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Material Density Effect on Level Measurement Technology

December 10, 2013
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The density of the material being measured can have either a direct or indirect impact on the choices of level measurement technology for use on a given application, and also may impact the performance and reliability of that technology.  
    When speaking about the density of a powder or granular material the definition depends upon what density we are speaking of. There are several common references to “density”, including skeletal density, particle density, and bulk density. We are referring to bulk density in regards to level measurement and detection.  
    In a vessel the bulk density of the material is the ratio of the mass of the material to the volume it displaces. The bulk density includes the material particles, as well as voids between particles and voids within the particles themselves. Because the voids are included, the bulk density can be when the material is in a loose or packed condition.  
    The bulk density of a material can be determined by filling a container that has a known volume (a cubic foot), with the material and then weighing it. The loose bulk density is determined by weighing the full container without having tamped the container or packed the material during and after filling. The packed bulk density is calculated the same way, but while filling you tamp the container to pack the material in order to fit as much of the material as possible in the container, minimizing voids between the particles. When evaluating the use of a level measurement or detection device, it is important to understand what bulk density is used, or if you are just given the range of bulk density between loose and packed.
    Bulk density can impact the choice of sensor technology. In regards to point level sensing (presence/absence detection), some sensing methodologies directly respond to material density. Rotary paddle and vibrating element technology are two examples. The size of the paddle used in a rotary paddle bin level indicator is dependent upon the bulk density of the material. The larger the paddle, the greater the sensitivity of the unit and the lower the minimum bulk density is that the unit can detect. The smaller the paddle, the higher the minimum density of the material will be. This is because the rotation of the paddle is more easily stopped by material with a higher bulk density. Therefore, the bulk density, loose (such as at the top of the material pile; high level) and packed (such as at the bottom of the material pile; low level), is an important material characteristic to know in order to properly apply rotary paddle bin level indicators, specifically in selecting the most appropriate paddle.
    With a vibrating rod or fork point level sensor the mechanical structure is placed in vibration at its natural resonant frequency with known amplitude. The frequency changes when material present at the probe dampens the vibration. Based on a given amplitude and frequency the minimum material density is determined. Most have a minimum bulk density in the 2-3 lb/cu ft range. Knowing the material bulk density is a key to the successful use of vibrating rods and forks.
    An example of indirect impact of material density is in regards to the dielectric properties of the material. The dielectric constant or permittivity of a material is an expression of how the material is affected by an electric field. Air has the lowest dielectric constant, 1.0. The lower the dielectric constant of a material the more challenging is its detection by an RF admittance or capacitance sensor, and the shorter the effective measuring range will be for radar based continuous level transmitters. Bulk density is related to dielectric because lighter weight materials typically have more air present and a lower dielectric constant as a result. Therefore, the lower the bulk density of the material is, the more challenging for RF probes to detect and the lower the measuring range might be for radar based units.
    Whether you have a point level sensor or continuous level sensor application the bulk density of the target material should always be considered.
    Joe Lewis is co-founder and managing director of BlueLevel Technologies Inc. Lewis authored “Solids Level Measurement and Detection Handbook.” He has a BS in electrical engineering from Roger Williams University, and a Masters in Business from Bryant University.

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