The nitty gritty on displacer level transmitters
Last leveling of the month! Yup, we’re taking a break from level topics on Visaya. We’ll talk about them again soon enough, but we can’t move to a new topic without talking about the displacer level transmitter!
You can find plenty of displacers still working in the field, especially in chemical or oil and gas companies, even though some vendors want to change them to radar devices. Of course, if it works for you, then why change, right? Well, let’s learn more and find out if you should change.
Frankly, I’ve never had a good experience with them. I used to diagnose, repair, and calibrate them, and most of the time their problems were hard to find, much less solve. I still remember the standard weight to check the device response and set up the zero trim and all the structure necessary to test it. Amazing what sticks with you.
Anyway, let’s dig in and see what comes up!
Displacer level transmitters use Archimedes’ principle, or the physical law of buoyancy, to measure levels. Time to hop in the way back machine and revisit high school physics! Archimedes’ principle states that fluid exerts an upward force on a body immersed in it, whether partially or completely submerged. This force will equal the weight of the fluid the body relocated upward at the center of mass of the relocated liquid.
Displacers come in different sizes, but they all have a cylindrical shape and regular cross section. Usually, you’ll install it in a bypass and connect it to a spring to limit its movements. If the tank level increases or decreases, then the displacer rod experiences more or less buoyant force. The transmitter will read that force, translate it to a level measurement, and send that data to your control system.
You can apply a displacer to a level or interface measurement, although it can become an expensive solution with a span higher than 12 meters. Furthermore, new technologies bring a bunch of advantages that displacers lack.
For example, non-contact radars have no moving parts and no contact with the product, which makes maintenance much easier. A displacer’s moving parts create many headaches in the field, so a device without moving parts has a lot of appeals.
Now, if you stay on top of your maintenance and calibration, then you’ll have a reliable and accurate device for a long time. However, reality makes this kind of dedication hard, which limits this technology.
And of course, if you want to use a displacer, you’ll need to scale it out. Yep, no standard, so you need to calculate the diameter for the greatest possible force across the measurement range.
So obviously I believe we have better options on the market today. But maybe a displacer fits your process perfectly. Or maybe you don’t want to take the time to learn the new solutions. Up to you. Just make sure you scale out to find out if it’ll work for you.
This video has more info about displacer calibration: