Abstract: Said device comprises a substantially tubular mixing chamber (1), a first inlet orifice for a stream (4) of the first gas at one end (2) of said chamber, a second inlet orifice (6) for a stream of the second gas, located downstream of said first orifice in the direction of flow of the stream of the first gas, means (10) for homogeneously mixing said streams of the first and second gases, said means being located downstream of said second inlet orifice, an outlet orifice (3) for the mixture, located downstream of said mixing means (10), at the other end of said chamber, and an orifice (11) for sampling said mixture of the first and second gases, said orifice being located between said mixing means (10) and said outlet means (3) for the mixture, said device furthermore including means (5, 8, 9) for sending the stream of the first gas into said chamber with a controlled flow rate and for sending the stream of the second gas into said chamber with a controlled flow rate.