Everything under control
Non-contact level measurement is currently very popular

Level detection is used in a variety of applications.Sensors protect the environment from tank overflow, prevent containers from running dry and damaging a pump, and they are also used for high-precision tank gauging. They perform vital control functions throughout the process control industry. A wide range of solutions is available to meet the broad spectrum of applications. This article provide an overview ofuseful solutions.

Users in the process industry often do not need high-precision level measurements. What they need is early warning when limits are reached, and they want an alarm to be raised when a container overflows or is completely empty. As a result, simple mechanical detectors are still in widespread use, and this is fine for many applications. They usually have a long track record in the field and provide reliable service. The task is more challenging from the engineering point of view, however, when level detection is needed in separators or under extreme process conditions, or when calibrated data are transferred to a higher level tank management system. These applications reflect the general trend and current preference for high-precision equipment that is simple to operate. Which technology is right for the job always depends on the measurement scenario and the medium. We are, however, seeing increasing demand for radar based solutions.

“Although we have perfected the performance, precision and reliability of our servo technology at Enraf, we see further potential in radar tank gauging. We are developing new software to increase the reliability of our equipment. This opens up wide-ranging opportunities in all types of applications. Our goal is, for example, to achieve level detection right up to the antenna if possible to make optimal use of tank capacity,” explained Wilfried Landerer from Enraf. “Even in the future, we are not going to see a “one fits all” situation in radar technology. The application, in other words the mechanical and physical environment, will determine which measurement method is best.”
Nobody wants to have a complicated system
Users want things to be as simple as possible. Production operators no longer have the time to look after the various probes on a system. Günter Pinkowski from Krohne Messtechnik put it this way: “The user wants to install a meter on the container, turn it on and answer a few relatively simple questions. After that, they expect the meter to work flawlessly.” The market is looking for simple commissioning, simple operation and long, maintenance-free service. To meet this need, Krohne is currently concentrating on the weaknesses which users often complain about on current radar level detectors, namely suitability for the particular application and ease of operation. Optiwave, Krohne’s latest generation of FMCW detectors, delivers better signal dynamics and significantly more information from the container. It is able to distinguish between very weak measurement signals and strong reflections. The same applies to the completely new Optiflex, which is a contact reflex radar device. “The way we see it, measurement techniques which offer genuine advantages to the user will become increasingly popular. This means methods which are suitable for universal use and which are both simple and reliable.”
Rainer Waltersbacher from Vega Grieshaber explained that undirected microwave (radar) and guided microwave (TDR) are currently best suited to meet these needs. In addition to purely technical development, the company will focus on device handling and operation. The Vega range of antennas for special applications includes a parabolic antenna designed for fluids with poor reflective properties. The parabolic antenna systems are suitable for temperatures between –40°C and +200°C and pressures between vacuum and 3 bar.
Andrew Blazey, Product Level Measurement Marketing Director at the Siemens field office in Peterborough, Canada, also believes that radar is a key technology. “Non-contact level detection has by far the greatest potential for continuous level measurement applications, and at Siemens we will be concentrating on this technology in the near future. It shows the highest growth rates and offers the biggest advantages compared to other solutions.” In addition to the ultrasonic Sitrans Probe LU, a 2-wire radar-based meter was introduced in the summer.
2-wire meters with digital interface
Emerson Process Management is also backing radar technology, not only for tank gauging but also for process applications. Jens Weselmann, Radar Level Product Marketing Manager explained that “process engineering is the biggest field of application for compact 2-wire meters that offer a digital data interface. Today’s product families offer plenty of performance for most applications. Nevertheless, Emerson Process Management wants to cover the entire process engineering market.”
The Rosemount 5400 level meter is a very recent addition to the product line, and it uses free propagation. The 2-wire meter has two antennas and offers many of the familiar Rosemount features including a removable cover and a swivel display. The device uses Saab Rosemount’s patented dual port technology and circular mode technology borrowed from the high-precision REX series of products which are approved for calibration. The term dual port refers to the separate send/receive modules which provide greater discrimination between the target echo and interference signals. Circular mode technology enables the 5400 to filter out multiple reflections returned from container walls, etc. before they are captured by the software.
Potential applications in small tanks
Many large storage tank farms as
well as small storage, buffer and supply containers do not have any level detection at all. The Liquicap T from
Endress+ Hauser is designed to fill this gap. This product has a capacitive two-rod probe for continuous measurement in conductive fluids (30 µS/cm and above). The measurement range is 150 mm to 2500 mm, and it can handle process temperatures in between a range of –40°C and +100 °C. The probe length is pre-set in the factory for the specific order, eliminating the need for the customer to calibrate the probe during installation. Development activity at E+H will be concentrated on two areas in 2005. Stefan Rejda, Level Measurement Marketing Manager at E+H, shared some of his future plans with us. He told PROCESS that the company wants to introduce technical innovation to push the application boundaries of ultrasonic non-contact technology. It also wants to add new features to vibration-based bulk goods limit switches. Areas to be addressed included sensor geometry, higher process pressure and a Namur interface. All of these features will significantly widen the spectrum of applications for these sensors.
There is a need to come up with unconventional solutions to handle extreme conditions or media properties. Pepperl+Fuchs has developed the Pulscon LTC to address this need. According to Martin Holdefer, the company believes that the Pulscon LTC guided wave meter and the new LUC-M, which will be introduced in the second quarter to address the non-contact product space, offer the greatest potential in the existing product range. The new two-wire Pulsocn LTC uses guided microwave based on the TDR (time domain reflectometry) principle. Pulses are guided along a rod or cable and are not influenced by surface movement or superimposed layers of gas or dust. A coax probe will provide reliable results even in media with a very low DK value. There are, however, viable alternatives to propagation time techniques. Walter Hein, who works at Turck, points out the usefulness of direct analogue level detection. “We believe this technology will continue to have great potential in the future. The emphasis will increasingly be focused on continuous measurement rather than pseudo-analogue measurement based on a sequential arrangement of reed contacts. Magnetorestrictive sensors, which are rugged and have proven their effectiveness over a period of many years, offer a viable solution. He believes that the main challenges for the future will be integration of temperature sensors and the ability to withstand high temperatures.
At the end of the day, there is no universal level meter, and there can be considerable differences in both price and performance. High tech at any price is certainly not always the right solution. High precision level measurement is generally not a problem. The great unknown is what the process conditions will be when the sensors are mounted into tanks or are exposed to difficult products, foam or extreme temperature or pressure. Manufacturers are focusing on preventing the outside influences from falsifying measurement results.