I missed your six second comment from eight days ago!
The meter I bought says in the manual that it needs a five second breath sample of adequate strength. It shows a 10 second count down timer after its initial warm up and I have to start blowing within the 10 second window or else the meter shuts down. There's a beeper that beeps every second when it's ready for the breath sample, then it switches to a continuous tone as I blow for five seconds. Once it stops, then I also stop blowing. When I watch the count down timer, let say I blow on 7, it starts at 10 and goes 9, 8, (and this is when I blow) 5, 4, 3, 2, 1, 0 (stop blowing). If I don't blow hard enough, I don't hear the five second continuous tone. I'll have to try tomorrow blowing past the five second tone to see if that alters the readings.
Edit: I tried blowing well past the end of the continuous tone and sure enough the reading dropped significantly (0.07 -> 0.02). Also, doing a couple of quick in/out breaths before blowing signifcantly dropped the reading and conversely, holding my breath for a few seconds before blowing raised the reading a bit (0.07 -> 0.09)