Technology overview - pressure sensors

The ceramic-capacitive measuring cell consists of a base body, two capacitor layers, a glass solder and a membrane.
The capacitor layers on the base body and membrane are similar to a plate capacitor and form a measurement and reference capacitor. When pressure is applied, the distance between the membrane and the base body changes and this changes the capacitance between the electrodes. This change in capacitance is evaluated and then processed into a signal common in industry.

Features:

• Pressure ranges from 100 mbar to 600 bar, vacuum to -1 bar
• Extremely robust with high pressure rating and burst pressure
• Drift-free operation > 100 million pressure cycles
• High long-term stability and repeatability
• Dry measuring cell without filling medium

Pros:

• Ceramic does not age or fatigue (long-term stability)
• Resistant to dynamic pressure peaks and cavitation
• Minimal temperature influence on the ceramic measuring cell
• High chemical resistance
• Counter electrode (base) supports the diaphragm in case of overpressure

Cons:

• Elastomer sealing is necessary - for gases only below 25 bar
• Structurally complex installation of the measuring cell in the sensor, therefore large housing and higher costs

In these measuring cells, strain gauges in the form of resistors are attached to the back of the stainless steel membrane and connected as a Wheatstone bridge. This can be implemented as a thick layer or a thin film. As soon as the measuring cell is pressurised, the membrane deforms. This changes the output signal of the bridge circuit proportionally, which in turn is measured by the electronics and converted into an output signal common in industry.

Features:

• Pressure ranges from 6 bar to 600 bar
• Suitable for gas up to 600 bar
• High long-term stability

Pros:

• Small size, wrench size 19 or 22 mm
• Seal-free installation, measuring cell is welded to the process connection
• High burst pressure
• Favourable for large quantities

Cons:

• Lower overload resistance than ceramic-capacitive measuring cells
  ⇒ damping element strongly recommended for applications with dynamic pressure and cavitation

These measuring cells make use of the piezoresistive effect. It describes the change in the electrical resistance of a material due to pressure or tension. This change in resistance is recorded with a bridge circuit and converted into an output signal that is common in industry. The piezoresistive silicon measuring cell is bonded to a printed circuit board, which is coated to be universal media-resistant (polluted air, oil, water, cooling lubricant, etc.).

Features:

• Suitable for pneumatic sensors
• Differential pressure measurement possible

Pros:

• attractively priced
• small sensor element

Cons:

• not resistant to aggressive media without additional effort