Answering 7 questions about level measurement!

Have you seen what people ask Google? I have, and I found some interesting questions, among other things. So I’ve decided to leave the other things alone and answer the questions here at Visaya! You’ll see a range of topics from basic principles to advanced applications.

Today, I’ve selected seven questions about level measurement. If you have questions, you can reach me on Visaya’s social media. And now, in no particular order, our seven questions!

1. What’s the best level measurement principle?

Short answer: It doesn’t exist.

Long answer: You won’t find a perfect technology or device that covers all situations. Nonetheless, you can find options for your application that provide reliable measurement, good accuracy, and relevant information.

For measuring level, plenty of technologies come to mind, and each will have pros and cons depending on the application. Some users still prefer to deploy traditional devices, such as differential pressure (DP) transmitters, even though radar or a capacitance device could be a better option.

You have two relevant points to consider when choosing a device. First, you need to check its performance. You do this by scaling out, using your process data to find your accuracy, precision, and installation requirements.

Second, you need to feel comfortable using it in your plant. Some companies still keep DP transmitters for this very reason. Why buy the fanciest, most up-to-date gadget if no one knows how to use its best features? You’re wasting money doing that. However, the internet can teach you with tutorials, videos, articles, free training, and more. So make sure you know how to use it before buying it!

2. How does radar level measurement work?

Very relevant question! We actually have content already up on Visaya addressing this one. You can watch our video here explaining the principle using a squash ball. We also have a definitive guide that’ll give you an in-depth explanation. But we’ll keep it simple for now.

Radar level transmitters work with the time-of-flight (ToF) principle. “Time-of-flight” means the time it takes a signal to travel through a medium from the sensor to the product. Radar devices use electromechanical waves.

Level measurement
Courtesy of Emerson Automation Solutions

However, to apply a radar solution, the product needs a minimum dielectric constant. That means the product can carry an electrical current. The radar sends a signal, and when the signal hits the product surface, it reflects back. The transmitter calculates the level using the time between the sending and the return of the signal. Objects in the tank may affect the readings.

3. What’s a level switch?

In a level application, you can measure continuously or use level detection. Level detection works like a smart light switch. If the switch detects the level, then it sends a signal to the control system. You can set up the device to send “off” if the level rises to a certain point and “on” if it falls below another point, or vice-versa.

Courtesy of Endress+Hauser

A level switch in an application can turn on a pump or open a valve to prevent overflow or add product. To detect high levels, you install the sensor at the top of the tank, and for low-level detection, you install at the bottom.

Often you find level switches in continuous applications as a fail-safe. If the standard transmitter sending continuous level data goes down, the switch keeps the product from spilling over or the tank from running dry.

These switches use a variety of principles – vibration, capacitance, and others.

4. How does ultrasonic level measurement work?

The ultrasonic device is like radar but with a different signal. Radar devices emit electromechanical signals, and ultrasonics use mechanical signals. Ultrasonic signals will reflect from the product surface, but the product doesn’t need a minimum dielectric constant.

This option has problems with certain conditions such as foam, vapor, and ambient noise. You also have dead zones, areas where the devices won’t measure anything. Using ultrasonics to measure level has declined in popularity because of low-cost radars that don’t have these issues.

5. Why do changes of density affect level measurement?

Density changes won’t affect all level devices. You only see this problem in devices where the equation to find the level needs the density of the product. Makes sense, right? So this problem crops up a lot with DP transmitters.


However, you have companies where the control system calculates the level using data from the DP device plus density from a secondary device. That will give you a correct reading, although it takes more effort.

On the market, you’ll find a wide range of technology that can fit in your process without this problem. Of course, you need to scale out to make sure you don’t hop out of the frying pan into the fire!

6. What’s the best accuracy in a level measurement?

Many devices on the market offer similar accuracy. Usually, you’ll find highly accurate devices, like servo level transmitters, in custody transfer or storage applications.  These devices can have an accuracy anywhere from 0.7 to 1 millimeter.

For a process that doesn’t need a lot of accuracy, you can choose a radar with an accuracy around 2 millimeters or an ultrasonic around 10 millimeters. The best choice will depend on your process needs. So again, remember to scale out and find the level principle that suits your process best.

7. What’s the best level technology for foam?

Foam presents a challenge to level measurement because of its lack of consistency. Radar devices generally perform better than ultrasonics, but some radars can read false levels too. Sometimes you can fix this issue with echo analysis.

If you have foam in your application, then talk with your vendors to find the right instrument for your application. You might discover that new tech has eliminated this old problem!

This video reviews level measurement in an open tank:

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