  Using an artificial HI Lyman alpha spectrum we simulate the corresponding
HeII forest with fixed values of eta and a Doppler parameter consisting of a
thermal and a turbulent part. In addition metal lines with line strengths and
line density as expected in the case of HS 1700+6416 are superimposed.
FUSE-like noise is added. The analysis of the simulated spectra in terms of
Doppler profiles recovers the input eta = N(HeII)/N(HI) with a scatter by a
factor of 10. About 10 % have significantly lower eta-values for various
reasons. The majority of the extremely high eta-values (up to 1000) are
evidently caused by metal lines. We conclude that part of the scatter in eta of
previous analyses of the HeII forest in HE 2347-4342 can be regarded as an
artifact. However, we confirm that the correlation between small column
densities (in voids) and high eta-values is not a methodical artifact and
appears to be a true phenomenon.
