Davidson Measurement Tech Tips

Be aware of Natural or Resonant Frequency

Natural or resonant frequency, the frequency at which a cavity, sensor or structure goes into dynamic resonances, often goes unnoticed when taking pressure measurements.

When pressure transducers or transmitters are installed on piping the slower response due to the installation can often mask spikes or frequencies that could be creating errors in readings, even damage.

Natural frequency can be graphed as shown below:

As a rule of thumb, to make true measurements it is recommended measurements be made to 1/5^th of Fn (this can be stretched to 1/3^rd if the sensor’s frequency response is known. Note, when installing pressure transmitters which are not flush to the media there is a small pipe effect from the wall of the pipe to the sensor diaphragm as shown below:

To confirm you are able to detect the true response and not miss peak or dynamic fluctuations, you can calculate the resonant frequency of the passage with the formula:

Fn = (approx.) C/4L

where Fn is the natural or resonant frequency of the passage.

C is the speed of sound in the medium

L is the length of the passage.

The above formula is often referred to as the organ pipe effect. Just make sure the "C" and "L" are in the same units. Don’t forget the effect of the process connection on the calculations as that will also add to or introduce the passage length.

If you would like to know more about the effect of resonant frequency on measurements and structures simply ask one of our engineers, they will be happy to explain it.