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Calibration is the process of determining the relationship between the readings obtained by a measuring instrument or system and the applicable units of some defined system of measurement. To ensure that measurements being made or output provided by equipment are accurate, they need to be compared against a reference that is known to be accurate. This is what the process of calibration is meant to achieve, a comparison between measurements of known and unknown. The piece of equipment or device with the known or assigned accuracy is called the standard. Standards vary from country to country depending upon the type of industry. It is important to understand what is being calibrated and how the calibration is being performed.
For example, consider a digital thermometer that uses an external temperature probe. Here, a test instrument is attached to the digital thermometer and a voltage equivalent to a specific temperature is applied to the digital thermometer. The result is then recorded and the thermometer is considered to be calibrated. The formal definition of calibration by the International Bureau of Weights and Measures (BIPM) is the following: “Operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties (of the calibrated instrument or secondary standard) and, in a second step, uses the information to establish a relation for obtaining a measurement result from an indication. This definition states that the calibration process is purely a comparison, but introduces the concept of measurement uncertainty in relating the accuracies of the device under test and the standard. Calibration Methods Calibration methods can be both manual and automatic, depending upon the device being calibrated.
Manual This is a complex procedure which is performed by the user itself. It requires the complete knowledge about the system or equipment being calibrated so that the calibration process is exact and without any error. · Automatic Automatic calibration is achieved by automatic calibrators. These automatic calibrators consist of control unit housing the electronics that drive the system. There are a few different formats for calibration sessions: Method #1- Review and grade first, then discuss
Method #2- Review together and grade together
Method #3- Review together with agents Concept of Calibration Calibration is essentially a process of comparison. An instrument is used to measure or is measured by a calibration standard, and the result is compared to two things: the known value and the uncertainty of the standard and the performance specifications required by the customer. The concept is simple, but the work is in the details. Some of the details include: The assigned value of the measurement standard, which is usually determined from its calibration history.
The known uncertainty of the standard, which comes from several places, including the historical reports of the calibration and the internal statistical process control methods many calibration labs have for their measurement systems.
The environment of the calibration activity which includes temperature and relative humidity. · The methods and equipment used to make the comparisons. ·
The uncertainty of measurements system relative to the published performance specifications of the item being calibrated or the customer’s requirements.
Importance of Calibration Calibration is used to define the quality of measurement parameters, like accuracy, range or precision, which are recorded by a piece of equipment. It’s necessary for processes like testing, manufacturing and quality assurance. ·
The quality of measurements of just about every tool will deteriorate to some extent. Ensure that these shifts in accuracy are tracked, and measures are taken to prevent them from affecting the final product quality. ·
Equipment that operates on certain technologies, or measures shifting parameters like humidity, temperature and pressure, is more likely to be affected by a ‘drift’ in accuracy. ·
In situations where the quality of the measurements is imperative for maintaining quality, you need to ensure that the instrument is operating within an acceptable range of error. ·
In order to ensure that you enjoy complete confidence in the measurements and the output of any piece of equipment, calibration of instruments need to be performed on a periodic basis.
Factors Affecting Calibration · Using the Wrong Values- Disregarding the instructions and selecting the wrong calibrator values will “teach” the instrument incorrectly, and produce significant errors over the entire operation range. ·
Calibrator Formulation Tolerance- It is important to use calibrators that are formulated to tight tolerance specifications by a reputable manufacturer. The tolerance can affect the mean value obtained when using the calibrator. ·
Ambient Temperature Effects- It is important to periodically calibrate an instrument at a temperature close to that at which it will be operated. Environmental factors, such as the ambient temperature, can introduce errors that may not be readily evident when testing samples with unknown values. Overall, Calibration is a process that ensures that accuracy is maintained in the measurements produced by your equipment. Calibration performance of any equipment is compared against a reference standard. It assures calibration of measurements. We must calibrate our instruments periodically, and identify if there is a ‘drift’ in the measurements and eliminate it through calibration.
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