Dec 12, 2005

Sensors Play Critical Role in Level Control

The list of reasons why companies need sensors in their manufacturing operations is almost as long as the list of companies that produce them. Yet, boiled down to their most basic function, level sensors exist for one reason—to measure how much material is in a tank. Without the precise measurement provided by a quality sensor, there is no way to accurately determine this value.

It sounds elementary, but the ramifications of operating without a sensor, or with an inaccurate one, in certain manufacturing operations range from inconvenience to disaster. Certainly, in a chemical processing plant where two materials are being mixed, an inaccurate measurement of each material could result in poor product quality. If one of the materials has volatile properties, an imprecise mix could result in an unstable end product that might even cause an explosion.

Not knowing the precise amount of inventory also can have a negative financial impact. An oil company estimating how much product is in a tank might be off on its level measurement by mere milli-meters, but depending on the size and number of tanks, this can translate to millions of gallons of oil.

A true comprehension of sensor and level measurement begins with an understanding of the various types of sensor technologies:

  • Radio frequency capacitance uses the electrical characteristics of a capacitor in several different configurations for level measurement;
  • Conductance (conductivity) is based on the electrical conductance of the measured material;
  • Hydrostatic tank gauging is one of the oldest and most common methods, and measures the pressure exerted by a column, or head, of liquid in the vessel;
  • Radar, or microwave, operates on the principle of beaming microwaves downward from a sensor located on top of the vessel; and
  • Ultrasonic uses sound waves to determine fluid level.
    These technologies can be further categorized in two areas: contact (cable) and contact-free (guided radar). In order to determine which technology is appropriate for a particular operation—radar and ultrasonic are the most popular—there are five criteria that should be considered:
  • Process material. What is the generic name of the material(s) being measured?
  • Material characteristics. Is the user measuring a liquid, slurry, solid, interface, granular or powder? What is the consistency of the material—watery, oily, abrasive? Specific technical values of the material, such as its conductivity properties, are also helpful.
  • Process information. What are the normal temperature and pressure, as well as the minimum and maximum? Is there process turbulence? What is the vessel material—metallic, non- metallic or lined?
  • Vessel function. What is the main function of the vessel—sump, reactor, storage? What is the vessel shape and size? Does the vessel contain agitator or other internal obstruction?
  • Power requirements. What are the power requirements of the process— 115, 230 or 24 VAC, or loop-powered?

The above criteria show that there are a variety of applications and conditions under which a level sensor might operate. Yet, whenever possible, it is advisable to find a sensor with application stability—that is, a sensor that can work with virtually any application. This provides the user with much-needed flexibility. As requirements change or as different materials are mixed in a tank, the need to change sensors is reduced or eliminated.

The technological advances in today’s level-measuring instruments are impressive, especially given the sensor’s humble origins. Years ago, the function of level measurement was accomplished with mechanical floats indicating the height of a material’s surface. Later, the floats were equipped with a length of wire for more accurate measurements, and now ultrasonic devices with acoustical bandwidths and radar devices have entered the scene.

One of the most recent developments has been Time-Domain Reflectometer (TDR) technology. TDR technology allows users to measure two or more products in a tank; more specifically, TDR measures the interface between the various materials. TDR is useful in measuring materials in tanks with a large number of internal components.

The value of this technology cannot be overstated. Certainly, if there are two materials with highly divergent properties in a tank, such as oil and water, a TDR-based sensor will accurately distinguish between these two diverse liquids. Yet, a more practical application involves the TDR sensor indicating whether two materials have been properly mixed. Furthermore, the sensor can detect an emulsion—which may have been created during the mixing process—from the pure product. In chemical processing or pharmaceutical operations, where the effective blending of multiple ingredients is critical, this capability is indispensable.

In all likelihood, the technological advances will continue unabated. Today, some sensors include diagnostics as well as configuration and process data that can be communicated over a network to remote monitoring and control instrumentation. Most high-quality sensors are also constructed to provide years of uninterrupted service with no maintenance. In addition, the need for constant or even periodic calibration has been eliminated, as sensors are precisely calibrated by the suppliers during the production process.

It should be noted that different systems do possess different strengths and display specific characteristics.

For instance, dynamic range, bandwidth (pulse width) and stability are important factors of radar and TDR systems. High dynamic range, in particular, is essential for reliable measurements and critical in the detection of level surfaces under foam. A high dynamic front end is also integral in picking up extremely small or weak signals. Radar has its strength in large- distance measurements and in contact-less applications. A low noise floor is also important for stable measurements with little averaging.

The online diagnostic capability of many sensors is also an attractive feature. Sensors with these capabilities can detect various issues responsible for decreased sensor performance, such as electrode leakage and corrosion, fouling and scaling, and liner deformation (generally caused during installation). Some companies have reduced the incidence of these issues through sensor improvements, such as shielding electrode and coil cabling, and using stronger flange and housing materials.

Ultimately, as sensors become more technologically sophisticated, their role in the level-control process—and consequently, their importance in that process—will continue to expand.

About the author

Achim Bletz is a research and development manager for level products at Krohne. He can be reached by e-mail at [email protected].