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PM Pressure Transmitter

Technology overview

Ceramic capacitive

The most important element of the ceramic capacitive technology is the ceramic (Al2O3) measuring cell. After assembly, the ceramic cell element resembles a plate capacitor with a reference electrode and a measuring electrode placed 0.01 mm apart. The capacitance is inversely proportional to the distance between the electrodes. As pressure is applied, the distance changes by a small value and the capacitance changes proportionately. This signal is then converted into pressure by a microprocessor.

Features:

  • Pressure ranges from 100 mbar (40 “H2O) to 100 bar (1450 psi). Vacuum to -1 bar (-14.5 psi)
  • Extremely robust with high over pressure and burst pressure rating
  • Drift-free operation > 100 million pressure cycles
  • High long-term stability and repeatability

Pros:

  • Ceramic does not age or fatigue (long-term stability)
  • Minimal temperature influence on the ceramic material and measurement
  • High chemical resistance
  • Counter electrode (base) supports the diaphragm in case of over pressure
  • Easy to troubleshoot – crack in the diaphragm results in a positive offset (20 – 35% of measuring range)

Cons:

  • Support of the measuring cell is necessary; this leads to added cost

Applications

Tank level on CIP skid

Pressure sensors are used to monitor tank level on CIP skids. Often, these sensors are fitted diaphragm seals which are fragile and easily damaged. What if you didn’t have to worry about diaphragm fill fluid leaking into your process?

The direct process measurement via the PM’s capacitive-ceramic technology offers an accurate solution in a robust package.

Line pressure

Transmitters are used to monitor line pressure in various sanitary applications, including pasteurizers that are subjected to high-pressure wash down. What if your transmitter had a rotatable vent to protect the device?

ifm’s PM sensors have been designed with this in mind. The vent is rotatable, so the transmitter can be installed in any orientation.

Unlock sensor potential with IO-Link

Via IO-Link, the PM family can provide:

  • Current system pressure
  • Sensor diagnostics
  • Remote parameterization

FAQs

Q. Why is the vent on the housing rotatable?

A. The PM transmitters are rated IP69K to withstand the rigorous cleaning requirements of the Food and Beverage industry. This rating means that the sensor is protected against ingress, which can cause the sensor to fail. In many applications, process media or cleaning chemicals can splash onto the vent and clog it. While this media does not penetrate into the sensor electronics, it does cause an incorrect indication of atmospheric pressure and, therefore, an incorrect pressure measurement of the process. With the PM transmitters, you can rotate the vent to reduce potential splashing. Additionally, ifm offers accessories to further protect the vent. Refer to the Operating Instructions for details on the E30139 and E30142 vent covers.

Q. Why does my pressure sensor show a value without any pressure applied?

A. Typically, this is an indication that the measuring cell has been damaged and most likely due to pressure spikes. In these cases, the offset is 25 – 35% of the measuring range of the sensor. Because of the structure of our measuring cells, our Quality / Warranty Evaluation Team can diagnose a cell damaged by pressure spikes with 100% certainty.

Q. How do pressure spikes occur?

A. A pressure spike (or water hammer) is a surge of an incompressible fluid that is forced to stop or change direction suddenly. It commonly occurs when valves are opened or closed very quickly. The easiest way to understand the concept is to compare the fluid with a train. When a train is stopped suddenly, the back of the train carries momentum with it, continues to move forward for some time and sends a shock wave throughout the train. Since the water flow is restricted to the inside of a pipe, a shock wave of incompressible liquid travels through the pipe at much higher pressure than the typical system pressure.

This chart shows pressure changes over time. It shows clearly that pressure spikes can be 10 times or more than the system pressure.

Q. Can ifm pressure sensors detect these spikes?

A. No, pressure spikes generally have a very short (in the nanosecond range) duration. The response time of our pressure sensors is 1 – 3 milliseconds.

Q. How can I protect my pressure sensors?

A. There are several ways:

  1. Prevent the spike from happening in the first place. Review the piping system and adjust the opening and closing speeds of valves if possible.
  2. Add an accumulator in the piping system to absorb the pressure fluctuations. This has the added benefit of potentially enabling a hydraulic system to meet demands with a smaller pump.
  3. Use a pressure sensor with a higher measuring range. This will provide higher overload and burst pressure ratings, but it will decrease the achievable resolution of the sensor.

Q. What is meant by turn down ratio?

A. This is minimum difference between the analog start point and analog end point of scalable transmitters. ifm’s PM transmitters have a turn down of 1:5. This means that the smallest span the transmitter can have is 20% of the measuring range. For example, if the full range of the sensor is 10 bar, the minimum span is 2 bar, i.e. 0…2 bar, 1…3 bar, etc.

Q. How do I interpret the accuracy specifications on the datasheet?

A. The most important thing to keep in mind when calculating the deviations is that they are all referenced with a turn down of 1:1. This means that regardless of how the transmitter is scaled, the specifications are based on the full range of the sensor.

For example, the switch point accuracy is ± 0.2 % of full range. In the example above, if the range of the sensor is 10 bar, the accuracy is ± 0.02 bar. It does not matter if the transmitter is scaled 0…2.5 bar or 0…5 bar or 0…10 bar.

Q. What does “Characteristics deviation” mean?

A. “Characteristics deviation” is our most all-inclusive measure of accuracy. It includes linearity, hysteresis and repeatability.

Q. How does the process connection system work?

A. The PM family uses a flexible process connection system known as Aseptoflex Vario. The system provides more sealing options to meet the current 3A sanitary requirements. In addition to an o-ring seal (either FKM or EPDM materials), we offer a PEEK sealing ring. This PEEK ring provides longer stability compared to traditional elastomers that become hard and brittle over time.

Q. Why do some fittings for the Aseptoflex Vario system have holes in them?

A. The leakage ports, or weep holes, are required for 3A authorization. When an o-ring is in place (see above), the process medium will remain in the tank or pipe. If that o-ring is compromised, i.e., missing or damaged, the process medium will leak at the weep hole. This is an indication that the o-ring must be replaced.