Time for a comparison between two popular methods of non-contact level measurement: laser level transmitters vs ultrasonic level transmitters. Read on to find out how they work and when to use which one.
Ultrasonic level transmitter
Ultrasonic measurement is based on the time-of-flight principle. A sensor emits ultrasonic pulses which the surface of the medium reflects and the sensor detects again. The required time of flight is a measure for the distance traveled in the empty part of the tank. This value is deducted from the overall height of the tank to yield the level.
Radar level transmitters work with high-frequency radar pulses which are emitted by an antenna and reflected from the product surface. The time of flight of the reflected radar pulse is directly proportionate to the distance traveled. If the tank geometry is known, the level can be calculated from this variable.
Radar uses electromechanical waves, but the ultrasonic sends mechanical waves that travel at the speed of sound to find the distance between the sensor and the product surface.
Guided Radar Level transmitter:
Guided Radar level transmitters work with high-frequency radar pulses which are guided along a probe. As the pluses impact the medium surface, the characteristic impedance changes and part of the emitted pulse is reflected. The time between pulse launching and receiving is measured and analysed by the instrument and constitutes a direct measure for the distance between the process connection and the product surface.
And like the radar, the ultrasonic will do all your level calculations for you if you give it the data it needs. It’s almost magic! Here’s the formula:
Distance = (Speed of sound x time delay)/2
Once you tell the transmitter the distance to the bottom of the tank, then the transmitter can calculate the tank height minus the distance from the product surface.
And let’s not forget the dead band. All transmitters have a short unmeasured range from the sensor to the medium. So you need to determine your measurement range starting after that dead band.
Pros, cons, and applications
Like anything else, the ultrasonic level transmitter has advantages and disadvantages. While other devices can get complex in their setups, the ultrasonic makes setup easy. Furthermore, if you have density, dielectric, or viscosity changes, it works despite those changes.
However, foam, turbulence, steam, and vapors can cause problems. And if you have a vacuum, then you can’t use it there either. Yes, the Star Wars movies would have you believe sound travels in the vacuum of space, but no. Also, if you have objects such as agitators in your tank, they can interfere.
Now, let’s talk applications. We all know about its use for level measurement. However, you can also use an ultrasonic to measure flow. In fact, if you combine it with a Parshall flume, you can find the volumetric flow through the difference of the levels!
Laser level transmitter
Y’all love to play with lasers, don’t you? I do, too. Still, I’m not a big fan of lasers for level measurement. 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 much like ultrasonic transmitters, only lasers work with the speed of light instead of the speed of sound. Lasers provide a fairly simple solution, and you can use them to measure the levels of liquids or solids.
Pros, cons, and applications
Unlike with non-contact radars, the dielectric constant of your product will have no effect on laser measurement. Lasers also shrug off the propagation velocity issues in vapor that ultrasonics can have.
However, if you have fine particles in your tank’s atmosphere, then you should probably use something other than a laser device. The particles will interfere with your signal and thus your readings.
Depending on your process, an ultrasonic or laser level transmitter may fit perfectly in your application. Find out what kind of performance and accuracy you can get by scaling out these devices using your process data.
To know more about such products, you can Ask Alex!