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Locating Levels in Tanks

The liquid/air interface of an egg is usually obvious. Take a look and you’ll see!

Thermographers are often surprised by what we find and no wonder because we are looking at the world as it has never been seen by the average person! I can still remember the first time, poking around the kitchen with an imaging system (a great place to learn by the way), I happened to take a look at a carton of eggs. The cause of the remarkable thermal pattern seemed obvious but only after some “further testing” was I willing to say it was due to the liquid/gas interface formed by the membrane inside the egg. For the level to be revealed, the egg had to go through a temperature change—in either direction—and the air, with its lower thermal capacitance, always changed more quickly than the liquid.

As is the case with many things we learn in life, the “lesson” was not obvious at first—it simply seemed like something cool! Later, however, I realized that

Industry relies on tanks of all sizes to store materials of all kinds. In all cases you must either wait for or push the tank through a temperature change that causes the levels to reveal themselves.

the egg was basically a small “tank” that was nearly full of fluid, not unlike many I see in the industrial complexes where we often teach. We began to experiment with “big eggs” or real tanks and found they behaved in a similar fashion to the kitchen variety eggs—it was quite easy to locate the liquid levels even in huge tanks if conditions were good.

With more experience and paying more attention to the conditions, we found tanks that were out-of-doors had predictable patterns. Large tanks were warm on top by mid-day and cooler by early evening. The liquid, with its greater thermal capacitance, stayed nearly the same temperature night and day. Sludge levels were often apparent as well, typically showing an irregular shape. Exactly when they would distinguish themselves from the liquid depended on certain conditions and the type/density of the sludge. Even “floaters” like waxes or foams can often be seen—again with the right conditions and a temperature swing. Silos and insulated tanks are also “fair game” even if their exact thermal characteristics may be a bit different.

Sludge, liquid & gas are all visible in this thermal image of a large industrial tank. Exactly what the levels will look like relative to each other depends on where each is in its temperature transition.

If you’ve not looked at tanks, go find some and watch them over time. If you are lucky, you may even see them when no temperature difference can be detected at all! If that is the case, the tank could be completely full or empty, or it could be at the point where there is little difference between the liquid and the gas. You can either wait for a change in temperature or you can force that change to happen. We’ve had good luck simply spraying the side of a large tank with water and waiting for evaporative cooling to work its magic. In one instance, I also used an industrial heat gun to force the change with excellent results.

I honestly don’t care much about thermal images of eggs—other than it was a fun way to learn—but I have seen time and time again how lucrative it can be to locate or verify the level in a large tank or silo. Even if the existing level indicators are working accurately, process operators and industrial engineers like to validate their readings with a second methodology and thermography has proven to be a very easy and practical way to do that. Enjoy learning more about this important application.

Thinking Thermally,

John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner

2 comments to Locating Levels in Tanks

  • This is exactly the application that I am looking for a solution to. I would like to know what Infrared camera was used to produce the images on this tank?
    Thank you,

  • DJ

    Nicely picked details, a lot of thanks towards the author. It’s incomprehensive in my experience at present, nevertheless in common, the convenience and importance is mind-boggling. Regards and all of the best ..

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