About Pressure Sensors

1. Introduction to Pressure Sensors

A pressure sensor is a key device used in a multitude of applications for the measurement of gas or liquid pressure. At its core, pressure is defined as the force required to halt the expansion of a fluid, generally expressed in terms of force per unit area. A pressure sensor operates primarily as a transducer, generating an electrical signal in response to the pressure it encounters.

Pressure sensors play a vital role in control and monitoring systems across a wide array of sectors. They are frequently used to indirectly measure other parameters such as fluid or gas flow, speed, water level, and altitude. You may hear them referred to by other names including pressure transducers, pressure transmitters, pressure senders, pressure indicators, piezometers, and manometers.

The world of pressure sensors is vast and diverse, with variations in technology, design, performance, application suitability, and cost. A conservative estimate suggests there are more than 50 technologies and at least 300 companies manufacturing pressure sensors worldwide. Among these, some sensors are designed to measure dynamic pressure changes, capturing high-speed fluctuations, as seen in applications like measuring combustion pressure in engine cylinders or gas turbines.

Moreover, pressure switches, a category of pressure sensors, operate by turning on or off at a specific pressure. For instance, a water pump might be regulated by a pressure switch that initiates when water is released from the system, thereby reducing the reservoir pressure.

What Do Pressure Sensors Do?

In simple terms, a pressure sensor measures the pressure of gases or liquids and outputs a signal proportional to that pressure. In industrial settings, this allows for real-time monitoring of conditions like fluid pressure in pipelines, hydraulic systems, tanks, or machinery. The output signal is often electrical (such as a voltage, current, or digital signal) that can be read by controllers or data acquisition systems, enabling automated control or data analysis.

How Industrial Pressure Sensors Work:

Most industrial pressure sensors use a flexible diaphragm as the primary sensing element. When pressure is applied, the diaphragm deflects slightly. This deflection is then converted into an electrical signal.

2. Types of Pressure Sensors

Pressure sensors can be categorized based on the type of pressure they measure, the pressure ranges they cover, and their operational temperature ranges. Here are the primary types of pressure sensors:

1. Absolute Pressure Sensor: This sensor type measures pressure relative to a perfect vacuum, making it suitable for applications requiring a constant reference, such as monitoring vacuum pumps, industrial process control, liquid pressure measurement, and aviation inspection.

2. Gauge Pressure Sensor: Gauge sensors measure pressure in relation to atmospheric pressure. A common example is a tire pressure gauge, which reads zero when the measured pressure matches the ambient pressure. Most sensors measuring up to 50 bar are designed this way, as they otherwise would register atmospheric pressure fluctuations as measurement errors.

3. Vacuum Pressure Sensor: This term can be a source of confusion as it might refer to a sensor measuring pressures below atmospheric pressure or one measuring absolute pressure relative to a vacuum.

4. Differential Pressure Sensor: These sensors measure the difference between two pressures, each connected to one side of the sensor. They are used for assessing properties like pressure drops across oil or air filters, fluid levels, or flow rates.

5. Sealed Pressure Sensor: Similar to gauge pressure sensors, these sensors measure pressure relative to a fixed pressure rather than the varying ambient atmospheric pressure.

3. Strain Gage-Based Pressure Sensors

Strain gauge-based pressure sensors, also known as strain gauge pressure transducers, are one of the most commonly used types of pressure sensors in the field of industrial and scientific measurement. These sensors convert the physical parameter of pressure into an electrical signal by measuring the strain (deformation) caused by the applied pressure.

A strain gauge is a device that measures the amount of strain (change in shape) that an object undergoes when a force is applied to it. The strain gauge operates on the principle that the electrical resistance of a material changes when it is strained.

3.1 Working Principle

In a strain gauge pressure sensor, a pressure-sensitive element (usually a diaphragm or a bourdon tube) deforms under the effect of the pressure applied. This deformation is proportional to the pressure.

