Method for testing the function of a device

A method of using a human control for testing the function of a device for measuring the concentration of a gaseous substance in exhaled air, the method comprising the steps of:

• checking that said human control is in a normal condition, wherein a human subject is considered to qualify as a control and as being in a normal condition when fulfilling the following criteria:
• no ongoing cold, no airway disease, no asthma, non-smoker, no allergy, no non-seasonal allergy, no intake of nitrate rich food within the last 3 hours, and no strenuous exercise performed within the last hour,
• measuring the concentration of said gaseous substance in air exhaled by said human control to obtain a current value,
• determining if said current value of the measured concentration is within a specified range,
• determining a reference value,
• determining the difference between said reference value and said current value, and
• interpreting a deviation of more than a specified value as an indication that the device is malfunctioning.