I just bought a manual digital peak flow from Microlife to measure my fev1 for copd. I tried it and it shows values higher than the doctor's office. I wonder if someone has experience with it and can you please tell me if they accurate compare with other more professional spirometers. Am I wasting my time with it? Should I use the readings as a true value? How many litres/ml difference from others? Any info about this would be great, please.