How to Handle Electromagnetic Interference with Ultrasonic Level Meters

At a customer's site, a two-wire integrated ultrasonic level meter was directly connected to a variable frequency drive (VFD) to control motor speed based on the measurement data. However, significant discrepancies occurred between the measured data and actual values. When the ultrasonic level meter was connected to a switching power supply, measurements were normal, but connecting it directly to the VFD produced the echo pattern shown below. This confirmed that the interference was caused by the VFD.

Step 1: Ground the ultrasonic level meter.

The ultrasonic level meter has three terminals, one of which is dedicated to grounding. After grounding at the site, the echo pattern changed as shown below.

The image shows that interference was significantly reduced, demonstrating that grounding is a standard method for reducing interference in environments with electromagnetic interference. Generally, proper measurement can be achieved after grounding. However, at this particular site, accurate measurement was still not possible even after grounding. Therefore, isolation had to be used. A signal isolator was installed between the VFD and the ultrasonic level meter to block the interference from the VFD. The echo menu at this point appeared as follows:

At this point, the echo menu was very clear with no traces of electromagnetic interference. Compared with the actual liquid level on site, the level height measured by the ultrasonic level meter was accurate. From this example, we learned that when encountering strong electromagnetic interference sources such as VFDs, motors, and high-voltage power lines on site, the following measures should be taken:

First, enter the echo menu (press and hold the UP button, then press and hold the SET button for three seconds; the echo menu will appear).

1. Connect the grounding terminal of the ultrasonic level meter separately to a true earth ground, without sharing the ground with other equipment.

2. If grounding is ineffective, install a signal isolator between the ultrasonic level meter and devices such as PLCs and VFDs.

After resolving the electromagnetic interference, exit the echo menu (press and hold the DOWN button, then press and hold the SET button for three seconds; the echo menu will exit and return to the previous menu).

For more detailed information, please call our hotline: 0591-28082552. The technical staff at Fuzhou Dayu Electronic Technology will be happy to assist you!