A strain gauge, which is a thin strip of conductive material arranged in a specific pattern (typically a zig-zag or meander pattern), is attached to this pressure-sensitive element. When the pressure-sensitive element deforms due to the pressure, the strain gauge also deforms. This deformation causes a change in the length and cross-sectional area of the strain gauge, which in turn changes its electrical resistance.

This change in resistance can be measured using a Wheatstone bridge, a simple electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown resistance. The change in resistance of the strain gauge unbalances the Wheatstone bridge, and this imbalance can be measured as a voltage change.

This voltage change is directly proportional to the strain, which is in turn directly proportional to the pressure. Therefore, by measuring the voltage change, the pressure can be determined.

Strain gauge pressure sensors offer high accuracy and reliability, and they are suitable for a wide range of pressures, from very low to very high. They can be used in both static and dynamic pressure measurement applications. Due to their robustness and versatility, they are widely used in various industrial applications, such as hydraulic and pneumatic systems, process control, and fluid power systems. They are also used in scientific research and development, in applications such as wind tunnels, fluid dynamics, and materials testing.

In summary, strain gauge-based pressure sensors are a reliable and versatile technology for measuring pressure, providing accurate and consistent results over a wide range of pressures and in a variety of applications.

4. Piezoelectric Pressure Sensors

4.1 Introduction

Piezoelectric pressure sensors are a type of pressure sensing device that leverages the piezoelectric effect to measure changes in pressure. The piezoelectric effect is the ability of certain materials to generate an electric charge in response to applied mechanical stress or pressure.

The operational mechanism of a piezoelectric pressure sensor is relatively straightforward. When pressure is applied to a piezoelectric material, it undergoes deformation, leading to a shift in the charge distribution within the material. This shift generates an electric potential, and the resulting voltage or charge output can be measured and calibrated against the applied pressure.

The piezoelectric materials commonly used in these sensors can be natural, such as quartz, or synthetic, like certain ceramics (e.g., lead zirconate titanate, PZT) or polymers.

A key characteristic of piezoelectric sensors is their dynamic nature. They are excellent for measuring dynamic pressure changes, including pulse pressure, sound, or vibration, but less suitable for static pressure measurements. This is because the generated electric charge tends to leak away over time (even if this might take a significant time in high insulation materials like quartz), leading to a zero output from the sensor even if the pressure is still applied. Therefore, they are often used when rapid pressure changes need to be detected.

4.2 Advantages and Applications

Piezoelectric pressure sensors offer several advantages. They exhibit a very high frequency response, allowing them to detect rapid changes in pressure. They can also function over a wide temperature range, making them suitable for extreme environments. Additionally, these sensors are generally compact and robust, providing durability in challenging conditions.

Due to these characteristics, piezoelectric pressure sensors find wide-ranging applications in numerous industries. They are used in the automotive industry for engine combustion pressure measurement, in the aerospace sector for aerodynamic pressure testing, and in the manufacturing industry for machine condition monitoring and process control. Furthermore, they are used in the field of acoustics to capture sound pressure levels.

Piezoelectric pressure sensors offer a reliable solution for dynamic pressure measurement in a multitude of applications, owing to their high frequency response and robustness. Their ability to convert mechanical pressure into a measurable electrical signal, combined with their compact and durable design, makes them a popular choice in various fields from automotive to acoustics. However, their unsuitability for static pressure measurement due to charge leakage should be taken into account when considering their use.

5. Flush Diaphragm Pressure Sensors 

When dealing with certain industrial pressure measurement challenges, the physical design of the sensor is just as important as its internal technology. One such design is the flush diaphragm pressure sensor (also known as a flush mount sensor or flat diaphragm sensor). "Flush" means the diaphragm (the sensing surface of the sensor) is flush with the inner wall of the vessel or pipe where it’s installed, rather than recessed behind a port or opening. This design has significant advantages in both dynamic pressure measurement and general industrial applications involving challenging media.

