  Recent advances in identifying giant molecular filaments in galactic surveys
allow us to study the interstellar material and its dense, potentially star
forming phase on scales comparable to resolved extragalactic clouds. Two large
filaments detected in the CHIMPS $^{13}$CO(3-2) survey, one in the
Sagittarius-arm and one in an inter-arm region, were mapped with dense gas
tracers inside a 0.06 deg$^2$ area and with a spatial resolution of around 0.4
and 0.65 pc at the distance of the targets using the IRAM 30m telescope, to
investigate the environmental dependence of the dense gas fraction. The
N$_2$H$^+$(1-0) transition, an excellent tracer of the dense gas, was detected
in parsec-scale, elliptical clumps and with a filling factor of around 8.5% in
our maps. The N$_2$H$^+$-emitting areas appear to have higher dense gas
fraction (e.g. the ratio of N$_2$H$^+$ and $^{13}$CO emission) in the inter-arm
than in the arm which is opposite to the behaviour found by previous studies,
using dust emission rather than N$_2$H$^+$ as a tracer of dense gas. However,
the arm filament is brighter in $^{13}$CO and the infrared emission of dust,
and the dense gas fraction determined as above is governed by the $^{13}$CO
brightness. We caution that measurements regarding the distribution and
fraction of dense gas on these scales may be influenced by many scale- and
environment-dependent factors, as well as the chemistry and excitation of the
particular tracers, then consider several scenarios that can reproduce the
observed effect.
