Don’t Touch That Process!

AT A GLANCE

5 dominant technologies

Solid and liquid measurement

Cost misconceptions

Ultrasonic technology gaining acceptance

Early adopters of non-contact level technologies were often disappointed with the installed results. However, significant technology advancements and installation know-how acquired by manufacturers and end-users over the past several years brush away past non-contact level measurement disappointments.

Today, more than one technology can likely be successfully applied to straightforward level measurement applications. However, for those level measurement applications with special needs and operating environments, understanding the nuances of available technologies and closely following manufacturer installation guidelines are key to a successful installation.

A high-speed counter measures the time-of-flight for a pulsed laser to reflect off the substrate and return to the receiver.

Load cells have many forms. The majority use Wheatstone bridge circuits as the sensing element.

Currently, five technologies dominate the continuous non-contact level measurement arena: laser, load cells, microwave (radar), nuclear, and ultrasonic.

Laser

Laser technology works on the principle that light travels in well-defined and measurable ways.

Laser level instruments generate an optical pulse that is reflected by the liquid or solid material. A high-speed counter measures the time-of-flight from when an optical pulse is transmitted until its reflection is detected by the receiver. The time measurement is converted to a distance using the formula Distance = (Speed of light X Time of flight)/2. (See “Pulsed laser” diagram.)

The frequency differential from what is transmitted to what bounces back is directly proportional to the distance traveled.

Level is determined by how much of a defined strength gamma ray reaches the receiver.

Unaffected by agitation, off-gas layers, vapor pressure, vacuum, high temperature, acoustically absorbing materials, material with low dielectric constant, and falling polymer makes pulsed laser level technology suitable for a broad range of liquid and solid level measurement applications. Also, its small footprint allows installation in tight locations including hopper cones, angles, and through narrow openings.

To demonstrate pulsed laser’s diversity, level measurements have been successfully applied in areas classified as Class I Division I, with foam and agitator blades present.

Compared to some non-contact level measurement technology, pulsed laser instruments are generally more expensive. Because the technology relies on the reflection of light, anything that intermittently interferes with the line-of-sight, including some types and quantities of dust, can adversely influence performance.

Load cells

Load or force cells take many forms, including “S” and “Z,” bending, and shear beam. The majority of today’s industrial application designs use foil and semiconductor strain gauges as the sensing element. (See “Load cells” diagram.)

High-energy (40 – 60 KHz) transducers provide a level reading based on the single strongest echo.

Low-energy (20 – 30 KHz) transducers provide a statistical level reading based on the strength of multiple echos in solids measurement.

Foil gauges offer the largest number of choices, thus they are most used in tension, compression, and shear force load cell designs.

Though semiconductor strain gauges come in a smaller range of patterns, their small size and larger outputs per given stress input offer some advantages over foil gauge designs.

Because load cells are suitable for measuring liquids, solids, and slurries, they are frequently the technology of choice in laboratories, pilot plants, and other controlled environments. When applied in large production operations, load cells require extra attention to detail during design, installation, operation, and maintenance to achieve the required performance. For example, vessel agitators may need to be turned off followed by a vessel settling time to achieve accurate level measurements.

When three or more load cells are installed on a vessel, measurement systems are available that can detect and compensate for sensor degradation and/or failure, thus keeping a vessel online while failed load cells are repaired.

Microwave

Microwave (radar) technology uses continuous or pulsed (modulated) signals to determine level measurements. Though different manufacturers promote one signal type over the other, both will produce similar performance results, however continuous signaling uses considerably more power to operate—something to think about when calculating total cost of ownership. (See “Pulsed microwave (radar)” diagram.)

Microwave level measurements suffer from some lingering misconceptions. First and foremost, microwave technology isn’t a universal level measurement technology. Material dielectric constant, foaming, agitation, baffles, and vessel configuration are among application considerations necessary to achieve a successful microwave installation.

A second misconception is cost. A $5,000 microwave level measurement instrument of five years ago can be replaced today with a more capable $2,000 instrument.

Safety is a third lingering microwave misconception. Microwave level measurement technology is at least as safe as microwave ovens and cell phones.

Microwave level measurements are often used as the source of custody transfers, thus accuracy and repeatability become very important, but doesn’t come for free. Microwave level instruments accurate within 1 mm (0.04 in.) will cost about twice that of an instrument providing 3-mm accuracy.

Nuclear

Of the five level measurement technologies, nuclear probably suffers from the greatest number of misconceptions including that it’s an antiquated technology for level measurement.

Other nuclear level measurement misconceptions are that it’s expensive and dangerous, requires regulatory approvals, radiates (nukes) the contents it’s measuring, and is restricted to specific applications. All are incorrect. (See “Nuclear (radiation/gamma)” diagram.)

It’s true that nuclear level instruments require licensing, and though licensing may vary from state to state, licensing specifics are really a declaration of what the end-user company (owner) is willing to document and assume responsibility for, including activities such as calibration, and source replacement and disposal.

Most nuclear level instruments come with a general license that permits the owner to install the instrument. If the owner desires to replace the source, for example, they must ensure proper procedures, training, and tools are in place to support the additional functions planned by the owner and then apply for additional licensing, clearly indicating what they can and can’t do.

On the positive side, nuclear level measurements can be accurately made in liquids, solids, and slurries. Also, nuclear measurements are unaffected by high temperature or high pressure, aggressive materials, or agitator blades.

Ultrasonic

According to a recent report by Venture Development Corp. (VDC, www.vdc-corp.com ) ultrasonic level measurement technology’s leadership role is being replaced by microwave (radar) technology. Nevertheless, ultrasonic-based level instrumentation will continue to represent a valid level measuring technology for years to come and shouldn’t be discounted.

For several years, ultrasonic level measurement suffered from the misconception that it was a universal level solution. Unfortunately, this resulted in many installations failing to meet expectations.

Ultrasonic level measurements are generally unaffected by material viscosity, density, or dielectric constant; dusty conditions; sludge buildup; element contamination; product coating; or agitation. However, applications with foaming, extreme temperature fluctuations, or near vacuum conditions may make ultrasonic level instrumentation unsuitable.

The fact that ultrasonic level instrumentation manufacturers generally offer high- and low-energy generating transducers to address different application requirements provides clear indication that each application requires an engineered solution. (See “High energy ultrasonic” and “Low energy ultrasonic” diagrams.)

No silver bullet

The message is clear, when it comes to non-contact level measurements, there is no silver bullet, no universal solution. Every technology has its strengths and weaknesses, and all but the simplest applications need to be an engineered solution.

It’s also clear that technology advancements have been significant enough during the past few years to justify auditing the performance of level measurement and/or inventory tank gauging systems installed for longer than 10 years. With audit results in hand, spending time with knowledgeable level instrumentation application engineers may reveal ways to improve accuracy and repeatability and/or reduce operating and maintenance costs.

Also, don’t forget to evaluate the benefits of applying other technologies to level instrumentation. For example, VDC’s study indicates the majority of process level instrumentation and tank-gauging systems shipped to U.S. markets in 2002 included digital network protocols—the largest numbers being HART.

Another benefit worthy of consideration is ease of setup, calibration, and the diagnostics manufacturers have embedded in today’s level instrumentation. Using an integrated display or portable personal computer, users are guided through configuration setup, performing and documenting before- and after-calibration procedures, and executing diagnostic routines.

Among the many things control engineers and technicians do, messing with level measurements is one of the least glamorous. However, within many processes, level is one of the most important measurements. Perhaps it’s time to give it some attention.

For suppliers of level technologies, go to www.controleng.com/buyersguide .

Comments? Email: dharrold@reedbusiness.com

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