  Theoretical electron density-sensitive line ratios $R_1 - R_6$ of Ar XIV soft
X-ray emission lines are presented. We found that these line ratios are
sensitive to electron density $n_e$, and the ratio $R_1$ is insensitive to
electron temperature $T_e$. Recent work has shown that accurate atomic data,
such as electron impact excitation rates, is very important for reliable
determination of the electron density of laboratory and astrophysical plasmas.
Present work indicates that the maximum discrepancy of line ratios introduced
from different atomic data calculated with distorted wave and R-matrix
approximations, is up to 18% in the range of $n_e=10^{9-13}$cm$^{-3}$. By
comparison of these line ratios with experiment results carried out in electron
beam ion trap (EBIT-II), electron density of the laboratory plasma is
diagnosed, and a consistent result is obtained from $R_1$, $R_2$ and $R_3$. Our
result is in agreement with that diagnosed by Chen et al using triplet of N VI.
A relative higher diagnosed electron density from $R_2$ is due to its weak
sensitivity to electron temperature. A better consistency at lower $T_e$
indicates that temperature of the laboratory plasma is lower than
log$T_e$(K)=6.5. Comparison between the measured and theoretical ratios reveals
that 32.014~\AA line is weakly blended by lines from other Ar ions, while
30.344~\AA line is strongly contaminated.
