The Piezoresistive Effect and Measuring Pressure

Patrick Niedenführ
Patrick Niedenführ, February 2019

Today, innovative sensor technologies provide you with the right pressure sensor for every measuring task. Not only can you choose between relative, absolute and differential pressure sensors that work with different reference values, modern pressure sensors also allow you to choose between different measuring principles based on different physical effects. Piezoelectric and piezoresistive effects are mainly used in modern pressure sensors. 

 

In this blog post, we take a closer look at the special properties and practical advantages of a piezoresistive differential pressure sensor.

First – how does the piezoresistive differential pressure sensor actually work?

Unlike relative and absolute pressure sensors, differential pressure sensors measure the difference between any two pressures. A differential pressure sensor has two separate pressure connections (hose, threaded or manifold). And – depending on the calibration – it can measure both positive (p1>p2) and negative (p1<p2) differential pressures.

 

The piezoresistive effect

Piezoresistive (diaphragm-based) differential pressure sensors based on silicon in turn consist of a thin silicon diaphragm in which resistors in the form of a Wheatstone bridge are embedded. If there are pressure differences, the diaphragm pushes through and the resistors are distorted. The resistors connected together in the measuring bridge react to this distortion, resulting in a piezoresistive effect. This changes the resistance of the resistors and the electrical voltage. Finally, a pressure-proportional measuring signal is triggered.

Exciting aspects of diaphragm-based sensor technology:

• The resistor in the silicon diaphragm’s semiconductor reacts very sensitively to even the smallest pressure loads and changes. This makes the piezoresistive differential pressure sensor extremely accurate and enables measurement ranges of just a few millibars.
• Silicon has a monocrystalline structure, which means that the diaphragm always returns to its original state after stretching without becoming distorted.

 

Exciting aspects of diaphragm-based sensor technology

Basic design of a piezoresistive differential pressure sensors


The comparison: How do piezoresistive pressure sensors measure up?

An alternative to the piezoresistive differential pressure sensor is the piezoelectric pressure sensor. Unlike diaphragm-based sensors, piezoelectric pressure sensors consist of disks of crystal. Under pressure, charge separation generates a proportional electrical voltage in the crystal – the “piezoelectric effect.” A charge amplifier outputs the voltage difference and a digital circuit converts it to the physical value of pressure.

 

When compared, both the piezoresistive sensor and the piezoelectric sensor have advantages. Deciding which measuring principle to use depends simply on the application in question. For example, piezoelectric sensors can withstand high temperatures and highly dynamic pressure processes particularly well, while diaphragm-based differential pressure sensors boast extremely good measurement accuracy and sensitivity in particular.

 

Membrane based differential pressure sensor

Diagram of a diaphragm-based differential pressure sensor from First Sensor 

 

The main advantages of using a piezoresistive differential pressure sensor:        

+ Very small size
+ Extremely accurate measurements
+ Excellent sensitivity
+ Measures even the smallest differential pressures
+ Large measurement range: 1 mbar (100 Pa) to 10 bar
+ Hardly any material fatigue or hysteresis
+ Very good overpressure resistance
+ Analog and digital interfaces
+ Highly linear signal–pressure characteristic curves (no air flow)

New: Piezoresistive low differential pressure sensor for manifold mounting

First Sensor offers diaphragm-based low differential pressure sensors in a new miniature manifold design. The very small, flat sensors cover measurement ranges from 1 mbar to 7 bar and are ideal for space-saving installation in manifolds. The piezoresistive sensors feature digital signal conditioning, an SPI interface and an analog output signal, making them ideal for battery-powered applications in mobile or portable devices.

 

Read more about First Sensor’s diaphragm-based miniature sensor here. 

 

Features:

 

• Suitable for mounting in manifolds
• Surface area 8 x 13 mm, height <7 mm
• High-precision and long-term stability
• Pressure ranges from 1 mbar to 7 bar
• Calibrated and temperature-compensated (0–70°C)
• 3.3 V or 5 V power supply
• SPI interface with 15-bit resolution

 

Requirements:

 

• Manifold-compatible pressure ports
• Manifold-ready housing
• Straight connection, so that the mounting point can be sealed with an O-ring

Application:

When is it recommended to use diaphragm-based differential pressure sensors?

When pressure measurement requires highly sensitive, highly accurate measuring, pressure ranges from 1 mbar to 10 bar and a space-saving design, a piezoresistive differential pressure sensor is the perfect sensor.

 

Typical fields of application for diaphragm-based differential pressure sensors:


Medical technology: ventilators, anesthesia equipment, CPAP, spirometers, oxygen concentrators
Air-conditioning technology: flow regulators, filter monitoring, burner control
Measuring technology
Industrial control and regulation technology

Precise process connections and filters

In principle, differential pressure sensors should have process connections so that the sensor can be connected to the flow line (by means of hoses etc.). Additional filters in the supply lines are also important in many applications, to keep dust, moisture or bacterial contamination away from the sensor, for example.

And if a standard solution isn’t enough?

Even more precise, even faster, even more efficient, even more cost-effective, even more application-specific: Some applications and some market situations require special sensor solutions. That’s why, in addition to offering a wide range of diaphragm-based differential pressure sensors, leading manufacturers such as First Sensor also develop and implement application-specific sensor systems.

 

Can a differential pressure sensor be both highly sensitive and reliable in its measurements? You’ll find the answer in this Buyer's Guide:

 

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