a method of evaluating a measuring device for measuring , in normal operation , a diffusion capacity of a person &# 39 ; 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 : receiving the mixture in a compartment ; circulating gas from the compartment back to the compartment through a circulation circuit ; and 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 .