Calibration is a process through which you confirm that the measurements you have taken are true. The measurements can be of the components, final product or your process parameters. Calibration is done through measuring a standard e.g. a measuring device and ensuring that the error is within the required limits.
For example a measuring scale is being calibrated. You place a known weight of 10 KG on it. Your measurement reading is 10.05 Kilograms. This means that the error is 0.05 grams. Depending on the level of accuracy required in your organization, this may mean a pass or a fail.? If your required tolerance limit is +/- 0.03 grams then your scale has failed calibration test. A tolerance of +/- 0.10 grams means that your scale has passed the test. Tolerance of the measuring equipment is usually set by the manufacturer but it is also determined by your process or customer requirements.? A real calibration shall test the measuring equipment over a range of measurements which can be either the range over which it is to be used or the full range.
Reference material is a material answering to the definition of material having the quality of being sufficiently homogenous and stable with reference to specified properties which has been established to be fit for its intended use in measurement or in examination of normal properties. Reference material accompanied by documentation issued by an authoritative body is called Certified Reference Material. You use reference material to find out the trueness of your measuring equipment as a true standard against which your equipment is measured.
Your measuring equipment degrades over time and must be re-calibrated. There are calibration companies which will calibrate your measuring equipment such as weighing and measuring equipment or pressure and heat gauges against known national or international standards and provide you a Calibration Certificate which is valid over a specified period of time.
Suppose you have an on-off gauge which determines the conformance of an item. Over time the gauge will wear out and it can show more components as good or vice versa. It shall appear through its flawed measurement that the production is improving when the gauge is simply out of calibration.
The time interval between calibrations is dependent on a number of factors which are the manufacturer?s guidelines, the operating environment in your organization, the accuracy requirements and the historical data of the length of time the instrument gives true readings.
All companies do not require calibration and those that do are not required to calibrate everything. For example if you have a manpower company dealing in labour then you do not need to calibrate and can claim an exception to this part of the ISO 9001 standard. To find out which equipment requires Calibration you have to keep in mind a number of factors:
Is the calibration required by the customer?
Is the calibration required by the product specifications? If your measurements should be within tolerances then you are required to calibrate your measuring equipment to make sure that your measurements are correct.
What are the consequences of the equipment?s giving wrong measurements? E.g. processing returns, accepting bad components, rejecting good components, reworkand unhappy customers. You will find out that these costs exceed the costs of calibration.
The cost of replacement compared to the cost of calibration. You will still need to calibrate and monitor the new equipment so as to know when it is not serviceable.
The equipment not requiring calibration should be marked as such.
After calibration your calibration provider usually affixes a sticker on the equipment stating that the piece is calibrated and the time period for the calibration to stay effective. The Calibration provider also issues a certificate to this effect which you need to store in the calibration file so that you can show it at the time of the ISO 9001 certification audit and for your own records.
It is a nonconformance under ISO 9001 when your measuring equipment fails calibration. All the measurements that you have taken since the last calibration are suspect now. The effects of this depend on how critical this equipment is? How much out of tolerance is the equipment? And how much time has passed since the last calibration? The action required may be corrective action like re-measurement, product recall or rework.
You should make sure that your measuring equipment is safeguarded from adjustments, damage or deterioration that would nullify the calibration status and subsequent measurement results.
You should maintain documented information of the measurement as evidence of the fitness for purpose of the monitoring and measurement equipment.
The difference between calibration and verification is that a calibration provides the error of the instrument and compensates for any lack of trueness through applying a correction. A verification on the other hand indicates that a measurement error is smaller than the maximum permissible error. The maximum permissible error is defined by the user as the largest error he or she is prepared to accept.