Hiệu chuẩn là gì

Before we define  what calibration is, I want you to understand below  scenario:
When you go to a market and buy some meat, you bought 1 kilo and paid it with your hard-earned money but when you checked it at home, it is only ¾ of a kilo, what would you feel?

You just gassed-up good for 2 days as your regular routine but only a day had passed and your meter indicates near empty – would this not make you mad?
You are baking a  cake and the instruction tells you to set the temperature to 50  deg C for 20 mins, you followed the steps and specification but your cake turned into a charcoal – will you not be upset?

Now, what do you feel if the above scenario happens to you? Of course, you may get angry, dismayed or worse complain to the services or product that you received and paid for, all because of the effect of the un-calibrated Instruments or units. This is why calibration is important in our daily life not just inside a laboratory or within manufacturing aspect. There is the involvement of quality, safety, and reliability.
Now let us define What calibration is.

Basics of Calibration

There are definitions of Calibrations by NIST or ISO that you can look for. But to easily  understand, below is a simple definition:
Calibration is simply the comparison of Instrument, Measuring and Test Equipment (M&TE), Unit Under Test (UUT), Unit Under Calibration (UUC), a Device Under Test (DUT) or simply a Test Instrument  (TI) of unverified accuracy  to an instrument or standards with a known (higher) accuracy to detect or eliminate unacceptable variations. It may or may not involve adjustment or repair.
It is making the instruments performs what it displays by referencing or adjusting it based on a Reference  Standard.
to ensure that you get what you have paid for;
Satisfy your expectations;
Create a win-win situations

What is a Reference Standard?
A reference standard is also an Instrument, or an equipment, or a measuring device with the highest metrological quality or accuracy than the Unit Under Calibration (UUC). It is where we make comparisons between the UUC and where measurement values are derived. It is also calibrated and is calibrated by a higher level laboratory with a traceability to a higher standard (See traceability below).

Why Calibrate?

We see the importance of calibration by basing it on the example above but in technical reason,
We need to calibrate because as components aged or equipment undergo environmental or mechanical stress, its performance gradually degrades.
This degradation is what we call the ‘drift’. When this happens,  the results or performance generated by a certain equipment will be unreliable where design and quality suffer.We cannot eliminate drift, but through the process of calibration, it can be detected and contained.
While detecting inaccuracy is one of the main reason for calibration, some other reasons are:
Customer requirements – they want to ensure that the product they buy is the most accurate one.
Requirements of a government or statutory regulations – they want to make sure that products produce are safe and reliable for the public
Audit requirements- as a requirement before achieving a certification like ISO 9001 certification
Quality and Safety requirements- a reliable and accurate operation through proper use of inspection instruments provides a great deal of confidence for everyone
Process requirements – some operations will not be executed unless equipment are verified to be calibrated, used in equipment or product qualifications as part of quality control. To ensure that the product produced is the most accurate and reliable.

Importance of Instrument Calibration

To establish and demonstrate traceability( I will explain traceability below). Through calibration, the measurement established by the instrument is the same where ever you are, it means that a 1 Kg of weights in one place is also 1 Kg in other places or wherever it reaches. You can use instruments regardless of the units or parameters it measures on different occasions.

To determine  and ensure the accuracy of instrument readings- resulting to product quality and safety
To ensure readings from the instrument are consistent with other measurements
To establish the reliability of the instrument making sure that they function in the way they are intended to be – resulting in product quality and safety
Provides customer satisfaction by having a product that serves what they have paid for.

What needs calibration?

All inspection, measuring and test equipment that can affect or determine product quality. This means that if you are using the instruments to verify the acceptance of a product whether to pass or fail based on the measured value you have taken, the instrument should be calibrated.
Equipment which, if out of calibration, would produce unsafe products.
Equipment which requires calibration because of an agreement. An example is a customer before progressing into a contract will see to it that the equipment that will produce their product are calibrated.
All measuring and testing equipment (standards) having an effect on the accuracy or validity of calibrations. These are the master standards that we used to verify other instruments or equipment for their accuracy.

What does a calibrated Instrument Looks like?

When you have your measuring instruments calibrated, see to it that it has a calibration label  where details of its calibration
date and due date are seen, also includes a serial number, certificate number, and person-in-charge of the calibration which depends on the calibration lab. Also, if needed, avoid seal is placed to protect it from an unauthorized adjustment.
A calibrated Instrument with labels are useless if the calibration certificate is not available, so be sure to keep it safe and readily available once requested.
Be cautious to check calibration certificates once received, not all calibrated instruments are performing the same as you expected, some have a limited use based on the result of the calibration. you must learn how to read or interpret the results in a calibration certificate.

When is Calibration Not Required?

