Plastic is everywhere. Look around you! I bet that on your desk right now or within reach of where you're sitting, at home or wherever you are, is something made out of a plastic material and probably more than one something. But, what is plastic? A good definition is found at the preceding link, in summary a "plastic" is a polymer and derives its name (plastic) from the fact that these materials can be reformed into various shapes, they are malleable and these materials have the physical property of plasticity, a physical property meaning that the material has the ability to irreversibly change shape given an external force is applied to it.
Plastics come in many forms and types. Each is created for a particular purpose and the properties that they contain. Some have higher or lower temperature limits, some are more or less conductive and some have more less abrasion resistance and different chemical compatibilities. In addition, different plastics can have different results given the same "force" our shaping process. The purpose here is not to discuss details of physics in regards to plastic material but to discuss the use of a state-of-the-art level measurement technology to measure the changing level of a wide variety of plastics in storage silos.
Notable characteristics of plastics:
- Bulk density typically less than 40 pounds per cubic foot
- Dielectric constant of plastics tend to be < 2.0
- Silo storage vessels (at processing facilities) tend to be </= 60 feet overall height
- Most plastics stored in pellet (granular) form, but some (such as resins to formulate PVC) are not
- angle of repose not very steep
In the use of guided wave radar to measure the level of plastics in storage silos the two important characteristics to consider are the dielectric constant and the overall measuring range. When the dielectric constant of a material being measured by guided wave radar technology gets too low, then a mode of calculating material level is needed that does not require calculation of the time of flight of the reflected radar energy from the surface of the material. This is because, with very low dielectric material and "long ranges" the reflected energy is far too small. The needed method for low dielectric materials with moderate measuring range (30-52 feet) is called TBF (tank bottom following) where the radar energy penetrates the material and is reflected off the bottom of the probe's counterweight.
Material Dielectric Constant Mode Maximum Range
Polypropylene (40% GF) 1.8 Direct 33'
Polypropylene (generic) 1.6 Direct 22'
Polypropylene (generic) 1.6 TBF 50'+
PVC Compound 1.7 TBF 50'+
ABS 2.0 Direct 46'
I have two case histories that exemplify the use of guided wave radar on plastic materials. But note how the use of Direct and TBF modes are required, depending on the dielectric constant of the material and the range of measurement. TBF is an effective mode, but if the dielectric constant changes the instrument may require a parameter adjustment to maintain its original precision. The Direct mode is preferred but both are necessarily considered when the dielectric constant of the material being measured dips below 2.0. TBF is most effective the further below 2.0 the material is, down to about 1.4.
If you are needing to measure the level of plastic materials or manage your plastic material inventories drop a comment, email or give us a call. We probably have a good solution for you.
Joe Lewis
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