Also called a "Calibration Check" or "Proficiency Test" or "External Calibration Check".   The accuracy of any testing instrument should be checked periodically per the regulations governing your breath testing program.  An accuracy check is a procedure used to test your instrument's ability to read a known standard within a given tolerance.  This is accomplished by running a standard with a known alcohol concentration through the instrument's sampling system and verifying that the result is within an acceptable tolerance range of the expected value of the standard.  It is much the same as running a sample on a subject but, in this case, the operator knows what result the instrument should produce.  It is checking to see that a given known gas value registers correctly within tolerances defined by your testing protocol. This procedure is called an "accuracy check" and is sometimes referred to as a "calibration check" because it is a test (check) of proper calibration.  If the reading produced by the instrument is outside of the acceptable tolerance, the device must be calibrated. Accuracy checks serve 2 purposes: Validating Subject test results.  If your testing instrument reads the known standard gas within a defined tolerance range then it is reasonable to assume the device will provide accurate readings on a subject test.  Any subject tests performed between two successful accuracy checks are, by default, accurate. Verifying long term accuracy.  By documenting your accuracy checks in a calibration logbook, you can demonstrate the instrument's performance over time.