Adsorption of residual gas molecules damped the emission current of a W (310) field electron emission (FE) emitter. The damping speed was linearly proportional to the pressure gauge readings at pressure ranging from ${sim}10^{-8}Pa$ to ${sim}10^{-9}Pa$, and the proportionality constant was employed to measure pressure in the $10^{-10}Pa$ range. A time plot of FE current revealed the existence of an "initial stable region" after the flash heating of W(310) FE, during which the FE current damps very slowly. The presence of non-hydrogen gas removed this region from the plot, supplying a means of qualitatively analysing the gas species.