Q&A: Weighing Your Level Meas. Options

June 23, 2008

Gene Henry Gene Henry is a level product manager for Endress+Hauser USA. He has over 32 years of experience in the instrumentation field, starting his career as an instrument

Gene Henry

Gene Henry is a level product manager for Endress+Hauser USA. He has over 32 years of experience in the instrumentation field, starting his career as an instrument foreman in the phosphate mining industry. He has spent the last 14 years in instrumentation marketing, providing consultative services to industrial and municipal plants. In his current role, he works on level applications and the development marketing strategies for level products in the United States marketplace. Mr. Henry can be reached at 888-ENDRESS or [email protected].

Q: In general terms, what options does a user have available today when it comes to industrial level measurement?

A: There are a variety of level measurement solutions available to the industrial
user today. For example:

Capacitance – With this technology a capacitor is formed between a level probe and a metal tank wall. Any material between the probe and the wall changes the capacitance value allowing users to measure the level.

Vibrating Tuning Fork – A vibrating set of tongs or a probe are used to sense the presence or absence of process material. There are different versions available, which allow users to measure either liquids or solids.

Ultrasonic – Transmits an ultrasonic signal down to a product and measures the time it takes the signal to bounce off the product and return to the sensor.

Free-Space Radar – Transmits an electromagnetic signal from a horn or rod antenna to the product surface. The signal is reflected back when it encounters a change in dielectric at the product surface. The time it takes this signal to return is proportional to the distance to the level.

Guided Radar or TDR (Time Domain Reflectometry) – This works the same as free-space radar except the radar signal is guided by a rod or cable.

Hydrostatic Pressure – This technology is where a customer would measure the increase or decrease in pressure at the bottom of a vessel to determine the height of the liquid inside the vessel.

Gamma or Radioactive – Here a radioactive source is mounted outside a vessel and the amount of gamma energy that is received and measured on the opposite side of the tank is inversely proportional to the level in the vessel.

Q: How do each of these level measurement methods break down in terms of their relevance and usefulness for different applications?

A: Every technology will have its pros and cons for a given application. We
always stress finding the correct solution for each application, not just selling a
particular technology. In many applications there is more than one good solution
to an application, and it comes down to customer preference, price, or installed
base for the finial choice.

Capacitance – This is used for continuous and point-level detection in both solids and liquid applications. Capacitance is also a tried-and-true technology for doing liquid interface measurements.

Vibrating Tuning Fork – Tuning forks are for point level detection only, and there are specific versions for liquids utilizing a frequency shift technology and a version for solids, which uses an amplitude shift to make the measurement. This technology is very reliable and often used for tank overfill safety applications.
Ultrasonic – Ultrasonics are used in liquid and solids applications; mainly for continuous level measurement, but can also offer contact closures, which are set for a specific level or height.

Free-Space Radar – There are different versions of radar units; some designed to work on aggressive liquid medias with challenging process conditions like high or low temperatures, steam and high pressure or vacuum services. Other units are designed to support the long distances required for solids or granular application with dusty atmospheres.

Guided Radar – Like free-space radar, the guided radar will also do liquids and solids applications. It is used especially in liquid applications with foam or turbulent surfaces and high temperatures. In solid applications, it is not affected by the angle of repose or narrow silos and storage bins. Guided radar can also be used to provide interface measurements in liquids. The advantage of guided wave radar in interface is that it can provide multiple outputs in the same service — e.g., overall level, interface level, thickness of the upper layer, and signal amplitude.

Hydrostatic Pressure – This is used on liquid applications in enclosed vessels using differential-pressure transmitters or a gauge pressure transmitter can be used on vented vessels. To achieve the best accuracy, the density of the product should remain constant.

Gamma or Radioactive – Gamma is normally used in the applications where nothing else will work. High temperatures, high pressures, highly corrosive or toxic media, abrasive products, and reactors are typical gamma applications.

Q: When specifying a solution for a level measurement application, what are some best practices users can employ to ensure the solution they are employing will fit the needs of their application?

A: We recommend an application data sheet be filled out with every level application. This ensures things tend not to get overlooked in the application. The information will include process material information like density, temperature, pressure, dielectric, and build-up potential. Some technologies are unaffected by some of this information, but it may be very important for another technology. For example, density normally has no effect on a radar unit, but it has a huge effect on dielectric. Hydrostatic pressure does not care what the dielectric of a product is, but the density will have an effect. If the product is corrosive, the technology chosen should offer Teflon coatings or materials compatible with the process. The electrical or area classification needs to be specified up front, so if the unit is in a hazardous area it provides required FM or CSA XP or IS approvals. A drawing or sketch of the tank showing the shape and size of the tank and roof, as well as the process connections for the level device, are very helpful and necessary.

Q: What are some common pitfalls users fall into when designing level measurement systems?

A: Most pitfalls are not a result of a technology not working, but more from how it is installed or if the right technology was chosen in the first place. The diameter and height of the nozzle the instrument is mounted on has a huge impact on the performance of most level transmitters. Is there a fill stream falling under the unit? How far is the unit mounted from the sidewall or center of a tank? What are the properties of the process material (dielectric, foam, angle of repose)? All these things should be considered in making a technology selection.

Q: In general, what is the range of performance users can expect from different level measurement methods in terms of accuracy, processing speed, responsiveness, distance, etc.?

A: Most people always want to focus on accuracy when they look at a level application. The short answer is they can, dependent on the technology used and the application conditions, achieve an accuracy of one millimeter to 10 millimeters. This is never the whole story though. One should also look at the repeatability, stability, resolution, temperature influence, and long-term drift to determine the full extent the measurement will have on their process. Processing speed almost never enters into the equation when choosing a level technology, although responsiveness can have an effect. Some technologies do respond faster than others due to the amount of filtering and processing being done to evaluate the level correctly. The distance is always a factor in choosing the correct technology. Free space radars can be used for distances up to 230 feet depending on the product and the dielectric of the product. Ultrasonics, depending on the sensor chosen and the material being measured, can be used up to 230 feet. The maximum distances of some technologies are limited simply by the length or rod that can be shipped or the pull force generated against a cable in a silo.

Q: Where do you see level measurement technology in 10-15 years? What are some key areas you see where level measurement technology can improve?

A: Level technologies, like all instrumentation, are dependent on advancements in electronics and microprocessors. Level technologies will continue to perform more functions faster in smaller and more compact packages. Even as the transmitters offer more functionality they will also provide simplistic methods or configuration and setup. The power draw on future electronic packages will continue to decrease. This enables the advent of the battery-powered units complete with built-in radio or telemetry capabilities. The reduction in wiring costs especially in tank farms and remote locations is a huge benefit to the customer. Level technologies will continue the current trend in offering multiple fieldbus connectivity. This will help the customer to interrogate the individual instrument and retrieve valuable information on the status of the unit enabling better asset management. Ethernet and Device Net protocols should be more prevalent in future level devices.


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