Calibration

What is Calibration?:

• Calibration is comparison between a known measurement standard and the measurement using your instrument. It checks the accuracy of the instrument and it determines the traceability of the measurement.

Purpose of Calibration:

• The purpose of calibration is to gain confidence in the results being measured. Accurate measuring devices improve product quality and minimize risk of rejections during manufacturing processes. 
• The purpose of calibration is to minimize any measurement uncertainty by ensuring the accuracy of test equipment or measuring instrument.
• The accuracy of all measuring devices degrade over time caused by normal wear and tear. It may degrade very quickly or over a period of time and this depends on the type of the instrument and the environment in which it is used. Hence it is required to have the instrument or test equipment calibrated at regular intervals based on usage pattern.

When to do Calibration?:

• As recommended by the manufacturer.
• After any accidental drop / fall down during measurement (mechanical or electrical shock).
• Calibrate the instruments in highly critical areas more often than those available in less critical areas so as to prevent high rework costs / high rejections.
• A new instrument - manufacturers certificate is acceptable provided it is traceable to NPL.
• The operating conditions and workload of a measurement device also determine, how often it should be recalibrated.
• Stability history of the measuring device : If the instrument has a long history of being stable, then you can calibrate it less often. On the other hand, if the history shows that the instrument drifts quite often or fails frequently in recalibration, you must calibrate it more often.
• Calibration frequency defined in any regulatory requirement, customer requirement, as applicable.

Calibration Pyramid:

Calibration Agency:

NABL: National Accreditation Board for Testing & Calibration Laboratories

What is NABL?: 

• It is an autonomous society providing Accreditation (Recognition) of Technical competence of a testing, calibration, medical laboratory etc.,. for a specific scope following IEC/ISO17025 and few other relative standards. 
• It is one of the laboratories in India for quality and consistency in the results. 
• All the accredited laboratories have to follow ISO / IEC 17025 standard to maintain quality management system in calibration and testing.

Accreditation: 

• Accreditation is the third party attestation related to a conformity assessment body conveying the formal demonstration of its competence to carry out specific conformity assessment task. 
• NABL accreditation is not a one-time phenomenon. Laboratory accreditation is valid for a period of 3 years only. During these 3 years NABL conducts periodical surveillances of the laboratory on an annual basis. Accreditation can be revoked if the laboratory fails to maintain the standards of NABL requirements.
• NABL accreditation labs are rewarded more business, being an independent third party certification agency. 

What is the purpose of laboratory accreditation?:

• Laboratory accreditation agencies uses specific criteria and procedures to determine technical competence, thus assuring customers that the test, calibration or measurement data supplied by the laboratory or inspection service are accurate and reliable. 
• Laboratories are re-evaluated periodically by the accreditation body to ensure their continued compliance with requirements mentioned in the standard.
• Accredited laboratories usually issue test or calibration reports bearing the accreditation body’s symbol or endorsement, as an indication of their accreditation.

Important guidelines before submitting the instrument for calibration or material for testing to  outside lab:

• Laboratories are performing calibrations and tests outside the scope of accreditation, but the report / certificate issued does not have NABL logo. Hence customers availing services from laboratories should be vigilant and verify the scope of NABL before submitting the instrument for calibration or material for testing. 
• Laboratories can have either all or part of their testing and calibration activities accredited. They normally cover commonly used calibration services or tests in their NABL scope. 
• In most of the cases, people say, we are getting the instruments calibrated from NABL lab, but sometimes, it appears that the instrument range is not in the NABL scope of the lab. 

For example :

Vernier caliper is in the NABL scope for range 0-500 mm only.

Scenario 1:

In this case, calibration is performed by the lab and report with NABL logo is issued to the client. 

Scenario 2:

Let us say, we have Vernier caliper with max range of 1000 mm. In this case also, lab accepts the instrument for calibration, however, the certificate does not comprise of  the NABL logo. This means the client has to be vigilant on the scope of the lab before availing the services.

• Periodically, it is important to obtain the NABL scope from the laboratories in hard copy, however, most of the labs now a days are storing the updated copy of scope on their websites from where it can be downloaded. 

Calibration report Assessment:

1. Check presence of NABL logo on the lab report.
2. Check if the instrument or test equipment name / description, equipment code is correctly mentioned so as to avoid conflict issue during customer audits / internal conflicts.
3. Check for calibration date, due date, range of the instrument is correctly mentioned.
4. The master instrument used for calibration should not be due. For example: If the instrument is calibrated today, the due date of master instrument used for calibration should be after this date.
5. Depending upon the type of instrument, verify the error with respect to the acceptance criteria specified for that instrument.
6. If all above are found OK, mention "Verified and Found OK". The person verifying and conforming OK, should sign on the report with date.
7. If the report if not OK for above sr.no 1 to 4, co-ordinate with laboratory and get is corrected.
8. If the error is greater than acceptance criteria, decision has to be made usage of the instrument based on the application. Standard says, the instrument should be accompanied with error details so that anyone using the instrument considers the error into calculation.

Calibration In Industry:  

A person nominated for calibration activity in any industry has to perform below activities:

• Maintain a list of IMTE (Inspection, Measuring and Test Equipment) containing all the details of instruments, test equipment's, gauges used in the company.
• As per the due date, calibration plan is made and he / she retrieves the IMTE from respective location atleast a week before the calibration due date. Retrieve time can be decided depending upon the distance and lead time taken by the calibration agency.
• Ensure all IMTE are having calibration sticker at any point of point.
• Maintain history for each IMTE. This will help analyse the trends in error / repair history of the IMTE. It enables to take decision to buy the same equipment or go for other option based on the history of the equipment.