Ultrasonic and laser level transmitters
Hey! Everyone having a good week? We’re close to the end of this month’s journey through the world of level measurement. We still have many level topics left that we’ll discuss later, but you should have a good base of information now to work with.
We talked about time of flight (ToF) and the devices that use the concept. You learned about non-contact radar level transmitters and their pros and cons. In that article, we also talked about frequency bands and how to decide which frequencies will work for your process. As we dug deeper into radar level transmitters, we hit guided wave radar and went over its principle, installation, challenges, accuracy, and more.
Today, we want to chat about two more level transmitter types. First, we’ll discuss the ultrasonic level transmitter and explain its principle and applications. You may know that it measures level, but did you know you can use it for flow?
Second, we’ll talk about laser level transmitters. Frankly, I never had much experience with this sort of device in the field. I remember a colleague who liked to use a portable laser to read distances. Are they better than non-contact radar transmitters? Let’s discuss.
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Ultrasonic level transmitters
For one of my first field services, I had to set up a Siemens ultrasonic level transmitter. I still remember how concerned I was during the trip to the site. I read the technical manual a couple of times because I worried that I’d have a problem installing and setting it up. The device had a simple local configuration and nothing more. In the end, it all worked fine and I could chill until the next service.
The ultrasonic level transmitter works with the same ToF concept as the radar transmitter, but they differ in the signals they use. 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.
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 uses
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.
So far, so good? 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! Cool, huh? We can talk more about it in another article, if you’re interested. Just let us know.
Laser level transmitters
Yes, let’s talk about lasers now! 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. If you like them, then feel free to 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 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 uses
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.
Lasers show up in a zillion applications in daily life. But in 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.
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.