absolute pressure sensor of the device is desired. However, your potential use of the program is not limited to air or gas. Absolute measurement is possible by measuring the target pressure relative to the known pressure from the absolute vacuum (see diagram below). This can be compared to measuring the temperature in Kelvin, where the lowest possible temperature is 0 ° C. Using the vacuum as the reference by which everything is measured, all measurements deliver a value greater than the absolute minimum defined by the reference. This is necessary for accurate measurements, because Boyle’s law states that the pressure of a gas is proportional to its volume at a constant temperature. Therefore, anything other than a complete vacuum results in an absolute pressure sensor that measures temperature differently.
Achieving a complete vacuum is very challenging, especially if the sensor is to remain within an acceptable price range. Therefore, absolute sensors should usually be compatible with an approximate vacuum, usually in the range of 35 microbar (0.0005 PSI).
How absolute pressure sensors work
Using a sealed container as a reference point, a measurement technology is then applied to the surface of the plate, the electrical properties of which change with pressure. There are different approaches in this field.
One of the most common methods is a pressure gauge. They attach a resistor (the amount of which varies with mechanical strain) to a material such as silicon, polysilicon, metal foil, or metal spray on a thin layer. In order to maximize the output signal and reduce errors, the sensor typically uses four resistors in the Wheatstone bridge configuration.
With today’s high level of integration, it is not uncommon for your piezoresistor sensor to have compensating circuits such as resistors, all on the same bed (see right).
Other measurement technologies also use component value changes when deformed. For example, capacitors vary in capacitance when pressed. Sometimes a change in inductor inductance can be caused by a local diaphragm that moves in response to pressure changes. The piezoelectric effect is also a common pressure measurement technology. This takes advantage of the fact that some materials, such as quartz, produce a voltage-dependent voltage.
As a result of the extensive advances in silicon fabrication in recent years, some mechanical elements are made in silicon chips known as microelectromechanical systems, or MEMS.
They mostly use the same physical properties of the electronic components mentioned earlier, but use some moving parts that are machined in semiconductor materials. Such devices rarely provide the output signal of the sensing element. Instead, they pre-condition the signal electronically before leaving through the closed pin.
A quadruple ampere configuration shown in the diagram above provides an example of such an amplifier circuit. Such circuits must be carefully designed and may also require appropriate screening and low-noise design methods to ensure a reliable output signal.
Due to the variety of processes and component tolerances, your circuit may require a separate calibration for each circuit of your production circuit. Temperature compensation may also need to be considered.
It is much easier to connect sensors with digital output to the microcontroller. This includes all signal ventilation, amplification and temperature compensation. The measurements are then converted to a digital value and stored in an internal register.
The interfaces provided to the microcontroller are typically I2C or SPI. Some sensors may support both, allowing you to choose the one that suits your application. This is an example shown below.
If you want to measure rapid pressure changes, a sensor with a digital interface may not meet your needs. The SPI or I2C interface only supports a certain number of data transfers per second. With more than one device on the bass, the available bandwidth decreases as the number of devices hanging on the bass increases.
To measure pressure that changes rapidly, you will probably need to invest time in producing your own analog front end, along with ADC at the right conversion time.
Use of absolute pressure sensor in the design
Many absolute pressure sensors are provided in a small enclosure suitable for mounting holes or surface mounts on a printed circuit board (PCB). These are known as board level sensors. This makes them ideal for a consumer application where PCB testing can be done. In an altimeter or sports watch.
However, such sensors are not suitable for high temperatures of liquids or gases. They are also not adequately protected against dust, moisture or chemicals that are often used for cleaning in industrial environments.
Industrial sensors are usually tightly packed. They are likely to be made of non-corrosive materials such as stainless steel and are filamentous, allowing them to be installed in pipes and storage tanks.
Industrial engineers usually want to choose their hardware and connect it all. They are not very interested in making custom circuits to control the sensor output. As a result, industrial sensors fall into three main types: sensors, converters, and transmitters. We’ll cover these briefly below, but read Chapter 5 for more information on these types of sensors.
The term “sensor” usually refers to a device that produces a ratio output. This means that the output of your sensor will depend on the supply voltage of the sensor. Thus, a 10mV / V sensor produces an output of 0 to 50mV for a 5.0 VDC source. Such devices can be completely raw in their packaging, with a suitable pad or base for soldering on a circuit board or cable.
The “converter” is a complete sensor, including signal ventilation, designed for you to be directly in an industrial environment. Your output signal will typically be a voltage-related voltage, generally in the range of 0 to 10 volts. However, some converters produce alternating signals in the range of 1 to 6 kHz.
Some older converters do not have a “live zero” when the sensor is at its lowest point. This will make it impossible for you to tell the difference between a minimum pressure gauge and a broken sensor or cable. This is an issue that should be considered for systems with a high level of security.
A pressure “transmitter” usually indicates a sensor that uses an output signal of 4-20 mA instead of a voltage output. These devices often require only a two-wire interface (power supply and ground) and have good electrical noise safety (EMI / RFI). The supply voltage for such sensors is in the range of 8-24 VDC.
Such sensors are designed for use with other industrial equipment such as programmable logic controllers (PLCs). They then communicate with each other and other industrial systems via digital buses. Buses include Fieldbus, standardized as IEC 61158, IO-Link, PROFIBUS and CANopen. With more information on industrial sensors, understanding the readiness to connect directly to such industrial networks is becoming increasingly common.
Application of absolute pressure sensor
With the rise of smartwatches and navigation systems, absolute pressure sensors find homes wherever they rise above sea level (altimeter measurement). Meteorological stations also use them to measure pressure. It is better to read the article of pressure transmitter for more information.
Gasoline and diesel vehicles also use them to measure the pressure in the engine manifold. Such sensors are known as manifold absolute pressure sensors or MAP sensors for short. The Electronic Engine Control Unit (ECU) uses this information to determine the optimal combustion of the air-fuel mixture and the combustion time.
In industrial applications, it is often necessary to create a partial vacuum. This is the case in food packaging, where the residual pressure determines the shelf life of the product. The absolute pressure sensor can ensure that the pressure in each package is the same.
Alternatively, industrial absolute pressure sensors are available that support measurements of more than 300 bar (4400 PSI).
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