An air blank is a method of checking a breath alcohol tester prior to performing a test to ensure the instrument is starting from zero. The device takes a reading of its alcohol sensor and sampling chamber to confirm that there is no alcohol remaining from any previous test. The device must display a reading of .000 when performing the air blank. This documents that any trace of alcohol that a person may have blown into the device on a previous test has dissipated and that the instrument is ready for another test.
Some instruments require the BAT to manually perform this procedure. Other devices perform this procedure automatically. Some automated instruments will not allow the test to proceed if the air blank result is not .000.
DOT regulations require an air blank prior to a confirmation test, but do not require an air blank prior to a screening test.