The definitive guide to time of flight (ToF) devices

Here we go again with another guide! If you haven’t read our definitive guide to pressure transmitters, then definitely check it out.

This topic actually relates to what you learned in our previous guide. Time of flight (ToF) devices measure level, just like differential pressure (DP) transmitters. Of course they do it in a different way, meaning new pros and cons.

So to recap a bit about level measurement with DP transmitters, we talked about their use in open tanks and closed tanks. We also discussed the new technology of electronic DP transmitters and the old problems of clogs and leaks.

To avoid some of those problems, we can use Time of Flight devices. We have different principles based on this idea, so this article will cover them for you. Ready?

PART 1: What is time of flight?

Time of flight (ToF) is the time it takes for waves to travel through a medium from Point A to Point B and return. If you ever watched a Discovery Channel program that talked about how bats locate their prey, then it touched on this principle. Scientists call it echolocation, but it’s the same thing.

Echolocation Time of Flight
Courtesy of Arduino-Info

If you’ve never seen such a show, then here’s the gist. To locate their prey, bats send out high-pitched ultrasonic waves, inaudible to humans. These waves bounce from the prey back to the bat, telling the bat the size and location of the prey and even which direction it’s heading.

Like the bats, Time of Flight devices send electromagnetic or mechanical waves out, wait for the “echo” and calculate the time it took to return. Using this time, the device calculates the distance between the product and itself, which gives you the level of your tank. Easy, isn’t it?


time of flight
Courtesy of Digikey

Now that we know how Time of Flight works, let’s move on to the devices that use it. You probably figured out by now we call them radars. You have different types of radars on the market, though. We’ll explain the differences and their pros and cons. Off we go!

PART 2: Non-contact radar level transmitters

Some vendors call them non-contact radar level transmitters, some call them free space radar transmitters, but they’re all the same type of device. These usually go in the top of the tank and the waves fly freely through the tank, with no guided path. You’ll see what that means later. They use different frequencies and different antennas, both of which will change your beam angle and spread. Hold on, we’ll explain those now!

Frequency bands

Different devices use different frequencies. A radar gun, for example, operates on frequencies that change from country to country. The first radar guns used a frequency range from 8 to 12 gigahertz (GHz), called the X-band. The newer radar guns use the K-band instead (18 to 27 GHz), and you can find radar level transmitters using Ka-band, which lies between 27 to 40 GHz.

Frequency bands Time of Flight
Courtesy of Radar Tutorial

Most industrial radars work in one of four frequencies: C-band (6 GHz), K-band (26 GHz), X-band (10 GHz) and a few on W-band (75-85 GHz).

When it comes to W-band, vendors wage a battle. Those that offer W-band radar devices have rainbows and kittens in their sales pitches. And companies who don’t will throw tomes of technical explanations at you about why you need to stick to the other bands. We’ll go through this topic later.

Beam angle and spread

Your beam angle and spread depend on the antenna and frequency of your transmitter. Low-frequency radars with standard antennas have beam angles around 20 degrees. For example, the SITRANS LR200 works with 6 GHz. A 4-inch antenna creates a beam angle of 29 degrees, and an 8-inch antenna has a beam angle of 17 degrees.

Beam Angle Time of Flight
Courtesy of Endress+Hauser

High-frequency transmitters can have lower beam angles, even with small antennas. For example, the VEGAPULS 62 works on 26 GHz, so you can have a beam angle between 8 and 22 degrees. You can get small angles with low-frequency radars, but then you’ll need bigger antennas, sometimes too big for common processes.

But why small angles? So that the radar can measure in low tanks, or tanks with interference, or even close to the tank wall. VEGA has a terrific ad showing a device measuring the level of a water bottle. We’d love to replicate this, so VEGA, please send us a demo! 🙂

Vega Radar Water Bottle
Courtesy of VEGA

Endress+Hauser also offers high-gigahertz technology. The reps say 113 GHz, but that’s just a sum of the frequencies, so don’t get too impressed.

