Calibration defines the accuracy and quality of your measurement. But just buying a certified, calibrated equipment is not the answer; you need to service and maintain the calibration of your equipment throughout its lifetime for reliable, accurate and repeatable measurements
SHWETA DHADIWAL BAID
NOVEMBER 2010: Calibration defines the quality and accuracy of your measurement. It is the process of parameter verification of any equipment against a known standard so that the quantities around the world show similar results. “Accurate measurements play a vital role at each stage in development and manufacturing of a quality product for consistent and reliable performance,” says Anirudha Kela, managing director, Scientific Mes Techniq. A reliable and accurate result brings confidence in you as well as in your product.
The goal of calibration is to minimise the measurement uncertainty by ensuring the accuracy of your test equipment. S.M. Sulfikaruddin, vice president, support solutions unit, EMC, Agilent Technologies, says, “All the measurement processes are accompanied by errors and uncertainties, and they cannot be eliminated. Calibration quantifies and controls the errors or uncertainties of the measurement processes to an acceptable level.”
Repeatability is a great virtue of equipment even ahead of accuracy, which can be defined only by quantification.
“In measuring, we quantify a thing, which firms up our knowledge, while qualifying a thing is never precise, like something is very good, too small or very big. When we say the voltage is 1.203V, it is precise, and it is neither less than 1.202V, nor more than 1.204V. The moment an equipment is calibrated by a standard that is the best earmarked, accuracy hierarchy is set up, irrespective of its origin, cost and newness. We therefore say we pass on the accuracy of a million-rupee meter to this thousand-rupee meter with one stroke of calibration that may cost just a few hundred rupees,” explains S. Desai, managing director, Kalashri Electronics.
There are accreditation bodies around the world that have been evaluated by peers as competent to enhance the acceptance of products and services across national borders. This is to create an international framework to support international trade through the removal of technical barriers.
“The momentum is huge amongst world’s leading test agencies regarding calibration. Everyone is demanding ISO/IEC 17025 accreditation including measurement uncertainty data,” shares David Bisenius, an entrepreneur and internationally recognised regulatory expert in product safety.
Calibration is not a one-time thing. It’s a repeatable process to maintain the accuracies. The interval for re-calibrating your equipment is influenced by a lot of factors including the type, usage and environmental factors.
Traceability in calibration
Calibration is an unbroken chain of comparisons with national and international superior equipment. There are about 123+ countries bound to ISO 9000 international standards to get their equipment calibrated in such a way that they maintain the same quality standards.
“Calibration should be traceable to national standard with unbroken chain of traceability, i.e., the equipment which is used as master should be calibrated with better accuracy master until the traceability reaches to National Physical Laboratory (NPL) or international standards,” explains Desai.
In India, the highest levels of standards are maintained by NPL. “NPL gets their equipment calibrated from National Institute of Standards and Technology (NIST), Washington,” shares Desai.
There are four Electronic Regional Test Laboratories (ERTLs) and government labs like Electronics Test & Development Centre (ETDC) and Centres for Electronics Test Engineering (CETE) that get their equipment calibrated from NPL and provide calibration services to others. Apart from this, there are private labs, either owned by the original equipment manufacturers (OEMs) or some third party, which provide calibration services and are accredited by National Accreditation Board for Testing and Calibration Laboratories (NABL). NABL provides laboratory accreditation as formal recognition of technical competence, while ISO certification is on quality system management. Thus most of test and measurement equipment manufacturers maintain their NABL-accredited calibration laboratory.
The hierarchy followed in calibrating one equipment from the master—which, in turn, is calibrated from higher levels of accuracies and standards—is maintained around the world. “You must test your equipment by a master that is at least a factor of four better than the equipment being calibrated,” suggests Sulfikaruddin. If you are consulting a private laboratory, you must first ask for traceability of their standards and compare it with your equipment’s accuracy. “Without traceability, the calibration accuracy may be questionable,” comments Bisenius.
