A method of evaluating a measuring device for measuring, in normal operation, a diffusion capacity of a person's lungs by measuring a change of concentration of a gas species in a mixture, when the mixture is inhaled and subsequently exhaled by the person or imposed by an artificial ventilator, the method comprising the steps of: (a) receiving the mixture in a compartment; (b) circulating gas from the compartment back to the compartment through a circulation circuit; and (c) selectively reducing partial pressure of the gas species in the circulation circuit to a predetermined level. A device for implementing the evaluation method is also disclosed.