As with anything, DP transmitters have advantages and disadvantages. For instance, an 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 the article explains it and the various devices that use it.

Breaking down the concept

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?

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.

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.

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.

To know more about ultrasonic level measurement, you can check out the Visaya Product Reviews on Ecometer, Ecometer S and Ecometer P

Radar level measurement

Radar devices are the stars of level measurement. And guess what? Yep, they work with ToF too. They generate electromagnetic waves called microwaves.

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!

EcoMeter S Ultrasonic water cistern level sensor with separate digital display
in the shop from 123 €

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.

EcoMeter P Ultrasonic Level Sensor with integrated radio module and external power supply
in the shop from 139 €


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.

You can also find out more about level measurement here:

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