The calibration process:
In measurement technology and metrology, calibration 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, a sound tone, or a physical artifact, such as a meter ruler.
The outcome of the comparison can result in one of the following:
- no significant error being noted on the device under test
- a significant error being noted but no adjustment made
- 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.
The calibration standard is normally traceable to a national or international standard held by a metrology body.
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 this information to establish a relation for obtaining a measurement result from an indication."[1]
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.
In metrology labs, calibration is defined as comparing results between a tested instrument and a higher-precision reference. The reference may be another device exposed to the same conditions (as far as possible) to those to which the tested device is exposed. In some cases a calibration standard can be used instead of a measuring instrument. For example, when performing gas calibration, a gas cylinder is used at a known concentration and the same concentration is compared to that measured with the measured measuring instrument.
Common terms:
Error: The difference between the value of the reference and that of the device under test is defined as an error or "Error".
Adjustment: In many cases, especially when the measuring instrument is integrated in an industrial control system, it will be necessary to adjust the instrument under test. The adjustment operation corrects the errors of the device according to the values tested in the calibration process. It is important to note that not every device can be adjusted - for example, temperature sensors of the thermocouple type. In cases where you want to ensure proper accuracy after adjustment, it is recommended to perform additional calibration - As found As left.
Traceability: When a documented calibration of a device has been performed by a reference with traceability to a national standards laboratory, it can be said that the device has traceability to a national standard. The same instrument tested could be used as a reference for the documented calibration of a second instrument. The second device can be a reference of a third device, and so on. In such a case there will be a documented calibration chain that reaches the national standard. Each calibration in this chain is defined as a calibration that is consistent with a national standard.
Accuracy: The use of the term "accuracy" is common in the industry and is generally referred to as a maximum error. However this is an incorrect definition. Accuracy is a qualitative rather than a quantitative value, hence it can be determined that one measuring instrument is more or less accurate than another instrument, but accuracy cannot be quantified numerically. Emotion manufacturers indicate numerical value for accuracy. In order to determine the value, the manufacturers test a maximum error typical of new emotions used in standard applications. This value is determined empirically and usually does not refer to emotions that are in prolonged use or those that are in use in non-standard conditions.
Uncertainty: When calibration and metrology laboratories want to quantify the measurement accuracy, uncertainty is used. Uncertainty uses to estimate the deviation of the reference from the true value. In order to estimate this deviation, statistical calculations based on different empirical experiments must be used. The uncertainty value is a quantitative value that indicates the range in which the true value is at a 95% level of uncertainty (other levels of uncertainty can also be used). For example, reading a reference -temperature of 10C with an uncertainty of 2C +/- means that there is a 95% chance that the true value is between 8C and 12C.
The importance of calibration:
Factories that produce a product that includes measuring instruments use calibration in order to give validity to the integrity of the product.
Many measuring instruments accumulate deviations over time, so the risk of deviating from the quality of the measurement increases over time. Periodic calibration will ensure compliance with the specification and give an indication of the need for corrective actions such as adjusting or replacing the device.
There are various standards that require periodic calibration. For example, the Ministry of Health requires an annual calibration of the temperature sensations in food factories that carry medicines.
Types of certification for a calibration laboratory:
The Standards Institute is able to authorize a calibration laboratory to perform calibration according to the ISO9001 quality control standard. This standard examines compliance with internal procedures, with an emphasis on securing customer needs and does not specifically address the working methods in calibration or testing laboratories.
In most places calibration can be provided in this standard.
The ISO17025 standard is an advanced standard that specifically addresses laboratories that perform tests and calibrations. Laboratories with this standard are required to work in strict work procedures including a combination of accepted metrological methods for assessing uncertainty. Calibration facilities in pharmaceutical factories and accredited laboratories are required to work with suppliers with the ISO17025 standard.
Calibration in Controtec:
As a supplier with over 30 years of experience in the field of sensors for industry, Controtec offers calibration services for sensors of temperature, humidity, pressure and lighting (for photostability cells). Our calibration process is performed by a skilled team while using consistent references to national standards laboratories. Controtec calibration certificates bear the Standards Institute's standard for ISO9001. As representatives of international companies, we also offer calibration in the ISO17025 standard performed by our partnerships in Israel and abroad.
Controtec has professional LAB calibration equipment that includes a temperature and pressure humidity calibrator.
