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EOP solves dusty, noisy level applications

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article image Levelflex EOP algorithm.

THE patented End-of-Probe (EOP) algorithm incorporated into the Levelflex M device from Endress+Hauser enables accurate and reliable level measurement of low dielectric solids (flour, cement, PE and PP granules, powders, hops, lime) even during pneumatic silo filling and fluidised emptying operations.

Like many similar level devices, Levelflex M uses guided radar (Time Domain Reflectometry, TDR) technology. TDR means that a microwave signal is pulsed from a transmitter mounted to the roof of a silo or tank, down the probe into the material whose level is being measured.

The probe, in solids, comprises of a high tensile stainless steel rope of any chosen length up to 35m long, and during calibration, the actual length of the rope is recorded in the device, or the automated 'probe length' process determines the end-of-probe length.

The pulsed signal travels down the rope, normally in air, but also vacuum, until it is reflected by a medium with a higher dielectric coefficient (e).

At this interface the energy is reflected and travels back up the rope to the Tx, and the time-of-flight is recorded. Knowing the speed of propagation of electromagnetic waves, it is a simple calculation to determine the distance from the Tx to the surface, and if the rope end is at the bottom of the vessel, it is possible to calculate the level of the product in the tank or silo.

Although TDR technology has solved many historical problems when measuring levels of solids during noisy and dusty filling, it still performs poorly when the interface between the airgap and the material is not clearly defined - typically when the material is conveyed pneumatically during filling, or when the bin or silo is fluidised to ensure easy emptying.

This aerated product blanket has a dielectric constant similar to air, so there is no reflected signal, - energy simply continues along the rope and dissipates at the end of the probe, and without the EOP algorithms an incorrect, or no, level is recorded, with possible expensive consequences.

This is where EOP differentiates the E+H Levelflex M from TDR devices from other manufacturers. At calibration the End-of-Probe is recorded, assuming that the medium of transmission is air (e=1).

During the TDR process, when there is a 'fuzzy' material layer and hence no definite dielectric change, the signal continues beyond the material surface and continues to travel down the rope until the rope end, at which point it dissipates.

But, since e>1 in the material (grain, PE, PP etc) the signal's transmission speed is reduced, which has the effect that the rope end 'appears' to be further, and that the rope gets longer.

If the EOP position is tracked continuously, it is possible to relate the 'virtual' changes in rope length to the height of material in the vessel, and this EOP algorithm is not affected by dust or noise.

Hence, Levelflex M can provide accurate and reliable level measurements in the most difficult applications, where no other technologies will work, and should the dielectric constant of the material change due to variations in moisture content, or release of air during fluidised product transmission, these are compensated for by the EOP principle.

Levelflex M is available with SAA DustEx approval and is the only TDR level measuring device suitable for use in potentially explosive dusty applications such as grain, cement, hops silos. It is a cost effective level instrument for all solids industries, mining, chemical, food, agriculture.

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