Calibration is the process of comparison at which a known value of standard instrument with higher accuracy is applied to a device under calibration (DUC) and a corresponding output reading is recorded under the specified condition of DUC.
It is to bear in mind that Calibration is just the conduct of comparison, not relates to consequent adjustment.
Calibration inaugurates new credence of Measurement Uncertainty. Uncertainty is the result of measurement as the statistical scatter of values attributed to measured value that reflects the incomplete knowledge of quantity value. In order to evaluate and identify factors relates to calibration instruments that affect calibration accuracy and that’s why calibration is executed.
Through an unbroken chain of comparison, the property of measurement results whereby it can be related to the appropriate standards is referred to as Traceability.
All calibration should be executed traceable to nationally or internationally accepted standards like the International System of Unit, SI. To retain the primary standard of measurement, there subsists the National Metrology Institute (NMI) that supports metrological infrastructures in that country from the top level of standard to plant’s process instrument. As an example the NMI’s are NIST in the USA, NPL in the UK, PTB in Germany.
Figure-01: Traceability Pyramid
To realize Traceability pyramid from bottom to pick a level, a real-time example is given below:
From the traceability pyramid, it is noted that it is not essential the working standard is calibrated by primary standard rather than it is meant that calibration executed are traceable to NMI through all the standard used to calibrate the standard, no issue how many steps exist between working standard and NMI. For example, US customers can send their instruments to NIST directly before calibration from another calibration lab though it is an expensive job.
Normally, the working standard instrument is sent to a standard lab that has more accurate test instruments. When moving up along the pyramid, it is a best practice to maintain a 4:1 accuracy ratio. For example, if a customer’s Clamp Meter AC current accuracy is 0.4% FS (Full Scale, FS), it should be calibrated with a standard of 0.1%FS or better. It is significant to mind that to move up pyramid, the cost of calibration instrument and accuracy will increase.
In order to promote the quality of the calibration, it is necessary to establish traceability that is satisfied by a difference to a standard that is directly or indirectly anticipated to national standards, international standards or certified reference material. This may be performed by a national standard laboratory-like in Bangladesh BSTI (Bangladesh Standards and Testing Institute) maintained by governments or by the private companies offering metrology avail.
The quality management system offers a fruitful metrology system that annexes with methodical, repeated and documented calibration of all measuring equipment. To meet this traceable action ISO 9000 and ISO 17025 is required to set out how they can be quantified.
In order to achieve the quality of calibration services, traceable uncertainty statement to a defined confidence level often consort with calibration value. According to physical quantities measuring devices and instruments are denomination which varies internationally such as NIST 150-2G in the US, NABL-141 in India. By using two standards that covers different instrument which measure different physical quantities e.g. Electrical parameter (voltage, current, resistance, power, frequency, capacitance, inductance), mechanical quantities ( pressure gauge, vacuum gauge, pressure switch, limit switch, transmitter, balance), thermal properties ( temperature switch, temperature gauge, temperature controller), sound, light, vibration, time, electromagnetic radiation and ionization radiation etc. Different standard used for different test devices such as oil or gas deadweight tester is used for pressure gauge/ pressure switch calibration, IR (Infrared) calibrator is performed for Non-Contact temperature measurement device calibration and multiproduct calibration is used for Electrical instrument calibration.
To ensure the reliability of the instrument and increase our confidence level, it is necessary to calibrate our instrument.
There are some following reasons to calibrate instrument:
The instrument may damage very early or over a long period of time that actually depends on type of instrument and the environment where it is being used. So, generalization is the accuracy of measuring instrument is improved by calibration job and accurate measuring device rectifies instrument product quality.
5.1 Case-01 Process Plant
Figure-02: Plant Process
From the above figure-02, we noticed that there is process fluid inside the process vessel which is burnt by the burner. And there is two transmitter- pressure and temperature transmitter.
Pressure transmitter connects with pressure indicator (PLC Controlled) by 24V loop supply. By measuring pressure of process fluid, the pressure transmitter provides information to pressure indicator by 4-20 mA loop current. Suppose, pressure indicator operates 0-300 psi; when the pressure transmitter provides 4 mA, pressure indicator shows 0 psi; when the pressure transmitter gives 20 mA, the indicator shows 300 psi. And according to PLC output, the pressure valve will open or close.
And for same working principle for temperature issue. By sensing temperature inside of the vessel via a temperature sensor, the temperature transmitter provides information to temperature indicator by 4-20 mA loop current. Depends upon temperature indicator output Flow control valve is open or close accordingly and can control the flow of natural gas.
So, if error produces in temperature or pressure measurement at any instrument, error in fuel control, then inefficient use of fuel, finally huge gross loss arises in production. That’s why it is necessary to calibrate all of your instrument periodically to prevent your annual loss or to increase your revenue.
Figure-03 describes the flow diagram of process plant.
Food manufacturing plant totally depends on accurate temperature measurement by using different types of thermometer probes, thermocouple thermometer. If their thermometer is not calibrated properly by manufacturing or safety standard then it may provide invalid reading which could spoil or destruction the food as a result customer could be risk of food poisoning or other serious food injuries. This would lead to loss the revenue of company through food defrost and possible legal action is taken by government.
In shipping container, mines, petroleum, or other sophisticated lab- gas detector is used to measure precise concentration of gases. If inaccurate measurement of gas level is obtained from uncalibrated instrument then faulty data analysis would produce catastrophic effect which could build toxic gases and arises possibility of detonation.
So, after all of the discussion, we can judge that we need to calibrate our instrument to saves money, improves safety, gets certification and keeps instrument working longer.
Before starting calibration services, in order to verification of conformity with standard and regulation, demonstration of competence and risk assessment and its management- accreditation is necessary. In every country, there exists an accreditation body like BAB (Bangladesh Accreditation Body) in Bangladesh, NABL for India. BAB is the national authority body with responsibility of accreditation in Bangladesh and maintain APLAC/ILAC MRA status in compliance with international norms.
It is a matter of sorrow that many of us think- if we calibrate our instrument from outside Bangladesh, it will add more value; it is totally a wrong concept. This wrong concept firstly wastage of money as well as time-consuming tasks. Since as of now BAB has 10 accredited calibration laboratories in Bangladesh in different fields such as Mechanical, Thermal, Electrotechnical then why it is necessary to go outside Bangladesh? If we do calibration from our internal resources, it will save our foreign currency consequently reduce our instrument downtime.
The cost of calibration is too much lower than the hidden cost and risks relates with the uncalibrated instruments. To ensure the pitfalls associated with readings are in tolerance limit, therefore, it is highly recommended that measuring instruments are calibrated regular basis from a reputed company.