The unsupported length depends on fill weight
X axis: unsupported length in m FLB/FLG
Y axis: fill weight in kg/m
Lower scale: travel S in m
FLB = unsupported with permissible sag
FLG = unsupported with straight upper run
 
Reference example A:
 fill weight = 0.125 [kg/m]
 unsupported length FLB = 0.375 m
 travel = 0.75 m