Airflite Calibration Services:

Calibration Services Technical information:

Calibration in measurement technology and metrology is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy.

Such a standard could be another measurement device of known accuracy, a device generating the quantity to be measured such as a voltage, or a physical artefact, such as a metre ruler.
The outcome of the comparison can result in no significant error being noted on the device under test, a significant error being noted but no adjustment made, or an adjustment made to correct the error to an acceptable level.

Strictly speaking, the term calibration means just the act of comparison, and does not include any subsequent adjustment.

Calibration frequency:

Manufacturer-recommended calibration interval. Manufacturers’ specifications indicate how often to calibrate

their tools, but critical measurements may require additional intervals.

Before a major critical measuring project. Suppose you are taking a plant down for testing that requires highly accurate measurements. Decide which instruments are required. Send them for calibration, then “lock them down” in storage so they are unused before the test. 

After a major critical measuring project. If you reserved calibrated test instruments for a particular test, send that same equipment for calibration after the testing. When the calibration results come back, you will know whether you can consider the test complete and reliable.

After an event. If your instrument took a hit — something knocked out the internal overload protection or the unit absorbed a sharp impact — send it for calibration and have the safety integrity checked, as well.

Per requirements. Some measurement jobs require calibrated, certified test equipment — regardless of the project size. Note that this requirement may not be explicitly stated but simply expected — review the specs before the test

Monthly, quarterly, or semi annually. If you do mostly critical measurements and do them often, a shorter time span between calibrations means less chance of questionable test results. Many times, calibrating at shorter intervals will afford you with better specifications

Annually. If you do a mix of critical and non-critical measurements, annual calibration tends to strike the right balance between prudence and cost

What the experts say:

https://au.flukecal.com/blog/how-often-should-you-calibrate

http://www.rscal.com/must-calibrate-test-equipment/