Table of Contents

Calibration

The ability of all measuring instruments to measure accurately and reliably is to be proved, to get meaningful results. For this, the results of measurement are to be compared with higher standards.

The instruments, gauges to be used for measurement should be of known accuracy in order that the results obtained are meaningful. In order to identify the errors and rectification of errors, the instruments are compared with masters or standards. This act of comparison is known as calibration.

Definition of Calibration :

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

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

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

Calibration Process Example – Manual Calibration

Purpose of Calibration :

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

1) The calibration of any measuring system is very important to get meaningful results.
2) In the case where the sensing system and measuring system are different, then 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 carried out by making adjustments such that the readout device produces zero output for zero-measured input, and similarly, it should display an output equivalent to the known measured input near the full-scale input value.
4) It is important that any measuring system calibration should be performed under environmental conditions that are as close as possible to those conditions under which actual measurements are to be made.
5) It is also important that the reference measured input should be known to an as much greater degree of accuracy – usually, the calibration standard for the system should be at least one order of magnitude more accurate than the desired measurement system accuracy.

Steps or precautions to be observed during calibration of a measurement system :

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

Calibration procedure

The procedure for calibrating instruments is of two types namely

(a) Primary calibration
(b) Secondary calibration

(a) Primary calibration

As per this procedure, a system is calibrated against a primary standard.
While calibrating flow meters, if the flow is determined through measurement of time and volume or mass of fluid, then it is termed as primary calibration.

(b) Secondary calibration

As per this procedure, a device that has been calibrated by primary calibration is used as a secondary standard for further calibration of other devices of lesser accuracy.
A turbine type flow meter 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 calibration is done by comparing readings of the two devices over the desired range.

(ii) Indirect calibration

This procedure is based on the equivalence of two different devices adopting some similarity concept.
Example: Flow measurement-Requirement of similarity is ‘Reynold’s number should be equal’.

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

Errors Due To Calibration:

Any instrument has to be calibrated before it is put to use. Calibration is a process of giving a known input to the measurement system and taking necessary actions to see that the output of the measurement system matches with its input.
If the instrument is not calibrated properly, it will show reading with a higher degree of error. This is called a calibration error.
Calibration errors are fixed errors as they have been introduced into the measurement system because of improper calibration.

Advantages of Calibration :

Benefits of Calibration are as follows,

Calibration fulfills the requirements of traceability to national/ international standards like ISO 9000, ISO 14000, etc.
Calibration is proof that the instrument is working.
Confidence in using the instruments.
Traceability to national measurement standards.
Interchangeability.
Reduced rejections, failure rate thus higher return.
Improved product and service quality leading to satisfied customers.
Power saving.
Cost-saving.
Safety.

Reference: https://en.wikipedia.org/wiki/Calibration