PI sanitary pressure transmitter
Technology

Principle of operation for flush pressure transmitters

Flush pressure transmitters, such as the PI1 sanitary pressure transmitter, use ceramic capacitive technology, which is known for its robustness and long-term stability. At the core of this technology is the ceramic measuring cell made of alumina (Al2O3), functioning similarly to a plate capacitor. The ceramic cell contains two electrodes: a reference electrode and a measuring electrode placed 0.01 mm apart. The capacitance between these electrodes is inversely proportional to the distance between them. When pressure is applied to the sensor, the distance between the electrodes changes slightly, altering the capacitance. This change is then converted into a digital pressure reading by the sensor's microprocessor, enabling highly accurate pressure measurements.

Cell construction

The strength of the ceramic capacitive measuring cell comes from its robust construction. The ceramic diaphragm, made from very hard ceramic material, is supported by a ceramic base to withstand overpressure. Additionally, the entire measuring cell is centered and supported by a brass locating ring, providing a strong support system that enables the sensor to handle high-pressure ratings. In contrast, stainless steel diaphragms are supported by fill fluid, which is less rigid and robust.

Note the signal change in the animation below as pressure is applied to the sensor.

PCB microprocessor
Ceramic capacitive base
Outer ceramic diaphragm
Applied pressure

Main advantages of the ceramic capacitive measuring cell

Robust and direct sensing measurement technology
Eliminates possibility of contamination from diaphragm fill fluid
Long-term stability of ceramic cell design does not fatigue like stainless steel
Internal support construction creates very high pressure ratings
Ceramic cell is more resistant to abrasion, dents or dings that can create measurement offsets
Highest overload pressure rating, especially with dynamic pressure peaks

Relative pressure measurement and atmospheric vent

Relative pressure measurements, often referred to as gauge pressure, require a reference to atmospheric pressure or the surrounding ambient environment. The pressure measurement is the differential between atmospheric and system pressure, making it “relative” to atmospheric pressure. To accomplish this, most pressure sensors use a vent system that exposes the measuring cell to the atmosphere, allowing the system pressure to be measured as above or below (vacuum) this reference.

ifm has optimized the location and design of the atmospheric vent to improve machine availability and process performance.

Optimized atmospheric vent location

The vent is strategically placed 90° from the display to optimize its orientation while keeping the display in a preferred position.

By keeping the vent orientation optimal (with the membrane in a vertical position), the Gore-Tex membrane efficiently sheds moisture. This reduces the risk of clogging and measurement errors due to incorrect reference pressure.

Proven vent design

ifm has rigorously tested various vent design variations to improve cleanability, breathability, and media flow though the vent.

Considerations include the size and number of holes to improve flow, and a drip edge to prevent media from entering the vent. These optimizations prevent build-up in the vent, which could otherwise cause measurement offsets and reduce overall machine performance.

Technology comparison: ceramic vs stainless steel diaphragm

Summary of technology comparison

Metal diaphragms are fragile and susceptible to damage during shipping or normal handling. A damaged diaphragm can result in a permanent offset, impacting the sensor's accuracy.
Metal diaphragms rely on fill fluid to transfer energy to the measuring cell. If the diaphragm is damaged or worn, the fill fluid may leak into the process, contaminating the system. Even though many fill fluids are FDA approved, leakage can still affect product quality or lead to scrapped products. In some cases, a leaking diaphragm may continue to function temporarily as the process media replaces the fill fluid, but this poses a significant risk to the system.
- One advantage of the fluid-filled diaphragm is its thermal insulation properties, which help minimize the impact of temperature shocks on the sensor electronics. For this reason, ifm offers fluid-filled diaphragm models (series PI22 / PI23) that can handle continuous media temperatures up to 200 ºC (392 ºF).
Ceramic capacitive measuring cells do not require fill fluids and eliminate the risk of contamination. While a ceramic cell requires a stable seal between the measuring cell and process connection, ifm has optimized this design by using a very stable PTFE seal. This seal is hygienic and compatible with most media found in Food & Beverage applications. Additionally, this design meets all common sanitary approvals, ensuring reliable performance.