Every measuring instrument needs calibration but not all measuring instruments are required to be calibrated. Below are some of the reasons or criteria to consider before having an instrument calibrated. This may save you some time and money.
It is not critical in your process ( just to display a certain reading for a purpose of functionality check ).
It functions as an indicator only ( for example: high or low and close or open)
As an accessory only to support the main instrument. For example, a coil of wire used to amplify a current. Current is measured but the amplification is not that critical, used as an accessory only to amplify a measured current.
Its accuracy is established by a higher or reference to a higher or more accurate instrument within a group. (for example a set of pressure gauges that are connected in series in which one of them is a more accurate gauge  where they are compared or referenced to).
If the instruments are verified regularly or continuously monitored by a calibrated instrument that is documented in a measurement assurance process. For example, a room thermometer that is verified by a  calibrated thermometer regularly.
If the instrument is a part of a system or integrated in a system where the system is calibrated as a whole. For example, a thermocouple that is permanently connected to the oven ( some thermocouples are detached after usage then transferred to other units).

What is Accuracy of an Instrument?

Accuracy is a reference number( usually in percentage) that indicates how close the measurement results to the true value. There is a true value which means that you have a source of known value to compare with.
The closer your measuring instrument reads or measure to the true value, the more accurate your instrument. This is what we are looking when performing a calibration, to determine how close ( or far) the reading of our instruments in comparison to the reading of the reference standard.

Where can we find the Accuracy of Instruments?

Original Equipment Manufacturer (OEM) manual or brochure ( Specifications)
Publish standards or handbook
Calibration Certificates and records
A requirement from the process
When calibrating, see to it that the reference standard has a higher accuracy than the  Unit Under Calibration (UUC),  usually, a good rule of thumb is to have an accuracy ratio of 4:1.
This means that if your UUC has an accuracy of 1, the reference standard to be used should have at least a 0.25 accuracy, four times more accurate than the UUC (1/0.25 =4). To make it more clear, since most of the time, accuracy is expressed in percentage, the closer the value is to ZERO, the more accurate the instrument.

What is a Tolerance of Instruments?

Tolerance is closely related to accuracy at some point but for clarity, it is the permissible deviation or the maximum error to be expected from a manufactured component and expressed usually in measurement units ( example are psi, volts, meter, etc.).
We are performing a calibration to determine if the reading is within the tolerance specified, it may be less accurate but if it is within the tolerance limit that we have specified or specified through the requirements of the process, it is still acceptable. This is where a pass or fail decision can be brought out.
Tolerance is the combination of many factors which includes:
Process requirements
The capability of  measurement equipment
Manufacturer’s tolerance
Published Standards

What is Precision in Measurement?

Precision is the closeness of a repeated measurement to each other. Precision signifies good stability and repeatability of instruments but not accuracy. A measuring instrument can be highly precise but cannot be accurate, you can only correct this if you perform calibration. This is where tolerance is observed where reading should stay within the tolerance limit in order for the data to be acceptable.

Below are the relationships of Accuracy, Precision, and Tolerance to  understand better

What is Traceability in Calibration?

As it was stated, calibration is the comparison of an instrument to a higher or more accurate instrument. These higher accuracy instruments are called the reference standard and sometimes also known as the calibrator, a master or a reference.
This reference standard that is used to calibrate your instruments has also a more accurate master being used to calibrate it and so on and so forth until it reaches the main source of that certain highest accuracy (measurement result).
There is an unbroken chain of comparison being linked from top to bottom of the chain. It is passed to local from international  standards or the BIPM (Bureau International des Poids et Measures (International Bureau of Weights & Measures))
.This means that the 1 kilogram you have is also 1 kilogram no matter where you go. There is unity to every measurement. Traceability can be determined through its calibration certificate indicating the results and reference standard used to calibrate your instrument.

Why is Traceability in Measurements Necessary?

For companies engaged in manufacturing and engineering,  ensures that parts produce or supplied have the same or acceptable specifications when used by customers anywhere. Compatibility is not an issue.
Requirement by relevant  laws and regulations to guarantee product quality.
Requirement from a contract agreed by two parties ( contractual provisions)
Statutory requirements for safety

Where can we find the traceability information of a Calibrated Instrument?

Since traceability is very important, we should have a knowledge on how to determine or check the traceability information of a certain calibrated instruments. We can find it through its calibration certificate, this is one of check items in a calibration certificate once it is calibrated by an authorrized laboratory.
In a calibration certificate, there is a traceability information written usually in the middle part together with the reference standard used or at the bottom and or even both.
This is a requirement so it must not be neglected if the laboratory is a competent one. Moreover, the most important one is the result of its measurement uncertainty, it should be reflected  with the data results to ensure that you have a traceable calibration done by an accredited laboratory.

What is Measurement Uncertainty?

Calibration is not complete without Measurement uncertainty or Uncertainty of Measurement. This is where the unbroken chain of comparison is being connected or linked in.
Measurement Uncertainty is the value being displayed to quantify the doubt that exists on a specified measurement results. Since No measurement is exact, there is always an error that is associated to every measurement. And in order to determine the degree, effect or quantity of this error that exists in every measured parameter, we compute or estimate its  Measurement Uncertainty.
During uncertainty computation or estimation, we identify all the valid sources of errors that have an effect on our measurement system. It can be from our procedures, instruments, environment and many more. We evaluate and quantify the value of each error and combine them to a single computed value.