Time of flight! What does it mean for you?


Time of flight! What does it mean for you?

Hey, hey, time to learn about a cool technology! For those of you just joining the level journey on Visaya, we already chatted about the basics of level measurement using differential pressure (DP) transmitters in open tanks and closed tanks. Those articles include equations to calculate your values, so if you need the math, then check ’em out.

We also went over traditional options versus new technology, too. Now, we’ll explore still more new tech in level measurement. But don’t worry, we’ll come back to pressure transmitters, as they still have a lot left to talk about.

As with anything, DP transmitters have advantages and disadvantages. For instance, unstable product density can cause trouble with DP transmitters. You can also have problems with installation, leaks, and so on.

So what else can you use besides DP transmitters? That brings us to devices based on time of flight (ToF). You have different principles based on this idea, and today we’ll explain it and the various devices that use it. Ready? Let’s go then!

Time of flight what?

Okay, let’s break it down. Time of flight (ToF) refers to the time it takes for things such as acoustic or electromagnetic waves to travel through a medium from Point A to Point B and return. In our case, usually Point A is the device that creates the wave, Point B is the surface of your product, and the medium is air or vacuum. Pretty simple, right?

time of flight
Courtesy of Digikey

Well, maybe not. Your Point A may have particular issues, and your Point B may have different issues. Let’s start with potential Point A devices. Hold on, gang; we got a lot more coming! 🙂

Ultrasonic level measurement

For a long time, ultrasonic devices ruled the cost-benefit roost. However, new radar devices may make ultrasonics history. Of course, companies still sell them because they do still work.

Speaking of work, ultrasonic works with the ToF concept. The device emits mechanical waves with a piezoelectric sensor. These waves reflect off the product surface to return to the sensor. Then the device measures the level by calculating how long it took for the wave to touch the surface and return.

time of flight
Courtesy of Emerson Automation Solutions

Now, ultrasonic transmitters have a disadvantage called the dead zone or blocking distance. You need to take this zone into account when you scale out an ultrasonic, especially if you have a process condition which may attenuate your signal, such as foam or waves on the product surface.

Radar level measurement

Oh, I love radar so much I don’t have the words to express it. Radar devices are the stars of level measurement. And guess what? Yep, they work with ToF too. They generate electromagnetic waves called microwaves. You’ve probably heard of them.

The antenna transmits the microwaves to the product. When the product reflects the signal, the antenna receives that reflected signal. The time between emission and reception is proportional to the level in your tank, so a microprocessor calculates the level using that fact.

time of flight
Courtesy of Instrumentation Toolbox

Now, to use radar transmitters you need a dielectric constant (DC). The dielectric constant is the ability of a material to carry an electrical current compared to the ability of a vacuum to carry that current.

Some radars can operate in low dielectric constants, and others need high DC values. When the radar signal reaches the product surface, some of the signals will return to the radar. And the amount of energy sent back to the radar will be proportional to – guess what? Yep, the DC of the medium. Beautiful.

I could go on about radar, but I’ll do that in other articles. Let’s continue!

Laser level measurement

Lasers! You see lasers everywhere, especially in movies like Resident Evil and Star Wars. Of course, lasers don’t actually work like what you see in the movies. Too bad, but never mind. Let’s talk about how they do work!

time of flight
Courtesy of Sensorsmag

Once again we have the ToF concept. The device transmits a light beam that reflects from the product surface back to the device, just like the others. And like the others, you find your level by translating into distance the time it took for the signal to return. We’ll talk more about lasers and where to apply them in later articles too.


So there you have it, an overall explanation about time of flight and the most common devices that use it. Future articles will go through the details and explain how to find the right level device for your application.

Choose the best solution for your application!

time of flight
Courtesy of Giphy
Reference DC : Clipper Controls

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