An Introduction to Flow and Pressure Sensors
Image Source: Sergey Ryzhov/Stock.adobe.com
By Jake Hertz for Mouser Electronics
Published August 4, 2023
No matter what industry you’re working in, flow and pressure sensing is one of the most important and ubiquitous applications. Whether the use case is automotive, industrial, aerospace, or medical, monitoring and understanding the flow rates, pressure levels, and fluid dynamics in a system has become a necessary task.
Designed to capture and analyze vital parameters governing the behavior of fluids in a system, flow and pressure sensors provide system operators with increased transparency into the states of their systems. With this insight, businesses can improve system safety, optimize processes for improved efficiency, and leverage new functions such as predictive maintenance.
Yet, to achieve these benefits, designers need expertise to navigate emerging design considerations and to integrate flow and pressure sensors appropriately into a system. In this piece, I’ll provide a foundational understanding of both flow and pressure sensors, the design considerations for each, and some future trends surrounding the technology.
Flow Sensors
At a high level, a flow sensor is an electrical component that is designed to measure the rate of flow of a fluid in a system. More specifically, flow sensors exploit the electrical and/or mechanical behaviors of a system to convert the flow rate of a gas or a liquid into an electrical signal that can be interpreted by a processor or microcontroller.
Volume flow rate is defined by the equation \(Q={V\over t}=Av\), where Q is the volume flow rate in units of meters cubed per second (m³/s), V is the volume through which the fluid flows in meters cubed, t is the time in seconds, A is the cross-sectional area of the fluid, and v is the velocity of the fluid. Flow is also defined by the equation of continuity for incompressible liquids, which states that \(A_1V_1=A_2V_2\).
Several different types of flow sensors are available on the market, each with its own unique working principles, advantages, and disadvantages. Types of flow sensors include the following:
- Differential pressure flow sensors place a restriction in the way of the liquid flow and calculate the pressure difference that results across that restriction. The measured differential pressure is then converted into velocity data and flow rate. These sensors take advantage of the fact that the flow rate is directly proportional to the square root of differential pressure \((Q∝\sqrt{ΔP})\).
- Thermal mass flow sensors (Figure 1) exploit the relationship between heat transfer and liquid velocity. These devices consist of a heating element that transfers heat to the liquid and a temperature sensor that measures the heat absorbed by the liquid, which is then converted into flow data. In these systems, the mass flow can be calculated using the following equation:
\(W={H\over{ΔTC_p}}\), where W is the mass flow, H is the heat input, T is the temperature, and Cp is the specific heat at a constant pressure. - Turbine flow sensors place a spinning part in the direction of fluid flow. The spinning part is agitated by the fluid’s movement, and its rotation around its axis is measured and used to determine the flow of the fluid.
By converting these flow data into usable electrical information, flow sensors enable precise control, optimization, and analysis across a wide range of automotive, medical, and industrial applications. Some example applications include tracking the amount of air through an engine’s intake manifold in a vehicle, measuring the flow of gases in ventilators and anesthesia workstations in medical devices, and verifying fluid flow in manufacturing processes.
Figure 1: Mass flow sensor in an automobile. (Source: MariiaDemchenko/stock.adobe.com)
Because of the variety of options available, selecting the correct flow sensor for your design may be a challenge. Ultimately, the specific application you are building and its requirements determine which sensor to select. Some common requirements to consider include cost, accuracy, resolution, area, and power consumption. Generally, differential pressure flow sensors are the most popular due to their low cost, high performance, and versatility.
Pressure Sensors
Pressure sensors are devices that sense system pressures and convert them into readable electrical signals. Pressure is defined as an amount of force applied to a given area \((P={F\over A})\) and can be measured in units of Pascal (Pa), Bar (bar), Newton per square millimeter (N/mm²), or pounds per square inch (psi).
Generally, several types of pressure can be measured, including gauge pressure, absolute pressure, differential pressure, or vacuum pressure. The following are just a few of the devices used to measure pressure:
- Piezoelectric pressure sensors exploit the piezoelectric effect of materials, in which an applied force creates a proportional electric charge. This charge can be converted back to the force that was applied and hence can determine the pressure.
- Capacitive pressure sensors measure the change in capacitance between two plates as caused by the movement of a diaphragm, which moves in relation to exerted pressure (Figure 2).
Figure 2: Symmetrical output of a TDK capacitive pressure sensor. (Source: TDK)
The capacitance of a parallel plate capacitor is calculated as \(C={ε{A\over d}}={Q\over V}\), where \(ε\) is the dielectric constant of the material between the plates, A is the area of the face, d is the distance between plates, Q is the charge stored in the plates, and V is the voltage across the plates. From this equation, an engineer can relate the voltage difference between the plates and the distance between plates as it changes due to pressure to determine the pressure the sensor is experiencing.
- Strain gauge pressure sensors detect electrical resistance changes in response to the applied force. Through the help of supporting circuitry such as a Wheatstone Bridge (Figure 3), the resistive change in the strain gauge can be correlated back to the originally applied pressure on the device.