Calibration: Challenges to overcome
1. Identifying calibration needs and devoting time for calibrations
2. Lack of knowledge related to calibration cycle, calibration process and accredited laboratories
3. Lack of appreciation for the role of calibration
4. Calibrating the master/reference equipment and maintaining ideal environment for it
5. Calibration of equipment that don’t have a set procedure and you have to test them with first principles
6. Lack of training for the calibration technician/engineer
7. Undetermined traceability
Training the technician
All the measurement processes are accompanied by errors and uncertainties that cannot be eliminated. Sulfikaruddin says, “Calibration quantifies and controls the errors or uncertainties of the measurement processes to an acceptable level.” This makes calibration a challenging job that must be handled by a trained technician or engineer.
According to Kela, the uncertainty of results is affected by 4 ‘M’s—man, machine, method and material. Therefore utmost importance is given to train the people who perform calibration of equipment. They are also trained to use standard calibration methods and standard accessories to get accurate results.
Adjustment is not calibration
Most users are always confused between calibration and adjustment. It is important for the user to understand the difference between the two. When you are experimenting in a lab, and you do not see a ‘zero’ reading when the equipment is not connected, you make ‘adjustments’ to remove the offset. This is not calibration.
“Calibration is process of parameters verification of any equipment against a well-known standard. Calibration report is a data that shows what parameters of the equipment are within the manufacturer specification or what are out of specification. It does not include adjustment of parameters that are out of specification,” explains Gaurav Tyagi, senior engineer-application and support at Aimil.
“Adjustment is a procedure in which parameters are adjusted within the specification range and usually done by the manufacturer or user,” Tyagi adds. Although most companies maintain high levels of standards, they need to calibrate the equipment regularly.
Agreeing to this, Kela says, “Drift in accuracies over a period is unavoidable but can be detected in time, corrected mathematically and adjusted by calibration, i.e., by using a reference or a standard equipment. All calibration labs do not have capabilities of adjustments and fine-tuning.”
For how long is your calibration certificate valid? Or how frequently you need to calibrate your equipment? This is a very subjective question. The answer depends on many factors like the type of the measuring equipment, usage of the equipment, man-handling and atmospheric changes that may influence the reading. In all the cases, these are variables.
“The calibration cycle of the equipment depends on the sensitivity of the equipment, environmental condition, usage, etc. Hence the calibration cycle varies from six months to 36 months depending upon the type of the equipment. Calibration cycles are generally recommended by OEMs,” explains Sulfikaruddin.
“A regular user can get his equipment calibrated by having it checked by the OEM or sending it to an NABL-accredited laboratory with the details of the specifications of the product. Usually, it is required to be calibrated after an interval of twelve months from the date of previous valid calibration,” shares Chandmal Goliya, managing director, Kusam Meco. He recommends recalibration at the earliest to avoid further errors if the readings in between are suspect.
International Organisation for Standardisation (ISO) issued ICO/IEC 17025 as the primary standard used for testing and calibration laboratories. These standards specify the general requirements for the competence to carry out tests and calibrations using standard methods, non-standard methods, and laboratory-developed methods. There are many commonalities between ISO 17025 and ISO 9000 standards, but the latter does not include the competence to the equation.
All the accredited laboratories around the world follow these ISO 17025 standards to improve their ability to consistently produce valid results. ISO 17025 includes the technical requirements that determine the correctness and reliability of the tests and calibrations performed in a laboratory
Citing the case of how low-end, low-cost equipment are handled, Goliya says, “The user tends to handle these equipment with very less care and precaution. As these are already low-budget equipment, such mishandling disturbs the accuracy and needs frequent recalibration. Mishandling results in erroneous results apart from damaging these equipment.”
Some equipment have a highly sensitive electronic circuit. For example, a digital multimeter may be calibrated once in six months, while an automated test equipment or a tester may need full calibration of all its boards and resources once in two weeks. In equipment that have time-base oscillators involving frequency counters and sensors, signal generators, spectrum analysers, precision digital multimeters, network analysers and LCR meters, drifts are caused very easily by usage and environmental conditions. Such equipment need calibration at least once a year to meet their specifications and accuracy.
Desai feels, “The user has to fix the periodicity after seeing the results for three to four consecutive calibrations. For example, if every year it is exactly the same or within limits, he can extend the periodicity. If every year it becomes worse and he has to adjust or write down the correction chart, he has to compress the schedule. Generally, electronic equipment periodicity is one year, while for mechanical items like micrometers it is six months. Very precision equipment may require three months. If the equipment undergoes any kind of repair, it has to be calibrated after repair.”
