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How to Measure Liquid Level in Vessels

When plants are first automating a tank level measurement that has long been a manual or sight-glass operation, they can be overwhelmed with the number of techniques available for doing such a deceptively simple job. Some basic analysis can unpack the selection process.

 One of the first questions is often whether to measure from the bottom up or top down. This makes me think of an old joke from my grandfather’s generation. Carpenter: “Boss, should I should build a house from the bottom up or top down?” Foreman: “Why from the bottom up, naturally.” Carpenter: “Nuts, now I need to tear it down and start over.”

The whole question of top down or bottom up appears time and time again when talking to users about all sorts of tanks and applications. There are also questions as to how detailed a level measurement must be to satisfy the needs of a given process. This analysis and a few suggested solutions are the primary topics of my article in the September/October 2019 issue of InTech, How to Measure Liquid Level in Vessels. Naturally, data requirements are the primary determining factor.

Sorting through the instrument selection process begins by determining what data is needed for the application and how it can be obtained as simply as possible. Level measurements are generally concerned with these points: Level of liquid-How close is the vessel to being full or empty? Volume of liquid-How many liters or gallons is in the vessel? Level has reached a high limit-Will the vessel overflow? Level has reached a low limit-Will the vessel run dry?

 Providing data to answer those four questions does not necessarily require four different instruments. In fact, one instrument, if selected carefully and integrated into the process control system, can do everything.

Frequently, all these readings may be used with the continuous measurements provided to control room operators, while the high- and low-limit measurements are tied to alarms to avoid either extreme. Both continuous and point measurements can be used in safety-instrumented functions to prevent overfill, typically tied into a separate control system specifically for safety.

This is where things get more complicated. How many process systems need the level measurement? What kind of measurement do they need? What does the system need to do as a result of the measurement? Is it for inventory? Safety? The article goes into more detail, so give it a complete reading. For our purposes here, let’s concentrate on continuous readings, which brings us back to the bottom versus top question.

The bottom approach uses one technology for all practical purposes: static pressure. A pressure instrument reads through a penetration in the vessel wall and registers pressure created by the weight of the liquid. If the vessel contains water and is vented to atmosphere, a pressure reading of 4.34 pounds per square inch indicates there is 10 feet of water above the instrument.

Keep in mind that the bottom-up approach sounds easy but can be more complicated in the real world due to variables such as changing liquid densities and problems associated with pressurized tanks. These can all be overcome of course with a few practical products, including Emerson’s Rosemount 3051S Electronic Remote Sensor (ERS) System. So how about the other choice?

Over the past decade or so, radar level measurement options in particular have increased, because of improvements in cost and their ability to measure easily in many conditions. For all radar options, the common denominator is bouncing a microwave radar signal off the liquid surface and measuring the time necessary for it to go down and come back to a sensor. This can be accomplished by measuring time of flight for a microwave pulse, or the degree of frequency shift with a frequency-modulated continuous wave (FMCW) signal. In any case, top-down techniques determine the distance from the instrument to the liquid surface.

So switching to the top-down approach often means radar, and a prime choice is Emerson’s Rosemount 5408 Level Transmitter. This instrument uses FMCW technology combined with highly sophisticated software for accurate and reliable measurements. Whichever approach is selected, there is no shortage of choices.

You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Level Group and other specialty areas for suggestions and answers.


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