Low or high frequency?

Here’s where the battle begins. Emerson makes a point – three of them, in fact- of telling you why high-frequency devices can give you more headaches than low. On the other side, VEGA and Endress+Hauser sell positive messages for high-frequency radar.

To begin with, in the documents about level measurement from Emerson, it says that high-frequency devices have problems with interference, such as vapor, foam, or buildup. Later, Emerson states that devices with beam angles smaller than 4 degrees can have problems with antenna misalignment and signal loss. Finally, apparently waves and ripples increase signal loss and risk your accurate and reliable level measurement.

VEGA and Endress+Hauser have countered these arguments with videos and recommendations for stilling wells and bypasses. Go through Emerson’s documents carefully, and you’ll notice a decided lack of these options.

PART 3: Guided wave radars

Guided wave radars resemble free space radars in that they use the Time of Flight principle and have particular frequencies. But a guided wave radar has a probe which guides the wave produced by the radar. So, when do you use which?

If non-contact technology fits your process, then you should consider going that route. However, process conditions such as high turbulence or low dielectric constants can benefit from, or even require, guided wave radars.

Guided wave radars go at the top of the tank, and their probes extend into the tank to make contact with the product inside. The radar sends a microwave signal down the probe to the product surface. When the microwave hits the surface, the signal bounces back to the device. Then the transmitter can calculate that distance using this formula:

Distance = (speed of light x time delay)/2

And how can the radar calculate the level with the distance? Simple. You set up the transmitter with the minimum and maximum heights of your application. Then the radar can calculate the level as well.

Probe selection

You’ll have to choose an appropriate probe for your guided wave radar. Some probes have specific characteristics that can limit the guided wave radar, such as mounting restrictions or length. Speaking of mounting, you also need to make sure that the probe doesn’t touch metal, including the tank wall. And if you have high turbulence, then you’ll need to anchor the sensor.

There are three main probe types in guided wave radars – single element, twin element, and coaxial. Which will work better for you? Vendors usually provide charts or tables to help you pick the right probe for your application, but we’ll give you a quick overview.

guided radar
Courtesy of SMAR

Coaxial probes cover a vast array of applications, including those with dielectric constants in their products. Twin element probes are a good choice for long measurement range, and you can get either a flexible or rigid one. In viscous or sticky products, you should probably go for a single element probe.

Interface measurement

When you have to measure the interface of two products, such as oil and water, you can use a guided wave radar. For this application to work, you need products with different dielectric constants, of course.

When the radar signal reaches the surface with the lower dielectric constant, part of the signal returns to the transmitter. When the signal reaches the product with the higher dielectric constant, more of the signal returns. If you set it up properly for these conditions, you can calculate the level of each product.

Interface measurement time of flight
Courtesy of ISA

PART 4: Ultrasonic level transmitters

Ultrasonic level transmitters also use the time of flight principle, only with a different type of signal. Where radars use electromechanical waves, ultrasonics use mechanical waves that travel at the speed of sound to find the distance between the product’s surface and the sensor.

ultrasonic level transmitters
Courtesy of ABB

Just like the radar, the ultrasonic uses the distance between the sensor and the product surface to calculate the level. However, the two types of waves travel at different speeds. For ultrasonic sensors, use the following formula:

Distance = (Speed of sound x time delay)/2

The transmitter also needs the distance to the bottom of your tank. Then it can calculate the tank height minus the distance from the product’s surface. And remember the dead band! All transmitters have a short unmeasured range from the sensor to the medium. Therefore, you need to determine your measurement range starting after that dead band.

Ultrasonics, like anything else, have advantages and disadvantages. One advantage is definitely the easy setup. Moreover, it works in applications with density, dielectric, or viscosity changes.

On the other hand, turbulence, foam, steam, and vapor can cause problems. Also, if you have objects such as agitators in your tank, they can interfere. And if you have a vacuum, then it won’t work there, either.  Yes, the Star Wars movies would have you believe sound travels in the vacuum of space, but no.

