Police manipulate breath testing devices (breath alcohol machine) by controlling the way people blow into them. Usually, police officers tell a person taking an alcohol breath test to take a deep breath and to blow into the machine as long as possible. During the test the officers will encourage this by telling the test subject to “blow, blow, blow” until the person runs out of breath. The alcohol breath test results then produce a falsely high number.
The blow alcohol tester machines are designed to estimate your blood alcohol content or BAC by measuring your breath alcohol. To do this the machines must use certain assumptions about your temperature, blood particulate levels, and several other factors.
By far the most important factor is temperature. Henry’s Law says that the concentration of a substance contained in the gas found in the headspace over a liquid is directly proportional to the temperature at the point of exchange. In breath testing terms this means that the warmer your lungs are, the more alcohol will be in your breath, regardless of the concentration of alcohol in your blood.