First, I agree that learning to use the device (I have both a Ketonix and a cheap breathalyzer) is not as simple as it might seem. I have found that to empty my lungs sufficiently requires a LOT of effort engaging both abdominal muscles and diaphragm. This is a very uncomfortable, and I think necessary, procedure. Clinical and lab devices which use spectrometers, etc. don’t have this particular issue.
Second, if you can master the technique and do it consistently, then you can safely presume that you are emptying your lungs sufficiently to to get an accurate ppm acetone into the device.
Third, why do you presuppose that successive breath samples must be consistent and are not reliable if not? We’re dealing with a very dynamic system that changes continuously. For example, β-hydroxybutyrate concentration varies minute to minute. That’s why it’s not a very good biomarker of much other than that you’re in ketosis and synthesizing ketones. Most people only sample once or twice a day and mistakenly think what they’re seeing is a static concentration.
There is no valid reason to think that acetone would not also change minute to minute both from the ‘spontaneous’ breakdown of acetoacetone (which is technically ‘fat burn’) and utilization within cells and organs (what we would most likely think of as ‘fat burn’). Additionally, in the case of BrAce if you empty your lungs taking a sample, it’s going to take a few minutes for the deep alveoli to build up the acetone concentration again from blood acetone migrating into the lungs. Thus, you could sample too frequently and not get accurate results.
The importance of BrAce as a measure of ‘fat burn’ are the trend and moving average. The Ketonix mobile app tracks both in addition to the sample points.