Star Wars Time of Flight
Courtesy of Giphy

Now let’s talk about applications. We all know level measurement, but did you know that an ultrasonic transmitter can also measure flow? If you combine it with a Parshall flume, you can find the volumetric flow through the difference of level. Cool, isn’t it? We can talk about it more in another article if you’re interested.

PART 5: Laser level transmitters

Yes, let’s talk about lasers now! We all like to play with lasers, don’t we? For level measurement, however, I’m not a big fan. If you like them, then tell me why in the comments. I only used lasers to find the distance between an installed radar and the product surface, to prove that the radar worked properly.

level transmitters
Courtesy of Sensors Mag

Laser level transmitters work similarly to ultrasonic transmitters, but lasers work with the speed of light instead of sound. A laser provides a fairly simple solution, and you can use it to measure liquids or solids.

Unlike the non-contact radars, the dielectric constant of your product doesn’t affect laser measurement. Lasers also don’t have the propagation velocity issues in vapor like ultrasonic transmitters. However, if you have fine particles in your tank’s atmosphere, laser devices won’t suit. The particles will interfere with your signal and therefore your readings.

You can probably think of thousands of applications for lasers in daily life, but when it comes to process automation, most of the time they stick to level measurement. If you do something different with your lasers, let us know in the comments! I’d love to hear about a new approach for lasers in instrumentation.

PART 6: 9 free space radar sensors you must know!

So now that we know all about Time of Flight devices, let’s check out these examples of good free space radar sensors. They’re listed in no special order, and there isn’t any relation between them.


The VEGAPULS 64 works with a frequency of 80 GHz, which means you can fit it in tanks with mixers inside. The smaller beam angle reduces problems with interference.

Courtesy of VEGA

You can use the VEGAPULS 64 to measure levels up to 30 meters. Moreover, it supports temperatures from -40 to 200 degrees Celsius and a pressure of -1 to 20 bar. For harsh environments, the device comes with different housing materials, wetted materials, and most of the certifications you might need.

The website is okay, and they have landing pages with plenty of details about this sort of free space radar sensor. You can read more about the VEGAPULS 64 here.

Siemens – SITRANS LR560

The SITRANS LR560 works with a frequency of 78 GHz instead of the usual 80 GHz. Why? No idea. You may think that 78 GHz is worse than 80, but it’s not. Just a bad marketing choice, that’s all.

The SITRANS LR560 is a niche device when it comes to solid level measurement. It has a measurement range up to 100 meters and can work in process temperatures from -40 to 200 degrees Celsius and a pressure range up to 3 bars.

Courtesy of Siemens

The SITRANS LR560 also has a narrow signal, allowing you to install it in tanks with interference inside. Moreover, you have different wetted parts and housing materials, along with different ways to integrate the device into your control system.

The website and documentation definitely need work, but if you dig hard enough, you should find what you need. Check out the SITRANS LR560 here.

Emerson Automation Solutions – Rosemount 5400

Here comes the low-frequency crew. As mentioned before, whoever writes the documentation at Emerson must hate high-frequency radars. The Rosemount 5400 can work in either 6 or 26 GHz. It has a measurement range up to 35 meters and can also handle harsh environments, such as process temperatures from -40 to 150 degrees Celsius and pressure ranges up to 16 bars.

free space radar
Courtesy of Emerson Automation Solutions

When it comes to housing materials and wetted parts, you have a couple of options, pretty much the basic requirements on the market.

Unlike other vendors, Emerson has a really good website and documentation. They even have blogs and other sorts of media to give you information on how to apply the device. You can visit Emerson’s site here.


Contrary to Emerson’s stance, KROHNE believes in the high-frequency free space radar sensors. The OPTIWAVE 7500 C has a frequency band of 80 GHz and small beam angles, allowing for implementation in many kinds of tanks.

