A transportable meter for environment radon measurement with a 40 1 decay chamber is designed on the principle of thomas two-filter radon content absolute measurement. The sensitivity is 0.37 Bq.m

-3

with 95% confidence level. This paper describes the experimental method of it's intrinsic uncertainty measurement. The typical intrinsic uncertainty (for n×3.7 Bq•m

-3

radon concentration) is

<

10%. The parameter of exit filter efficiency is introduced into the formula

and the verification is done for the case when the diameter of the exit filter is much less than the inlet one.