Measurement chamber device for an optical sensor for measuring the concentration of a substance which is part of an interstitial fluid of a mammal, wherein the measurement chamber device comprises a measurement chamber, which is designed to be inserted into the body of the mammal, which is filled with a measurement fluid and which contains a wall, which enables at least in a wall section a better diffusion of the substance than for other components of the interstitial fluid, characterized in that - the measurement chamber comprises - a first measurement chamber window section, which is prepared to be connected to a transmitter unit for emitting optical radiation into the measurement chamber, and - a second measurement chamber window section, which is prepared to be connected to a receiver unit for detecting optical radiation transmitted through the measurement chamber, wherein - the wall and the measurement window sections (5, 6) enclose the measurement fluid leakproof, - the measurement chamber device comprises a sterilization envelope, wherein - the sterilization envelope encloses the wall section of the measurement chamber, but does not block the optical radiation at the measurement chamber window sections (5, 6) and does not obstruct to connect the transmitter unit or the receiver unit, - the measurement chamber envelope is filled also with measurement fluid to enclose the wall section in measurement fluid and - the sterilization envelope is removable before use of the measurement chamber. - the measurement chamber comprises - a first measurement chamber window section, which is prepared to be connected to a transmitter unit for emitting optical radiation into the measurement chamber, and - a second measurement chamber window section, which is prepared to be connected to a receiver unit for detecting optical radiation transmitted through the measurement chamber, wherein - the wall and the measurement window sections (5, 6) enclose the measurement fluid leakproof, - a first measurement chamber window section, which is prepared to be connected to a transmitter unit for emitting optical radiation into the measurement chamber, and - a second measurement chamber window section, which is prepared to be connected to a receiver unit for detecting optical radiation transmitted through the measurement chamber, wherein - the wall and the measurement window sections (5, 6) enclose the measurement fluid leakproof, - the measurement chamber device comprises a sterilization envelope, wherein - the sterilization envelope encloses the wall section of the measurement chamber, but does not block the optical radiation at the measurement chamber window sections (5, 6) and does not obstruct to connect the transmitter unit or the receiver unit, - the measurement chamber envelope is filled also with measurement fluid to enclose the wall section in measurement fluid and - the sterilization envelope is removable before use of the measurement chamber. - the sterilization envelope encloses the wall section of the measurement chamber, but does not block the optical radiation at the measurement chamber window sections (5, 6) and does not obstruct to connect the transmitter unit or the receiver unit, - the measurement chamber envelope is filled also with measurement fluid to enclose the wall section in measurement fluid and - the sterilization envelope is removable before use of the measurement chamber.