About Pressure Sensors & Scanners

1.0 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.

2.0 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.

3.0 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.

4.0 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.