Values from the POWERbreathe device and Breathe-Link® software should not vary more then +-10% + 5 cmH20. To consistently breathe at the same rate and effort for each new session is impossible, consequently variations of results will be observed. Ideally if you are performing a test or training session with or without the Breathe-Link® software you should try to perform them at the same time and same effort to get a accurate overview of your breathing muscle performance.