In a Wheatstone Bridge with external excitation voltage \(V_{ex}\), the output voltage of the system can be calculated as \(V_o=[{R_3\over {R_3+R_4}}-{R_2\over {R_1+R_2}}]V_{ex}\). From this output voltage, the pressure applied to the strain gauge can be determined.
Figure 3: A strain gauge pressure sensor requires external circuitry like the Wheatstone Bridges shown here. (Source: Schneider Electric)
With so many options available, pressure sensors are one of the most common sensors used throughout the world. Today, these sensors are used in the industrial automation, aerospace, and aviation industries to track and control the movement of hydraulic and pneumatic systems and to check the pressure of critical vehicle fluids in spacecraft and airplanes.
The kind of pressure sensor to use will also depend on the specific requirements of your application. The first step is to understand what kind of pressure you need to measure (e.g., absolute, differential) and then balance that against the tradeoffs of power, accuracy, and cost.
Design Considerations for Flow Sensors
When evaluating flow sensors, you must consider several design issues. First, designers must consider the different requirements of their project and how those relate to the sensor specifications. Sensor specifications such as flow range (i.e., the highest and lowest flows the sensor can measure), accuracy (Figure 4), and response time will all factor into sensor selection and the success or failure of the design.
Figure 4: Accuracy of a Sensirion SLI-1000 liquid flow meter. (Source: Sensirion)
Second, from a mechanical perspective, a designer must consider factors such as the sensor’s intended placement, the size of the monitored channel, and how the sensor will be mounted to the system. Even if a sensor offers all the performance criteria necessary for an application, a mechanically unsound sensor is not right for the job. Additionally, a selected sensor must work reliably in a given system environment. Here, designers should consider temperature, pressure, and general media compatibility.
Third, designers must consider the electrical specifications of a sensor to determine whether the device will work within their larger electrical system. For example, the output signal type of a sensor (i.e., digital versus analog), the component power requirements, and the printed circuit board area will all influence a designer’s choice of sensor.
Finally, a device’s calibration and maintenance requirements are significant factors in a designer’s choice of sensor. For example, in projects that require long lifetimes, a designer may want to avoid mechanical flow sensors, which are more likely to wear down over time. On the other hand, if regular maintenance is acceptable, then a mechanical solution may be cheaper and simpler for the designer.
Design Considerations for Pressure Sensors
Like flow sensors, pressure sensors have unique design issues that designers should consider when selecting the correct component for their project.
First, designers should consider whether the sensor’s specifications fit the requirements of their intended application. Some of the most important specifications for a pressure sensor include resolution, linearity (Figure 5), accuracy, and range, though the hierarchy of importance may vary depending on the application. For example, if your system requires the ability to measure very small changes in pressure, resolution will be the primary concern.
Figure 5: Accuracy, tolerance, and error in a pressure sensor’s output. (Source: TDK)
Second, from a mechanical perspective, the sensor’s sealing, port size, orientation, and mounting options will also influence design choices, which will be defined by the physical constraints of the system. Other related factors may include the system’s environment such as temperature ranges and media compatibility (i.e., whether the sensor is suitable with the media into which it will be incorporated).
Third, some of the most important considerations surrounding pressure sensors pertain to their compatibility with the existing electrical system. For example, a given pressure sensor must have a compatible output signal type and operating voltage, and the device’s current requirements must fit into the capabilities of the system’s power supply.
Finally, calibration and maintenance are particularly important design considerations because pressure sensors are used to measure forces. Continually applying forces to the pressure sensor may quickly degrade the device and necessitate frequent maintenance and recalibration. Thus, designers should consider the reliability and robustness of a given sensor over time to understand if it meets the lifetime requirements of their application.
Future Trends
Both flow and pressure sensors are subject to much technological innovation.
In the world of flow sensors, one of the most notable emerging technologies is the use of microelectromechanical systems (MEMS). MEMS-based flow sensors are devices that integrate microfluidic channels, sensing elements, and electronics onto a single chip. Through this integration, MEMS technology miniaturizes flow sensors and reduces cost and power consumption. This could lead to more affordable and versatile devices by reducing power supply, size, and cost, as well as to smaller form factors.
An emerging field of research in pressure sensing is the use of flexible pressure-sensing devices. These devices can be bent and stretched without impacting performance or device lifetime. Flexible pressure sensors enable designers to leverage new applications such as soft robotics as well as to create new form factors and versatile solutions.
More generally. cloud connectivity and machine learning are enabling new use cases, such as predictive maintenance, for flow and pressure sensors. These technologies could create systems that have longer lifetimes, greater performance, and improved reliability.
Conclusion
Flow and pressure sensors are two of the most important types of available sensing technology; both are applied frequently in applications across automotive, aerospace, aviation, industrial automation, medical, and other industries. Yet, designing with these sensors requires a deep understanding of the technology as well as the electrical, mechanical, and environmental specifications of a system.
With a greater understanding of the fundamentals and design considerations surrounding both flow and pressure sensors, designers can build better systems with more ease and help create a future from which all industries may benefit.