  Jiangmen Underground Neutrino Observatory (JUNO) is the next generation
neutrino experiment which aims at neutrino mass hierarchy problem along with
many other cutting-edge studies concerning neutrinos. Located 700m underground
in Jiangmen China, JUNO's central detector is an acrylic sphere filled with
20kt liquid scintillator with linear alkylbenzene(LAB) as scintillator solvent.
To ensure that an unprecedented energy resolution of $\sigma_E/E \leqslant 3\%$
can be reached, LAB used in JUNO must have excellent transparency at the
wavelength ranging from 350nm to 450nm.
  In the past decade much effort has been devoted to the development of high
transparency LAB based on the measurement of light attenuation length. Through
a close cooperation with Jingling Petrochemical Corporation in Nanjing,
transparency of LAB samples prepared for JUNO has been improved progressively.
However, this improvement is also pushing our apparatus towards approaching its
measuring limit, undermining the credibility of our measurement. In order to
get a result accurate and precise, an apparatus upgrading and a more detailed
error analysis is inevitable.
  In this article, we present an analysis of how apparatus upgrading helps with
decreasing measuring errors, and we conducted measurements using the new
apparatus on several samples. A detailed error analysis is followed to validate
the results. We propose to apply statistical methods featuring Monte Carlo
simulation to estimate systematic uncertainties. Deviations caused by fit
models is also considered and the overall uncertainty is obtained by combining
two independent measurements. We finally report the light attenuation length of
a newly improved LAB sample to be $29.90\pm 0.95$m, which gives a new high of
all the preceding samples we tested. This study may provide a strong evidence
of JUNO's feasibility to reach its energy resolution.
