How often should I re-calibrate my instrument?
Consideration should be given to the frequency of use, the parameters of the application (temperatures at which it is used, the severity of the use) and the requirements of the regulatory agencies and/or the quality system you may be using. In general, for most laboratory and industrial applications a re-calibration interval of one year is considered a reasonable and prudent time frame.
Most of our clients who maintain an ISO 9000 or QS 9000 series program are using a one year recalibration interval, but one year may be too long (or too short!) for your particular application. By all means consult your quality department or your quality consultant.
Are there any recommendations for calibration intervals from respected sources?
If you perform petroleum testing, consider that the American Petroleum Institute (API) publication ‘Manual of Petroleum Measurement’, Chapter 7, recommends that liquid-in-glass thermometers and electronic digital gauging thermometers be recalibrated annually by a qualified laboratory. This document can be purchased from the American Petroleum Institute.
ICL tries not to recommend calibration intervals, but offers the following logical guidelines:
Start with a prudent calibration interval, following industry norms or recommendations.
When a suitable history of calibrations, and changes in the instrument’s indications, is available, (for example, three years of annual calibrations), review that history and the trends in the instrument’s indications and make a judgment accordingly. Perhaps the device has proven sufficiently stable that the recalibration interval can be extended to two years.
Remember that calibration should take place with a frequency sufficient to prevent out-of-tolerance conditions from occurring.
Try not to make generalizations about types of instruments, but consider all the aspects of each particular application. That little glass thermometer in the plastic bottle in your lab refrigerator may not experience much change in the course of a year, but the glass thermometer used weekly for melting point determinations at high temperatures is entirely a different matter.
Remember too that thermometers and hydrometers are dynamic with use. Shock, contamination, exposure to extremes in temperature, exposure to aggressive fluids or vapors, very rapid cooling or heating, mechanical stress, or any number of factors may cause an instrument to drift out of calibration prior to the expiration of its assigned calibration interval.