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PN pressure sensors

Technology overview

The PN family incorporates the best combination of over pressure, burst pressure and long-term stability for each measuring range. At 1450 psi and below, the PN sensors use the ceramic capacitive technology. Above this range, the thin film metal strain gauge technology is used. 

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 600 bar (8700 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
  • Only suitable for low pressure gases (<25 bar / 363 psi) due to the elastomer seal construction

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:

  • Elastomer sealing is necessary -  for gases only below 25 bar / 363 psi; quickly decreasing pressure can lead to mechanical failure of the seal due to its permeability
  • Support of the measuring cell is necessary; this leads to added cost

Stainless steel strain gauge measuring cell

Strain gauges are attached to the stainless steel measuring cell. Pressure of the medium on the measuring cell causes deflection of the strain gauges, which creates a change in resistance. Tension (positive strain) increases resistance and compression (negative strain) decreases resistance. The change in resistance is proportional to the pressure applied.

Features:

  • Pressure ranges from 6 bar (87 psi) to 600 bar (8700 psi)
  • Suitable for gas up to 600 bar (8700 psi)

Pros:

  • No elastomer seal required
  • High burst strength at high ranges
  • Fully welded all stainless steel diaphragm

Cons:

  • Lower over pressure and burst pressure limits at low ranges
  • Temperature compensation is necessary

Applications

Hydraulic power units

Pressure switches are used to monitor the system pressure of hydraulic power units. Mechanical switches are cumbersome, difficult to install and require a reference gauge for local indication of pressure. What if you could configure a pressure switch in less than 5 minutes?

The PN family offers a large 2-color LED display, raised pushbuttons and easy-to-navigate menu for fast setup without the need to applied pressure.  

Metal forming

Grippers are used to position metal blanks into a hydraulic press. The switch point of mechanical switches drifts over time due to wear and fatigue. This results in damage to the work piece and system. What if your pressure switch did not wear out?

The ceramic capacitive measuring technology of the PN family has no moving parts to break or stick as well as long-term stability and excellent repeatability.

Unlock sensor potential with IO-Link

With IO-Link, process sensors have the ability to transmit multiple sensor values. Via IO-Link, the PN family can provide:

  • Current system pressure
  • Sensor diagnostics
  • Remote parameterization

FAQs

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 is a representation of 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. Even though the PN family has a “Hi” and “Lo” memory to store the highest and lowest measured values, 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. Add a snubber / damping screw to the process connector of the pressure sensor. These accessories will slow down the response time of the sensor somewhat.
  4. 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 PN2 transmitters have a turn down of 1:5 (shown on our datasheets as “programmable 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 100 bar, the minimum span is 20 bar, i.e. 0…20 bar, 10…30 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 ± 0.4 % of full range. In the example above, if the range of the sensor is 100 bar, the accuracy is ± 0.4 bar. It does not matter if the transmitter is scaled 0…20 bar or 0…50 bar or 0…100 bar.

Q. What does “Characteristics deviation” mean and what is the difference between “BFSL” and “LS” values?

A. “Characteristics deviation” is our most all-inclusive measure of accuracy. It includes linearity, hysteresis and repeatability. “Best Fit Straight Line” is calculated by first measuring the pressure deviations from the ideal value. Then, a straight line is drawn to minimize the deviation. “Limit Setting” uses the same technique, but in this case, the straight line must include the zero and span points.  The deviation is generally higher when the end points are included in the measurement.