Question 1

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

Accepted Answer

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.

Question 2

How do pressure spikes occur?

Accepted Answer

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.

Question 3

Can ifm pressure sensors detect these spikes?

Accepted Answer

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.

Question 4

How can I protect my pressure sensors?

Accepted Answer

There are several ways:

Prevent the spike from happening in the first place. Review the piping system and adjust the opening and closing speeds of valves if possible.
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.
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.
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.

Question 5

What is meant by turn down ratio?

Accepted Answer

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.

Question 6

How do I interpret the accuracy specifications on the datasheet?

Accepted Answer

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.

Question 7

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

Accepted Answer

“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.

Question 8

Why does the LED on my PN4xxx sensor turn on, but my output does not?

Accepted Answer

The PN4 family of sensors have an AC triac output. This is a solid state relay that must be externally powered to fire. The LED on the sensor gets its signal from the microprocessor separately. When the system pressure reaches the set point, the LED turns on. But, unless the triac is powered, the output does not turn on. The easiest way to wire this sensor is to jumper power from pin 1 of the sensor to pin 4 of the triac.

PN pressure sensor FAQs