Time-of-Flight (ToF) is one of the measuring principles applicable to level, just like differential pressure (DP) transmitters. However, the similarities end there. This article will explain the working principle of Time-of-Flight sensors and discuss the various types of these devices commonly used for level measurement in tanks.
The working principle of a Time-of-Flight sensor
Time-of-Flight (ToF) refers to the time it takes for acoustic or electromagnetic waves to travel through a medium from Point A to Point B and return. Time-of-flight sensors measure this time and use it to determine the distance of the object reflecting the waves, which can be used to measure level if the time-of-flight sensor is located on top of a container and pointed down, at the contents.
If you ever watched a Discovery Channel program that talked about how bats and cetaceans locate their prey, then you have heard of how they use ultrasound waves for echolocation. Technically speaking, these animals also use a kind of time-of-flight sensor.
If you’ve never seen such a show, then here’s the gist: to locate their prey, bats and cetaceans send out high-pitched ultrasonic waves, inaudible to humans. These waves bounce off the prey and travel back to the bat, telling the bat the size and location of the prey and even the direction it’s heading.
Like bats and cetaceans, Time-of-Flight sensors do the same using electromagnetic or mechanical waves instead of ultrasound and a software to calculate the properties of the object the waves bounce off.
Now that we know how Time-of-Flight works, let’s move on to the devices that use it.
Types of Time-of-Flight sensors
Non-contact or free space 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 on the top of the tank, pointing down, and the waves fly freely through the tank, with no guided path. They use different frequencies and different antennas, both of which will change your beam angle and spread.
Different Time-of-Flight 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.
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.
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! 🙂
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.
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.
Selecting a probe for a guided wave radar
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.
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.
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.
Ultrasonic level transmitters
Ultrasonic level transmitters also use a time-of-flight sensor, 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.
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.
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.
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.
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.
Time-of-flight sensors are 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.
To know more about such products, you can get in touch with our engineers!