Understanding Level Readings & the Truth about Level Measurement Instruments


In order to understand level readings, you must first comprehend how the instrument works. Three of the most common level-measuring techniques involve using a displacer, float, or differential pressure instrument.

Here’s the catch.

While each of these instruments can be used to report a level reading, none of them actually measure level.

I know what you’re thinking…

If none of these instruments measure level, how do we end up with a level reading?

Here’s how it works:

A displacer uses a solid body attached to a torque gauge to measure buoyant force. As liquid rises inside a vessel, the displaced liquid creates a force that is converted to a level reading using the density of the displacer body and the density of the liquid inside the vessel.

Similarly, a differential pressure instrument uses assumed liquid density to determine a level measurement. A DP instrument measures the pressure differential between two points and reports the difference as a calculated level measurement.

A float, on the other hand, works entirely different. A float sits on the top of a liquid, moving as the level of the liquid changes. A float’s body must have a density between that of the light and heavy phases of the liquid in the vessel. If the float is too dense, the float will sink, producing a minimum level reading. Likewise, if a float is not dense enough, it will jump to the top, reporting a maximum value.

The location of the float can be determined by using a mechanical gear or magneto restrictive measurement system.  Level readings from all three devices are typically expressed as a height or a percent of range.

Now that we understand how these instruments work, let’s talk about how they respond to unusual conditions, so you can better interpret readings.

What happens when a level is below/above the range?

When a level is below the range, a displacer, float and DP instrument will all give similar results. When the level is below the range, all of these instruments will sit at a minimum value, regardless of how far below the range the true level falls.

When a level is above the range, a displacer and DP instrument will measure density and report the density difference as a liquid level. A float in this scenario, will read a maximum value.

What about when the density is lower/higher than expected?

The following chart will help you understand what happens to level readings in each of these scenarios, using the various measurement techniques.

Situation Technology
Displacer & Differential Pressure Float
Level Below Range Sits at minimum value
Level Above Range Measures average density, but reports a level reading Sits at maximum value
Density Lower Than Assumed Level reported is lower than actual Level accurate unless float density is too high- if so, goes to minimum value
Density Higher than Assumed Level reported is higher than actual Level accurate unless float density is too low- if so, float will report a maximum value

In general, a displacer and DP instrument will behave similarly, since both instruments rely on measuring force to report level.

Something else to keep in mind is that a displacer and float both sit in the liquid, causing the instruments to sometimes be mistaken for one another. As you have learned from this post, a displacer and float behave differently. Be careful not to make this mistake as it can lead to an improper interpretation of level readings. And in extreme cases, consequences can be catastrophic.

Lesman Instrument Company provides level instrumentation to meet your application needs. We can provide you with everything from the simplest float-type mechanical switch to radar level transmitters that work in tanks obstructed by ladders, mixers, and baffles. Recent additions to our level catalog include WIKA’s magnetic level indicators and Honeywell Enraf servo gauges and radar tank gauges for custody transfer and storage applications.

To see our complete line of level instrumentation products, visit: http://www.lesman.com/unleashd/level.htm

Lesman is the Midwest US authorized representative for Siemens (Milltronics) range of technologies in level instrumentation. Capacitance, radar, and ultrasonic technologies for point level switching and continuous level control. Not sure which will fit your process best? Lesman can help. Just download a level application datasheet and fax it to us. We’ll review your information and help you find the perfect instrumentation to fit.

To see our complete line of Siemens products, visit: http://www.lesman.com/unleashd/vendors/siemens.htm

Questions? Call (800) 953-7626

Source: http://www.chemicalprocessing.com/articles/2015/interpret-level-readings-right/

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