The invention relates to an apparatus for the acquisition and interpretation of expirograms comprising: a gas measuring probe that is designed to determine the gas concentration ƒmess of a gas in exhaled respiratory air; a reading device that is connected to the gas measuring probe via signal and is designed to read for a plurality of values x1, . . . , xN of an exhaled volume of the exhaled respiratory air the respective determined gas concentrations ƒmess(x1), . . . , ƒmess(xN) from the gas measuring probe; a storage device that is designed to store the values x1, . . . , xN assigned to the gas concentrations ƒmess(x1), . . . , ƒmess(xN); a function fitting unit that is connected to the storage device via signal and that is designed to determine a non-linear fit function ƒ(x)=g(x)·h(x)+OffsetGas  for the stored gas concentrations ƒmess(x1), . . . , ƒmess(xN) by determining functions g(x) and h(x) wherein h(x)=a+b·x g(x) is a continuously differentiable, non-linear function with g ⁡ ( 0 ) = 0 ⁢ ⁢ and ⁢ ⁢ lim x -> ∞ ⁢ ⁢ g ⁡ ( x ) = const , a, b and const being constants and OffsetGas being a constant, mean concentration of the gas in room air.