Abstract: A primary flow (Q) of a carrier gas is divided into a series of flows of predetermined flow rate (q.sub.1 -q.sub.10). There is introduced into a first of these flows (q.sub.1) a predetermined quantity of gas (A) to be analyzed, so as to obtain a first mixture (M.sub.1). This first mixture (M.sub.1) is diluted with a carrier gas flow (q.sub.3) to constitute a second mixture (M.sub.2). A portion (d.sub.1) of this second mixture is diluted with a carrier gas flow (q.sub.5) to constitute a third mixture (M.sub.3), and so on. A calibrating gas (U.sub.5) that contains very little of the second gas is provided. These flows feed a series of outlets, usable in parallel, of controlled flow rates of different mixtures over different ranges from very low content to higher content of the gas to be analyzed. The invention is useful particularly for the calibration of high sensitivity hygrometers.