Innovation in Level Measurement - End-of-Probe Algorithm

Signal Propagation Delay

Every product has a dielectric. The dielectric of a vacuum is one, and all other products are related to this vacuum. An electromagnetic wave travels with a certain speed through a vacuum. When the wave has to travel through a medium other than a vacuum, the speed of it will change. The higher the medium’s dielectric, the slower the wave’s speed will be. For example: A medium with a dielectric of three will have a slower wave speed than a medium with a dielectric of two. A hydrocarbon (i.e. diesel or LPG) is considered to have a "low" dielectric of around two, which seems low, but this is still 100 percent higher then the dielectric in a vacuum (one).

End-of-Probe- Reflection Shift

Each sensor has a fixed cable/rod length. The end of a cable/rod gives back a huge reflection as the waveguide ends there and all energy must travel back to the electronic. According to this, the portion of energy that is not reflected at the surface will travel down and come back as an end-of-probe signal. However, there will be a time shift as the electromagnetic wave is slowed down traveling through the media as mentioned in the above paragraph. This time delay between a signal traveling to the end of the probe through air and a signal traveling through a medium is proportional t the level of the material and the dielectric of that material.

End-of-Probe Algorithm

The end-of-probe algorithm serves as a way to calculate the dielectric of the material under process conditions using the shift in propagation speed and the surface reflection. When the surface reflection is lost, it calculates the level using the end-of-probe time shift and the last measured dielectric until the surface reflection returns. This guarantees an accurate measurement signal even under harsh process conditions.

Dielectric Influence

The dielectric of a product is never the same. Changes occur due to temperature, humidity, and sometimes even pressure. In practice, even products carrying the same name can have changing dielectrics. That is the reason why capacitance-measuring systems are less accurate than devices with end-of- robe algorithms (i.e., because they do not compensate for changing dielectrics).

The higher the dielectric, the slower the electromagnetic wave will travel through the process medium. Therefore, it is important that the dielectric is constantly determined when you want to use the end-of probe algorithm and measure accurately. The dielectric is determined every time the surface gives a reflection (on average two times per second).

Application

In Solids

As noted earlier, an end-of-probe algorithm system determines the level in a silo or tank in two ways; using the first reflection coming from the surface and by using the end of the probe. During the pneumatic filling of a silo, the product blown in can create a lot of disturbances (noise) to the measurement signal. Dust is the most commonly known disturbance. The signal coming from the surface of the product with a low dielectric constant can be lower than the noise created during the filling. In an end-of-probe algorithm-based system, only the end-of-probe signal is used to determine the level instead of using both the reflection from the surface and the end-of-probe. This level is as accurate as the level determined by the reflected surface since it calculates with the product properties under process conditions.

In Liquids

The same difficulties can be experienced in liquid applications. In this case, mist, strong agitation, and/or waves in a tank with a process medium with a low dielectric (i.e. hydrocarbons) can make a surface reflection disappear. As such, the end-of-probe will still give a reliable and accurate level signal.

End-of- robe, guided-wave radar systems are currently offered by a number of manufacturers. Each have different setup requirements and offer different levels of accuracy. Research and analysis of competitive offerings should be performed to identify the appropriate end-of-probe system for a given application.

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This article was provided by Endress+Hauser.  About the Author - Rob Vermeulen has been a part of the Endress+Hauser team for twelve years working in the Netherlands, Germany, Switzerland, and now in the United States as a Level Product Manager. Rob is a certified electrical engineer and is actively involved in developing and applying radar technology for the process industry. He has taught post academic courses on instrumentation sharing his knowledge and skills with others. In addition, Rob has lead many of the free technical seminars concerning guided radar technology that Endress+Hauser offers. A group of individuals at Endress+Hauser have worked diligently to patent the unique end-of-probe algorithm for level measurement. For more information on the end-of-probe algorithm or radar level technology at Endress+Hauser, contact Rob either by phone, (317) 535-2168, or by e-mail, Rob.Vermeulen@us.endress.com.

For more information on Endress+Hauser products, you may Request their Product CD or visit their web site at www.us.endress.com.