For additional physical protection and ease of handling, a secondary coating of polyvinyl chloride (PVC) or Hytrel (a thermoplastic elastomer that has desirable characteristics for use as a secondary buffer) is extruded over the 250um-coated fiber, increasing the outside diameter up to 900um. This type of construction is referred to as 'tight buffered fiber'. Tight Buffered may be single or multi fiber and are seen in Premise Networks and indoor applications. Multi-fiber fusion splicer, tight-buffered cables often are used for intra-building, risers, general building and plenum applications.
Loose tube fiber' usually consists of a bundle of fiber optic splicer enclosed in a thermoplastic tube known as a buffer tube, which has an inner diameter that is slightly larger than the diameter of the fiber. Loose tube fiber has a space for the fibers to expand. In certain weather conditions, a fiber may expand and then shrink over and over again or it may be exposed to water. Fiber Cables will sometimes have 'gel' in this cavity (or space) and others that are labeled 'dry block'. You will find many loose tube fibers in Outside Plant Environments. The modular design of loose-tube cables typically holds up to 12 fibers per buffer tube with a maximum per cable fiber count of more than 200 fibers. Loose-tube cables can be all-dielectric or optionally armored. The armoring is used to protect the cable from rodents such as squirrels or beavers, or from protruding rocks in a buried environment.