Question 1

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

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

Question 3

3. Can ifm pressure sensors detect these spikes?

Accepted Answer

No. Even though the PI 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

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.
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

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 PI transmitters have a turn down of 1:4 (shown on our datasheets as “programmable 1:4”.) This means that the smallest span the transmitter can have is 25% of the measuring range. For example, if the full range of the sensor is 10 bar, the minimum span is 2.5 bar, i.e. 0…2.5 bar, 1…3.5 bar, etc.

Question 6

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 ± 0.2 % 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.

Question 7

7. What does “Characteristics deviation” mean?

Accepted Answer

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

Question 8

8. How does the process connection system work?

Accepted Answer

The PI 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.

Question 9

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

Accepted Answer

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.

Question 10

10. Why does the PI display flash "W532"?

Accepted Answer

If the OU2 parameter is programmed for analogue signal current (I) and the output wire (pin 2) is not connected (resistance = infinite), the error message W532 flashes on the display. The measuring result remains unaffected. To get rid of the error, if an analog output is not being used, change the OU2 parameter from (I) to a switching output, such as (HNO).

Question 11

11. Which PI17xx or PI18xx model is the replacement for the PI27xx or PI28xx model?

Accepted Answer

Old PI27xx	New PI17xx
PI2714	PI1714
PI2715	PI1715
PI2717	PI1717
N/A	PI1718
PI2789	PI1789
PI2793	PI1703
PI2794	PI1704
PI2795	PI1705
PI2796	PI1706
PI2797	PI1707
PI2798	PI1708
PI2799	PI1709

Old PI28xx	New PI18xx
N/A	PI1814
PI2815	PI1815
PI2817	PI1817
N/A	PI1818
PI2889	PI1889
PI2893	PI1803
PI2894	PI1804
PI2895	PI1805
PI2896	PI1806
PI2897	PI1807
PI2898	PI1808
PI2899	PI1809
PI2602	PI1602

PI Sanitary pressure transmitter: FAQs