Calibration of scientific instruments such as thermometers and hydrometers refers to comparing the measurement performance of the test instrument to the measurement performance of a reference standard instrument. The reference standard instrument has been determined to be accurate or has been assigned accuracy by an accredited laboratory. Laboratories which issue calibration certificates must use reference standard instruments which are traceable to NIST (National Institute of Standards and Technology). Proper calibration involves proper documentation according to GLP (Good Laboratory Practice) methods. Calibration of a thermometer or hydrometer must be performed a specified points. The instruments can be calibrated at one two or three points. The points chosen would depend upon the temperature range or specific gravity range at which the instrument is being used. Instruments can drift with time and thermometers which require calibration should be recalibrated yearly; thermometers used as standards should be recalibrated every 6 months.
A statement of accuracy traceable to an NIST laboratory comes with many thermometers and hydrometers. This statement declares that the instrument was tested against a reference standard instrument and found to have accuracy within the specifications stated for the test instrument. This level of documentation is sufficient for many applications.