NIST recommended practice guide :

Certain equipment, instruments or materials are identifi ed in this paper in order to adequately specify the experimental details. Such identifi cation does not imply recommendation by the National Institute of Standards and Technology nor does it imply the materials are necessarily the best available for the purpose. iii FOREWORD Stopwatch and timer calibrations are perhaps the most common calibrations performed in the fi eld of time and frequency metrology. Hundreds of United States laboratories calibrate many thousands of timing devices annually to meet legal and organizational metrology requirements. However, prior to the publication of the fi rst edition of this guide in May 2004, no defi nitive text had existed on the subject. This NIST Recommended Practice Guide was created to a fi ll a gap in the metrology literature. It assists the working metrologist or calibration technician by describing the types of stopwatches and timers that require calibration, the specifi cations and tolerances of these devices, the methods used to calibrate them, and the estimated measurement uncertainties for each calibration method. It also discusses the process of establishing measurement traceability back to national and international standards.