According to Wikipedia, the slinky was invented in the 1940’s.  The original dimensions were 2.5 inches tall and composed of 80 feet of metal steel wire in 98 coils.  The contents of a fiber optic cable are also wound in a certain pattern to provide strength.  Like the slinky, the glass-composed fiber strands are longer (when unwound) than the outside cable.  Schneider Electric’s Fiber Manager™ documents this parameter in an attribute/field called TwistFactor, and is utilized to calculate the glass length of a cable.  It is this length that is the “law” of subsequent calculations and reporting. 

There are four ways of glass length calculation (the first three utilize the Twist Factor):

The length type (above left column) can be cable specific based upon information available.  It is assigned to the field with the SHEATHLENGTHSOURCE model name (typically CableLengthSource).  This field is assigned the Fiber Optic Cable Length Source domain which enables the user to quickly choose from the four types above.

It should be noted the OTDR Length option was added to the product about one year ago.  Older original Fiber Manager™ implementations that have since been upgraded may not have this option available.  If this is the case, an administrator will have to add the coded value to the Fiber Optic Cable Length Source domain.

Where can the Twist Factor parameter be found?  Fiber manufacture specification sheets often contain this information or sometimes the manufacturer needs to be contacted.   Twist Factor can also be manually calculated by conducting an OTDR on the uninstalled reel.  In the absence of information, a value of 1.02 or 2% twist can be used as an approximation.
 
Certain cable configurations that involve an inner core and an outer core can have different strand twist factors within the same cable.  Buffer tubes containing strands in the inner core have lower twist factors than the buffer tube containing strands in the outer core.  In this event, it is not suggested to average the values because all the resultant glass lengths will lack accuracy.  It is suggested to choose the twist factor value applicable to the majority of the strands.   An administrator can also add a twist factor field to the buffer tube object, but it will only be utilized as reference.

So…..What is the twist factor of a slinky? 

We first need to convert 80 feet to inches for common units.  That measures out to 960 inches.  We take that value (960 inches) and subtract the unstretched length (2.5 inches). We lastly divide that result by the original unstretched length (2.5 inches).  That leaves a value of 383 or 38,300% twist.  Significantly higher than the approximate 2% found in fiber optic cables.

If you found this blog post interesting, please see the associated posts below, which are specifically related to Fiber Manager™ field attributes.