Peekay4 it may be a difficult bit of maths but a zero alcohol reading IS 0.00%
No it is not. All breath analysis machines have tolerance levels specified by the manufacturer. A
positive test result is one that exceeds this tolerance level. A typical tolerance would be 0.01.
Nowhere in the Indian procedure document linked above requires a reading of 0.00% other than for air-blank calibration.
They often produce low readings
No, they don't. Again it seems many are confusing so called "breathalyzers" vs. breath analysis machines used for these tests. Two different things.
"Breathalyzers" (to use the term loosely) are portable units often used by law enforcement for preliminary screening tests. They are not very accurate, and subject to many errors cited here, such as detecting alcohol in the mouth (from mouthwash, cosmetics, etc.) These testers can also produce false-positives due to underlying medical conditions like diabetes. Diabetics produce
acetone which can show up as high as 0.06% BAC on a preliminary tester. So it doesn't matter if the tolerance is 0.00% or 0.04% anyway.
But breath analysis machines are different. These are calibrated, lab-grade gas spectrometers tuned to detect
ethanol. They also have specialized algorithms (called "slope detectors") to reject mouth alcohol and other causes of false-positives (MTBE additives, paint thinners, etc.). They are operated by specialized technicians following strict legal procedures.
Since these machines are detecting ethanol specifically, they are extremely accurate. False positives are rare. Due to their accuracy these machines are typically acceptable in a court of law in lieu of a blood test.
A disease which could produce
ethanol in detectable levels is also extremely rare (known as auto-brewery syndrome). Most of us may produce a trace amount of ethanol in our guts, yet in the entire history of humankind there are less than 100 recorded cases of auto-brewery syndrome. If you are unlucky enough to suffer from this disease, you may lose your flight medical anyway because you are essentially drunk all the time.