On the level: Dealing with non-linear shapes

A big advantage automation lends is the the ability to acquire data continuously rather than as discrete points in time. If consumption varies predictably, acquiring data at a couple of points in time can be fine. Otherwise, continuous measurement is practical and cost-effective.
By Mike Mendicino, Pepperl+Fuchs March 1, 2009

Many process applications require volume/level measurement to be automated. A big advantage of automating is the data can be acquired continuously rather than as discrete points in time. If consumption varies predictably, acquiring data at a couple of points in time can be fine. Otherwise, continuous measurement is practical and cost-effective.

There are many technologies available to measure level either continuously or in discrete steps. There is even a larger variety of sensors that can measure level continuously. For example, some mechanical systems have been used for centuries.

If a cylindrical tank is installed on its side, the non-linearity of the volume is caused by the constantly varying diameter of the fluid as the level changes. The challenge is to compute the volume with respect to this change. The answer lies in geometry that calculates the level based on known parameters of height and diameter.

Deriving the equations isn’t necessary; one can go to a variety of sources and come up with an equation in terms of radius, liquid height and tank length. Typically, users don’t have to do the calculations. Sensors using several different measurement technologies have the capability to calculate volume of irregular geometries among other conversions for weight, mass, etc. By entering the known parameters %%MDASSML%% for example, vessel diameter and units %%MDASSML%% and the maximum scale corresponding to 100% output, the sensor can apply a standard algorithm for those parameters adjusting the output based on the level. When applying these algorithms, there is a reduction in the accuracy of the reading, albeit small. However, in situations requiring a low level of uncertainty such as inventory control applications, one may choose not to use these internal calculations.

Level technology

Level measurement technology is a key task of many control systems in chemical, petroleum and environmental technologies. In addition to determining limit values to protect against overfilling, min and max control systems, it is also protection against overflow or running dry. Continuous level measurement is of considerable significance for accurate inventory management.

In addition to availability and reliability, the important benefits of continuous level measurement are the accuracy of the measurement and cost of ownership. Today more than ever, each decision must be preceded by a careful evaluation of which measurement principle goes with which process and medium, and what the trend will be for long-term operating costs of the measurement system. No one technology satisfies all situations.

Guided microwave sensors

Guided microwave systems are offered in today’s market with increasing popularity. In contrast to radar or ultrasonic systems, measurements are performed in contact with the medium. The microwave pulses are not radiated freely, but are guided on a sensor rod or cable. The time-of-flight of the pulses corresponds to the distance measured between the process connection and the surface of the product.

Guided microwave sensors are time-of-flight instruments and work with the time-domain reflectometry (TDR) principle. They transmit repetitive pulses at microsecond intervals with pulse widths in the nanosecond range. These pulses are reflected off the surface of the medium and are evaluated by sampling with time offset as an echo profile.

Special algorithms and multiple sampling of the echo profiles allow an exact representation of the spatial situation between the source of the wave and the reflection. The echo is converted to a proportional distance or level signal at a resolution of just a few millimeters. The measurement is insensitive to temperature, pressure or gas layers in the container. The measurement accuracy is independent of changes of the medium in density or moisture. The advantages of the guided signal and the practical absence of external influences compensate for the disadvantage of being in contact with the medium.

Guided microwave sensors are well suited for use in almost all process-related systems in which filling levels of liquids or bulk goods must be measured. They are being used more frequently in applications involving chemicals, petrochemicals, water/wastewater and the basic materials processing industry. The advantages of guided microwaves are found in areas where reliable measurements have been problematic until now %%MDASSML%% for example, in small process containers with formation of froth or turbulence, or in media with a low dielectric value. These include:

  • Reliable measurements of powdery media even during filling

  • Measurements in liquids even with froth and drop formation

  • Reliable and accurate measurements in a bypass or still pipe

  • Practically no interference caused by installed elements or supports, textured container walls such as corrugated sheet metal or narrow silo cells

  • Independence of media properties in liquids/bulk materials such as density, dielectric value, chemical corrosiveness or conductivity

  • Largely independent of process influences such as pressure, temperature, movable surfaces or froth, mist and dust.

    • Economic considerations

      Versatility and range of applications, give guided microwaves a clear advantage and a promising alternative to other continuous level measurement technologies. Important considerations include uncomplicated operation, flexible installation and maintenance requirements. Simple front panel control with a menu-driven user interface and PC-interface make it possible to adjust other features such as saving a characteristic curve for tank linearization.

      Application conditions and the price/performance ratio are critical factors for the economical and successful use of measurement systems. No one level measurement system offers the optimal solution for all applications. However, most applications can be implemented with time-of-flight measurement systems.

      Go to www.plantengineering.com to read the full text of this article.

      Author Information
      Mike Mendicino is a product manager at Pepperl+Fuchs (