There are different methods used for calibration, depending on the type of device or instrument being calibrated. Some common calibration methods include:

1. Traceability to a National Standard: This method involves calibrating the device or instrument against a reference standard that is traceable to a national standard.

2. Inter-laboratory Comparisons: This method involves comparing the measurement values obtained from the device or instrument to those obtained from a different laboratory using the same or a similar device or instrument.

3. Self-Calibration: This method involves the device or instrument calibrating itself by comparing its measurement values to its own internal standard values.

4. Reference Material Calibration: This method involves calibrating the device or instrument using reference materials of known values.

5. Accredited Calibration: This method involves having the device or instrument calibrated by a third-party calibration laboratory that is accredited to perform calibration services.

The frequency of calibration depends on the type of device or instrument being calibrated, the manufacturer’s recommendations, and the specific industry regulations. Some devices or instruments require daily calibration, while others may only need calibration once a year.