Level Measurement Selection Considerations

ControlGlobal.com hosted a recent whitepaper, Guided Wave Radar vs. Differential Pressure Transmitters for Liquid Level Measurement. The paper was developed by an automation supplier, which offered guided wave radar level measurement devices. The message seemed to be that the newer technology, Guided Wave Radar, could displace much of the more established, differential pressure level measurement applications.

Since Emerson offers both differential pressure (DP) and Guided Wave Radar (GWR) technologies, I turned to Emerson’s Lori Egbers, a Rosemount level measurement application specialist for her thoughts. She shared some background information that looks at the various level measurement technologies and their best application fits.

The choice of level measurement technologies is vast. This list includes both manual and continuous measurement methods such as DP, tuning fork, bubbler, dipstick, capacitance, ultrasonic, radar, sight glass, nuclear, displacer, gage glass, ultrasonic gap, hydrostatic, and weight. Obviously, for so many level measurement technologies still to be in use, it stands to reason that they have advantages and limitations in application.

One area of consideration is the application condition. Some of these conditions include levels of steam, vapors, dust, liquid density changes, pressure changes, liquid viscosity, abrasiveness, foaminess, corrosiveness, temperature range, and amount of agitation. Another area of consideration is the physical installation. Agitators, blades, baffles, and other obstructions in the vessel where level is being measured will determine the subset of available technologies.

Lori notes that both DP and GWR level measurements are considered contacting since they “touch” the process. Other level measurement technologies in this category include capacitance, magnetostrictive, and displacer. Examples of non-contacting level measurement devices include radar, ultrasonic, laser, and nuclear.

A ControlGlobal.com article, Beginner’s Guide to Differential Pressure Level Transmitters offers a great description on how DP level measurement works:

Differential pressure level measurement technology infers liquid level by measuring the pressure generated by the liquid in the vessel. For example, a water level that is 1000 millimeters above the centerline of a differential pressure transmitter diaphragm will generate a pressure of 1000 millimeters of water column (1000 mmWC) at the diaphragm. Similarly, a level of 500 millimeters will generate 500 mmWC. Calibrating [ differential pressure transmitter for 0 to 1000 mmWC will allow it to measure water levels of 0 to 1000 millimeters.

The advantages of differential pressure in measuring levels are that it’s reliable (no moving parts), simple, and well understood. Its use is flexible in measuring level, density, liquid interface, and mass. It can be applied in vessels with agitation and foam. DP measurement can be used in vessels with internal physical obstacles. Diaphragm seals extend DP measurement to applications with high temperatures, corrosive liquids, dirty or viscous liquids, or sanitary considerations.

DP level limiting factors include measurement errors from changing density, temperatures beyond 600degF, trickiness of high-vacuum applications, highly corrosive processes, highly abrasive processes (potentially damaging diaphragms), and liquid-only measurement. From a physical installation standpoint, the bottom-mount technology is a potential source of leakage and DP measurement often requires two taps.

How the Guided Wave Radar technology works is:

…based on Time Domain Reflectometry (TDR) principles. Low power nano-second-pulses are guided along an immersed probe. When a pulse reaches the surface, part of the energy is reflected back to the transmitter, and the time difference between the generated and reflected pulse is converted into a distance, which calculates the total level or interface level…

Lori shares many GWR application advantages including direct measurement of solids and liquids, effective interface measurement, no moving parts, and no impact to changing densities/dielectrics/conductivities. It’s tolerant of foam, turbulence, heavy vapors, and condensation. From an installation standpoint, it’s a better fit for small tanks, tanks with difficult geometries, small connection sizes (< 2″), and threaded nozzles. Finally, GWR is proving to be an easy replacement technology for displacer and capacitance level devices as it is very easy to swap by using the same connections. GWR is better than non-contacting radar measurement technologies for applications with heavy vapors, such as anhydrous ammonia.

GWR is limited from applications with agitators, blades, and baffles which may interfere with the probe placement. Coating on the probe from very sticky fluids may affect the measurements. If the level has an emulsion instead of clearly defined interfaces between separated liquids, the readings may not be accurate or will require more care in configuration. Finally, for taller (>50 ft) solid measurement applications, the height and associated pull forces need to be evaluated to determine suitability of GWR measurement.

I know that these are a lot of things to consider. Clearly, there isn’t one specific level technology, which is appropriate in every tank. It’s important to carefully consider tank and process conditions to ensure a good technology match. To help you select the right level measurement technology, Lori pointed to a Level Selection Guide. It shares the advantages and limitations of these and other level measurement technologies to help you find the best ones for your application.

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Update: A reader pointed me to a broken link on the Level Selection Guide which is now fixed. I appreciate that help!

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