Working Principle and Construction

The operational principle of a flush diaphragm pressure sensor begins with its unique mechanical configuration. The pressure-sensing diaphragm—which may incorporate strain gauge, piezoresistive, or other sensing technologies—is installed such that its outer surface forms part of the sensor's external mounting surface or process connection face. When mounted to a pressure port, the diaphragm sits flush with the mounting surface, creating a continuous, smooth interface with no protrusions or recesses.

When pressure is applied to the flush-mounted diaphragm, it deflects inward proportionally to the applied force. This deflection is immediately transmitted to the underlying sensing elements (such as strain gauges bonded to the diaphragm's reverse side or piezoresistive elements embedded within the diaphragm material), which convert the mechanical deformation into an electrical signal following the same principles described in strain gauge or piezoelectric sensor operation.

The key architectural difference from standard sensors is the elimination of the internal pressure transmission path. In a conventional sensor, pressure must propagate through a bore—typically ranging from 1 to 3 millimeters in diameter and several millimeters in length—before reaching the sensing diaphragm. This pathway introduces several physical phenomena that affect sensor performance:

• Pneumatic capacitance: The internal volume acts as a pressure capacitor, storing energy and creating lag in pressure transmission
• Flow resistance: The narrow bore creates friction that dampens rapid pressure changes
• Acoustic resonance: The cavity and passageway can create resonant frequencies that distort dynamic measurements

By eliminating these internal structures, flush diaphragm sensors provide a direct mechanical coupling between the pressure source and the sensing element, dramatically improving dynamic response characteristics.

Advantages and Performance Benefits

Flush diaphragm pressure sensors offer several significant advantages over standard internal diaphragm configurations, making them the preferred choice for many demanding applications.

Superior Dynamic Response: The most notable advantage of flush diaphragm construction is the dramatically improved frequency response and reduced time lag. Without internal pathways to dampen pressure waves, flush diaphragm sensors can accurately measure rapid pressure transients, pulsations, and high-frequency pressure variations. This makes them essential for applications such as engine cylinder pressure measurement, hydraulic transient analysis, and blast pressure monitoring, where pressure changes occur in milliseconds or microseconds.

Reduced Clogging and Contamination: The absence of internal bores or recesses eliminates spaces where particles, sediments, or viscous materials can accumulate. In applications involving slurries, suspended solids, or highly viscous fluids, standard sensors with internal cavities are prone to clogging, which can lead to measurement errors, sensor failure, or the need for frequent cleaning and maintenance. Flush diaphragm sensors maintain their smooth, continuous surface in contact with the process medium, significantly reducing these risks.

Improved Hygiene and Cleanability: In industries with stringent sanitary requirements—such as food and beverage processing, pharmaceutical manufacturing, and biotechnology—flush diaphragm sensors provide critical advantages. The flush mounting surface eliminates crevices where bacteria, biofilms, or product residues can accumulate during processing. This design facilitates thorough cleaning and sterilization through clean-in-place (CIP) and sterilize-in-place (SIP) procedures, helping to maintain product purity and comply with regulatory standards.

Enhanced Accuracy with Viscous Media: When measuring pressure of highly viscous fluids, the flow resistance through internal sensor bores can create significant measurement errors and time delays. The viscous medium may not fully transmit rapid pressure changes through narrow passages, leading to damped or inaccurate readings. Flush diaphragm sensors eliminate this source of error by providing direct contact between the viscous medium and the sensing surface.

Reduced Temperature Effects: Internal cavities in standard sensors can trap pockets of air or process media that may experience different thermal conditions than the bulk process fluid. These trapped volumes can expand or contract with temperature changes, introducing measurement errors. The flush design minimizes these thermal effects by eliminating dead volumes and providing more uniform thermal coupling with the measured medium.

Compatibility with High-Temperature Media: The direct-contact design of flush diaphragm sensors often provides better thermal management when measuring high-temperature gases or liquids. Without an internal pathway that can act as a heat insulator or create thermal gradients, the sensor body can be designed with more effective heat dissipation or thermal compensation strategies.