“Any equipment which has to sustain shocks, vibrations, violent change of temperature or pressure, gets disturbed. Hence the master is kept in the lab in an ideal environment. Masters that are used for on-site calibration and are subjected to vibrations and change in environment due to travelling or handling, are checked and corrected with masters in the labs after they come back to base,” explains Desai.
“Normally, manufacturers of electronic instruments specify accuracy for a period of time. It is advisable that a user maintains and follows the calibration schedule and gets instruments calibrated at regular intervals recommended by the manufacturer,” cites Kela.
Advancements in microprocessor and microcontroller technology, sensor technology along with an intelligent software algorithm and connectivity to computers have led to the development of a new feature called ‘self-calibration’ and ‘auto-tuning.’
“Today, test and measurement equipment are available with inbuilt calibration check points that can be used as reference points for getting a calibration done. Calibration in itself is the next big opportunity in the Indian test and measurements industry as there are very limited options available for the industry,” cites Manish Kwatra, CEO, MetroQ Instruments.
In-house calibration vs third-party labs
It’s an ongoing debate in many manufacturing as well as R&D organisations whether to have an internal calibration lab or outsource the calibration activity to an external accredited lab. There are situations where you may require to re-calibrate the equipment every day. It is totally subjective to the nature of organisation and use of measurement equipment.
“Whether you manage internally or externally, boils down to a business assessment. Large shops are likely to save money by outsourcing, while small installations may find it less expensive to manage it internally,” comments Keith Kidd, director-packet systems testing at Vizon Labs.
Calibration is a process and needs to be managed like any other activity in the organisation. Kidd adds, “Regardless of how one manages the calibration process, it is necessary to ensure that your training and controls for the calibration system are well known to all. The most likely hiccup with calibration is getting the organisation to follow the process, which takes discipline.”
Philip Wright, director at QSO, a weighing equipment and scale service company, explains a case of his customer for whom his company carried out scale and weighing equipment calibration twice a year. The organisation had its own test weights which were also calibrated on a yearly basis.
“We used to carry out the equipment calibrations and issue a certificate after completion of test once or twice a year. However, on a daily basis, at the start of every shift the operator using the equipment would place the required amount of the test weights onto the equipment as a check and, if necessary, re-calibrate the equipment himself,” he tells. You may be thinking what was the point in having an outside company come in and calibrate the equipment in the first place then. Well, it all boils down to that little bit of paper and traceability for the audits. Although test certificates are based on the accuracy at the time of test, if you feel the need to have more frequent checks then the more affordable way is to have a trained calibration technician and an in-house lab.
Every R&D firm must have reference equipment against which samples are checked to have confidence in the measurement. Desai says, “It has to be a standalone reference equipment that can be used for verification. Firms producing rotating machines, tacho generators and motors must have a master precision tachometer that does have valid calibration all the time. Firms producing temperature sensors must have a calibrator or master precision resistor thermometer to check against that. Mechanical gauges require master slip gauges to check the wear and tear on instruments that are used to measure the finished goods very often.”
“Test and measurement equipment distributors follow a mixed regime for calibration. They have their own calibrating equipment which are traceable to ERTL or NABL. Also, there is dependence on the respective OEM for calibration of their own brand equipment,” shares Kwatra.
Importance can’t be undermined
In today’s world of competition, the only differentiator for your product is quality, which cannot just be assumed. To produce quality equipment, you need measurements that are accurate, reliable and repeatable.
“Not only for all the manufacturing processes but also for R&D firms, it is necessary to use calibrated equipment to ensure reliability of results. Correctly performed calibration increases the productivity, optimises resources, and assures consistency, comparability and compatibility of products, services and acceptability,” says Kela.
The importance of calibration cannot be undermined by any user. When performing any type of measurement, you must have confidence in your results that they are accurate and within specifications. Testing and measurement with calibrated equipment brings this confidence and is a form of quality assurance.
“There are many labs in India which provide calibration certificate and help maintain the accuracy of equipment. It’s just the matter of awareness at all the levels,” says Goliya.
The author is a senior technology journalist at EFY