Overview of ifm’s pressure measuring cell technologies

Design and function of the ceramic capacitive measuring cell

When pressure is applied, the distance between the diaphragm and the base body changes and, thus, the capacitance between the electrodes. This change in capacitance is analysed and then converted into a standard industrial output signal.

What makes the measuring cell special

• Extremely robust, high pressure resistance and high burst pressure
• Drift-free operation over 100 million pressure cycles
• High long-term stability and repeatability
• Dry measuring cell without diaphragm seal fluid for high process reliability

_Advantages

• Resistant to dynamic pressure surges and cavitation
• Resistant to aggressive and abrasive media
• Minimal temperature influence on the ceramic measuring cell
• Ceramic does not age or fatigue

_{Disadvantages}

• Complex installation of the measuring cell in the sensor, therefore large housing and higher costs
• Sealing of the measuring cell to the process connection required
• Gas applications above 25 bar only possible to a limited extent

Products with ceramic-capacitive measuring cell

Design and function of the diaphragm seal measuring cell

In diaphragm seal technology, the process pressure is not applied directly to the sensor, but is transmitted via a diaphragm with a filling liquid behind it. The diaphragm itself separates the medium in the process from the sensor system. It is flexible and deforms when pressure is exerted on it. Behind the diaphragm is a special filling fluid (usually oil) that transfers the pressure from the diaphragm to the actual pressure sensor.

What makes the measuring cell special

• High temperature resistance

_Advantages

• Compact front-flush design possible
• Low susceptibility to temperature changes

_{Disadvantages}

• Sensitive diaphragm
• Pressure peaks can cause irreparable damage
• If the diaphragm is damaged, the diaphragm seal fluid may escape, thus contaminating the medium.
• Metal diaphragms can corrode

Products with diaphragm seal measuring cell

Design and function of the stainless steel measuring cell with strain gauges

In this measuring cell, strain gauges in the form of resistors are attached to the back of the stainless steel diaphragm and connected to form a Wheatstone bridge. If pressure is exerted on the measuring cell, the diaphragm deforms. This causes the output signal of the bridge circuit to change proportionally, which is detected by the electronics and converted into an industry-standard output signal.

What makes the measuring cell special

• High long-term stability
• Fully welded measuring cell without seal
• Small design, low price
• Suitable for gases up to 600 bar

_Advantages

• Fast step response time
• High bursting pressure
• Ideal for individual and bulk orders

_{Disadvantages}

• A damping element or restrictor is recommended in applications with dynamic pressures and cavitation.

Products with stainless steel measuring cell

Design and function of the piezoresistive measuring cell

The piezoresistive effect describes the change of the electric resistance of a material caused by pressure or traction. This change in resistance is detected by a bridge circuit (R1 - R4) and converted into an industry-standard output signal.

What makes the measuring cell special

• Vacuum and contaminant-resistant measuring cell thanks to coating
• Suitable for pneumatic sensors

_Advantages

• Small sensor element

_{Disadvantages}

• Only suitable for pneumatics and inert gases

Products with piezoresistive measuring cell