Introduction to Calibration

Definition: Calibration can be defined as a test in which known values of measurand are applied to the transducer and corresponding output readings are recorded under specific conditions. Calibration is the process of comparing sets of measurements for accuracy i.e. comparing one measurement of known verified magnitude or correctness on one device with another similar measurement on a second device. Definition by BIPM is that “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.”

Calibration Processes: Calibration is the process of comparing measurements. Measurements are taken on two devices – the first measurement being that of a known magnitude or correctness and the second one being a similar measurement on another device. The device holding the known magnitude or correctness is referred to as the ‘standard’. The second device is commonly known as the test unit or test instrument.

Modern Calibration Processes: Predicting the future, it can be foreseen that the need of accuracy which is already known and the need to have consistency and the standards which are comparable. internationally has led to the starting of national laboratories. Moreover, in manifold countries a National Metrology institute will exist which will uphold measurement standards which are primary. and can be used to provide trackability of customer's instrument by process of calibration.

Purpose and Scope: Instrument error can occur due to a variety of factors: drift, environment, electrical supply, addition of components to the output loop, process changes, etc. Since a calibration is performed by comparing or applying a known signal to the instrument under test, errors are detected by performing a calibration. The process of calibration starts with the measuring instrument design which needs to be calibrated. The design should be able to "hold a calibration" through the interval of calibration. In different words, the design need to be able to perform the measurements which are in range of "engineering tolerance" when it is performed within the span of stated environmental conditions over some reasonable period of time. Having a design with these characteristics increases the likelihood of the actual measuring instruments performing as expected. Basically, the purpose of calibration is for maintaining the quality of measurement as well as to ensure the proper working of particular instrument.

Manual and automatic calibrations: Calibration methods for modern devices can be both manual and automatic, depending on what kind of device is being calibrated.

Manual Calibration: It is the process of calibration which is done by ourselves rather than just programming in any type of machine or something we need to take all readings of the instrument and compare those readings with the standard measurement. However the accuracy in manual calibration process is low compared to automatic calibration process.

The procedure is complex, but overall it involves the following:

• (I) depressurizing the system, and turning the screw, if necessary, to ensure that the needle reads zero,
• (II) fully pressurizing the system and ensuring that the needle reads maximum, within acceptable tolerances,
• (III) replacing the gauge if the error in the calibration process is beyond tolerance.

Automatic calibration: It is the process of calibration in which process of calibration takes place automatically without any human interference. we can use the example of a 3666C automatic pressure calibrator, which is a device that is made of a control unit which consists the electronics that drive the system, a pressure intensifier is used to compress a gas such as Nitrogen, a pressure transducer used to detect desired levels in a hydraulic accumulator, and accessories such as liquid traps and gauge fittings.

Types of Calibration: Pressure Calibration: This is widely used calibration process in which gas and hydraulic pressure are measured across a broad spectrum.

Electrical Calibration: This is used to measure voltage, current frequency and resistance.

Mechanical Calibration: Mechanical calibration is used to measure a number of dimensional, mass, force, torque and vibration elements.

Temperature and Humidity Calibration: It usually takes place in a controlled environment.

Calibration Determination: There are various methods to determine the calibration of different instruments, which are described below. All of the values have their own use.

Calibrator values: When you take the measurement of instrument with the calibrator the value obtained after the process is called calibrator values. Model and weight in calibration curve fitting. Number of replicate in each calibrator level. Stored calibration curve use.

Precaution: Whenever you are going to use a calibrator make sure that calibrator as well as the instruments is cleaned perfectly. If it didn’t then there will be chance of getting error.