Calibração – Definição, tipos, finalidade, Procedimento de Calibração

Calibration – Definition, types, purpose, Calibration Procedure

The ability of all measuring instruments to measure accurately and reliably must be proven to obtain meaningful results. To achieve this, measurement results must be compared with higher standards.

The instruments and meters to be used for measurement must be of known precision so that the results obtained are significant. To identify errors and rectify them, instruments are compared with masters or standards. This act of comparison is known as calibration.

Calibration Setting:

The process of comparing a device with unknown accuracy to a device with a known and accurate standard to eliminate any variation in the device being checked is called calibration.

Thus, calibration of a measuring system means introducing an accurately known sample of the variable that is to be measured and then adjusting the reading device of the measuring system until its scale accurately reads the introduced known sample of the variable, i.e. the Calibration procedure establishes the correct output scale for the measurement system.

Traceability:- Traceability is the property of the results of a measurement, not of an instrument or calibration or laboratory report. Traceability means that the result of a measurement can be linked to a reference through a series of calibration reports.

Example of calibration process Example of calibration process – manual calibration

Calibration purpose:

Why is it necessary to calibrate measuring instruments and unit meters?

1) Calibration of any measurement system is very important to obtain meaningful results.
2) In the case where the detection system and the measurement system are different, it is imperative to calibrate the system as an integrated whole in order to take into account the error-producing properties of each component.
3) Calibration is usually performed by making adjustments so that the reading device produces a zero output for the zero measured input and likewise it should display an output equivalent to the known measured input close to the background input value of scale.
4) It is important that any calibration of the measurement system is carried out under environmental conditions as close as possible to those under which the actual measurements will be made.
5) It is also important that the reference measured input is known to a much greater degree of accuracy – typically, the system calibration standard should be at least one order of magnitude more accurate than the desired accuracy of the measurement system.

Steps or precautions to be observed when calibrating a measurement system:

  • The specified environmental conditions must be maintained so that similar conditions prevail when the system is calibrated and when the
    actual measurements are taken.
  • The device to be calibrated is checked for any physical defects.
  • The standard measuring system used for calibration must be at least ten times more accurate than the desired accuracy of the measuring system, i.e. 10:1 accuracy ratio

Calibration procedure

The procedure for calibrating instruments is of two types, namely

(a) Primary calibration
(b) Secondary calibration

(a) Primary calibration

  • According to this procedure, a system is calibrated against a primary standard.
  • When calibrating flow meters, if the flow rate is determined by measuring time and volume or mass of fluid, it is called primary calibration.

(b) Secondary calibration

  • Under this procedure, a device that has been calibrated by primary calibration is used as a secondary standard for further calibration of other devices of lower accuracy.
  • A turbine-type flowmeter is used as a secondary standard to calibrate other flow devices.

Secondary calibration is of two types namely
(i) Direct calibration
(ii) Indirect calibration

(i) Direct calibration

  • In this procedure, a standard device is placed in series with the device to be calibrated.
  • Now the calibration is done by comparing the readings of the two devices in the desired range.

(ii) Indirect calibration

  • This procedure is based on the equivalence of two different devices adopting some concept of similarity.
  • Example: Flow measurement – ​​The similarity requirement is 'the Reynolds number must be equal'.

By comparing the discharge coefficient of two devices, calibration is done.

Errors due to calibration:

  • Any instrument must be calibrated before being put into use. Calibration is a process of providing a known input to the measurement system and taking the necessary actions to verify that the measurement system's output matches its input.
  • If the instrument is not calibrated correctly, it will present a reading with a greater degree of error. This is called a calibration error.
  • Calibration errors are fixed errors because they were introduced into the measurement system due to improper calibration.

Advantages of calibration:

The benefits of calibration are as follows,

  • The calibration meets the traceability requirements of national/international standards such as ISO 9000, ISO 14000, etc.
  • Calibration is proof that the instrument is working.
  • Confidence in using the instruments.
  • Traceability according to national measurement standards.
  • Interchangeability.
  • Reduced rejections, failure rate and therefore higher returns.
  • Better quality of products and services leading to satisfied customers.
  • Energy saving.
  • Cost savings.
  • Security.

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