The 7500 C has a measurement range up to 100 meters and supports temperatures up to 150 degrees Celsius. It can also support pressures up to 40 bars. Like most market options, you can also use the 7500 C in different liquid applications with its different housing materials and wetted parts. On the other hand, it’s very short on protocol options, just HART and nothing else.

free space radar
Courtesy of KROHNE

The website and documentation provide just as low a level of user experience as the rest of the pack. You may eventually get the hang of it on your laptop, although you can forget about your phone or tablet. Read up on the OPTIWAVE 7500 C here.

Honeywell – SmartLine RM Series

The Honeywell SmartLine RM series can fit in a measurement range up to 80 meters with a reference accuracy around +-3 millimeters. Not the best, but not that bad. It also works in K-band, which means the frequency is around 24 to 26 GHz.

Just like most devices in this list, you have different options for housing materials and wetted parts. It can handle pressures up to 40 bars and process temperatures up to 200 degrees Celsius.

free space radar
Courtesy of Honeywell

The Honeywell site is actually okay, although the documentation falls to the level of the others. Honeywell can tell you more here.

Magnetrol – PULSAR R86

The PULSAR R86 also works in K-band, more precisely 26 GHz. It can measure up to 40 meters in process temperatures up to 400 degrees Celsius and pressures up to 160 bars. Compared with the others, it supports a pretty wide range of temperatures and pressures.

It has a fancy – or just different – design which separates the cable connections and access to the display.

free space radar
Courtesy of Magnetrol

The PULSAR R86 also has housing materials, wetted parts, and certifications to fit in different environments. However, it only has HART and FOUNDATION Fieldbus for protocols. If you want PROFIBUS, then keep looking or buy a converter.

The website and documentation aren’t great, but you can find most of what you need. Learn more about the R86 here.

Valcom – KRG Series

The KRG also works at 26 GHz. It has a measurement range up to 30 meters, and you can apply it in process temperatures from -40 to 150 degrees Celsius and pressures up to 150 bars. It lacks housing materials, wetted parts, and digital protocols, though. Pretty bare bones, but it may fit your application and budget, if you want to scale it out.

free space radar
Courtesy of Valcom

The KRG has a reference accuracy of +-2 millimeters, which is decent. The website and documentation are really difficult to manage, though. Valcom needs to improve in this area as soon as possible. If you want to read more about this device, then click here at your own risk!

Endress+Hauser – Micropilot FMR62

The newest free space radar sensor from Endress+Hauser works with a frequency band of 80 GHz. The FMR62 has a measurement range up to 80 meters and supports process temperatures from -40 to 200 degrees Celsius and pressures up to 25 bars. Check out the reference accuracy –   around 1 millimeter! And it comes in different housing materials and wetted parts.

free space radar
Courtesy of Endress+Hauser

Endress+Hauser’s website doesn’t provide a good user experience on a laptop. The same goes for zooming in and out on a phone or tablet. Yeah, no. But you can find a nice microsite there with information about the company’s level portfolio. The documentation is average, so you’ll find most of your answers. If you want to learn more about the Micropilot FMR62, then click here.

Monitor Technologies LLC – Series 200

Last but not least, we have the guys from Monitor Technologies! Who are they? Dunno, but we’re gonna find out. This could be a good low-cost option. They popped up during my search, so I read up on the device and the company.

The Series 200 can measure up to 30 meters and has an accuracy of +-3 millimeters. It supports a maximum process temperature of 130 degrees Celsius and pressure of 10 bars.

free space radar
Courtesy of Monitor Technologies

The website looks straight out of the 90s, and it’s really hard to find info on it. At least they have an “Ask Monitor” button where you can request more deets. This button should’ve been called “Help me!” You can check out the new kid on the block here.


Time of flight is a widely used and reliable method of level measurement. However, many people still don’t know the technology in depth and can come across issues while setting up. At Visaya, we share our knowledge so that everyone can make the most of the technology in their applications.

And watch Germán demonstrate time of flight with a squash ball in this Visaya Weekly Episode:

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