The Czech Metrology Institute performs calibration and verification of noble gases stack monitors in nuclear power plants and nuclear research facilities. Together with Kr-85 and Xe-133, the radionuclide Ar-41 is measured using HPGe detectors and its activity is determined using a gamma-ray peak at 1293keV. The counting efficiency used in these measurements was calculated by the Monte Carlo method using the MCNP code. Radioactive gas standard is prepared by irradiation of argon in a high-pressure vessel by a Cf-252 neutron generator. The inner shape and thickness of the cylinder walls were determined by radiography. The argon volume under normal conditions is determined from the high-pressure vessel volume and by a precise gas pressure measurement. As a result, the activity concentration of Ar-41 at normal conditions is certified.