Blue Eagle - The portable breath analyzer technology I loosely described earlier - using tin-oxide sensor technology - actually has the sensor running at a fairly high temperature when in use. The heat has the effect of reversing - at a known rate - the chemical combination of sensed molecules which may have accumulated on the sensor surface... those organic molecules are being continuously boiled off, in effect. So the sensors cleanse themselves after a period of operation in neutral air.
Any reasonably designed instrument will start by 'normalizing' the sensor to a baseline value before the new sample gasses are introduced. This should, in principle, erase the effects of any and all prior tests, so each new test should be fair, no matter how flammable the previous customer may have been..
Notwithstanding that, portable instruments of any sort are much more likely to be out of calibration due to handling damage, temperature extremes when stored in unshaded vehicles, etcetera. Sensors can also be 'poisoned' - like catalytic converters on cars - by exposure to certain substances which might be encountered in the highway environment.
If one's career is on the line, it is likely worth the effort to note the id or serial number of the testing instrument at the time of test, and then have the authorities dig up the maintenance and calibration history. Paying for recalibration or calibration test by an independent lab might be a good investment also. With highest quality devices, 5 or ten percent might be out of calibration at any point in time. With cheaper, older ones, perhaps as much as 50 percent of them could be.
My impression is that most authorities appreciate these fallibilities. They use the instruments as a means to quantify violations by clearly over the limit 'clients', but might well be reluctant to fight over the small points of a test that reports a value right at the lower statutory limit. With some tact and alertness to detail, this result might be accomplished even at roadside.
I hope I never personally have to put this theory to the test.