The accuracy of all measuring devices degrade over time. This is typically caused by normal wear and tear. However, changes in accuracy can also be caused by electric or mechanical shock or a hazardous manufacturing environment (ex., oils, metal chips etc.). Depending on the type of the instrument and the environment in which it is being used, it may degrade very quickly or over a long period of time. The bottom line is that, calibration improves the accuracy of the measuring device.
There are three main reasons for having instruments calibrated:
Yes: Demonstrable control of measurement and test equipment is required. Part of this is ensuring that instruments are calibrated on a rational periodic cycle, and that records are maintained and reviewed.
Yes: even the most precise measurement instrument is of no use if you cannot be sure that it is reading accurately – or, more realistically, that you know what the error of measurement is.
By checking the instrument against known reference standards that have themselves been calibrated in a chain of measurements that can be traced back to agreed International Standards – the system of SI units – for example the Volt; Ampere; Watt; meter; litre. (These standards have replaced the “National Standards” that were for many years the traceability requirement).
This depends on how important the measurements being made are to your product or service; the degree of wear and tear that the instrument will experience in service; the stability of the instrument itself and a review of the calibration records that already exist to determine whether adjustment has been needed previously. OTC recommends a starting periodicity of 12 months for most instruments with an increase in calibration frequency (to 6 or 9 months) if adjustment is required, and a reduction in periodicity to 2 years after a sequence of annual calibrations has shown that adjustment has not been needed.
A measuring device should be calibrated:
Hidden costs and risks associated with the un-calibrated measuring device could be much higher than the cost of calibration. Therefore, it is recommended that the measuring instruments are calibrated regularly by a reputable company to ensure that errors associated with the measurements are in the acceptable range.
Measurements are part of quality management systems. According to clause 7.6 of ISO 9001, you have to determine which measurements are necessary and which measurement devices are needed to provide evidence of conformity of a product to determined requirements. Starting from the product you are manufacturing, you must identify the parameters to be measured and monitored during production.
You could begin with the inspection of incoming material and parts or components which should be checked for compliance with specifications.
If, for example, you ordered sheet metal of a certain thickness, you should check it before commencing production. You must decide whether a vernier caliper or a micrometer screw is needed to determine the thickness with sufficient accuracy.
In a similar way, compliance with specifications has to be measured during production. Therefore, processes have to be established to ensure that the necessary measurements are carried out in a manner consistent with the requirement.
According to ISO 9001: ‘Where necessary to ensure valid results, measuring equipment shall be calibrated or verified at specified intervals, or prior to use, against measurement standards traceable to international or national measurement standards; where no such standards exist, the basis for calibration or verification shall be recorded.
Subclauses of clause 7.6 also stipulate adjustments, identification and protection of the measuring equipment. Records of the results of calibration or verification must be maintained. In addition, measurement, analysis and improvement processes shall be planned, implemented and monitored (clause 8). The main goal is to demonstrate that all measuring processes comply with the requirements to demonstrate conformity with specifications, are documented and properly implemented.
Metrology is a technical term meaning all activities and procedures related to measurements. The ultimate goal of metrology, also defined as ‘the science of measurement and its application’ (International Vocabulary of Metrology, 2010), is to ensure correct, comparable and reliable measuring results.
Metrology can be subdivided into the following areas:
Typical issues are calibration procedures and calibration intervals, control of measurement processes, and management of measuring equipment.
Measurement enters into practically all commercial operations, from trading in bulk goods (petroleum, natural gas, metal ores) to the retail sale of goods to the public in the marketplace. In particular, international trade in manufactured goods and production processes using parts and components manufactured in different regions of the world require correct measurements based on an international metrology system. This can be illustrated by the examples below.
Suppose you want to know the diameter of a pipe. Do you want the dimension in centimetres or inches? What is requested by your customer? The centimetre and the inch are units belonging to two different systems of units. Whereas the centimetre is a submultiple of the metre, the unit of length of the International System of Units (SI), the inch, belongs to the so-called Imperial Measurement
System. The SI is the recommended system and is applied worldwide, whereas imperial units are used in a few countries or for specific applications.
‘Time’ is the quantity most often measured. With today’s technology, the correct time is disseminated via radio, television, the telephone, the Internet and by satellite. One example of ultra-precise time measurements is the global positioning system, GPS, where time signals of atomic clocks from at least three satellites are used to calculate the position of the receiver on the ground with an accuracy of a fraction of a metre. These extraordinarily accurate results can be achieved only under the condition that each clock produces time signals of the same accuracy. This technology helps to make shipment of goods faster and safer, it facilitates and accelerates the exchange and retrieval of information and constitutes the base of electronic trade and commerce (Secrets of Electronic Commerce, 2009).
In global markets, certificates are often required as evidence of the compliance of products or services with specified standards or regulations. In many cases, conformity assessments and the checking of compliance with standards or regulations require measurements and tests.
Measurements and tests must therefore be correct within specified limits, comparable and reliable to ensure confidence in certificates.
In general, the accuracy of measuring instruments is achieved through regular calibrations. Calibration involves comparing a measuring instrument with a more accurate measurement standard. National measurement standards, which usually provide the most accurate measurements in a country, are compared with international or other national standards to ensure the correct dissemination of units worldwide. If an unbroken chain of documented calibrations exists from the highest standard down to ordinary measuring instruments, then the measurements achieved are called ‘traceable’.
The costs and risks of not calibrating
Unfortunately, calibration has costs associated with it and in uncertain economic times, this activity can often become neglected or the interval between calibration checks on instruments can be extended in order to cut costs or simply through a lack of resources or manpower.
However, neglecting calibration can lead to unscheduled production or machine downtime, product and process quality issues or even product recalls and rework.
Furthermore, if the instrument is critical to a process or is located in a hazardous area, allowing that sensor to drift over time could potentially result in a risk to employee safety. Similarly, an end product manufactured by a plant with poorly calibrated instruments could present a risk to both consumers and customers. In certain situations, this may even lead to a company losing its license to operate due to company not meeting its regulatory requirements. This is particularly true for the food and beverage sector and for pharmaceutical manufacturers.
Weighing instruments also need to be calibrated regularly.
Determining the correct mass of a product or material is particularly important for companies that supply steel, paper and pulp, power, aviation companies, harbors and retail outlets, who invoice customers based on the mass of what they supply (fiscal metering). These companies need to prove not only that the mass is accurate but also that the equipment producing the readings was correctly calibrated.
Invoicing in these industries is often based on process measurements.
There is therefore a growing need to have the metrological quality of these weighing instruments confirmed by calibration.
Product manufacturing also depends on accurate masses and so laboratories and production departments in the food and beverage, oil and gas, energy, chemical and pharmaceutical industries, also need to calibrate their weighing instruments.
All measuring and test equipment should be calibrated. Calibration is used to:
Calibration has to be carried out because the performance of measuring and test equipment may change with time as a result of the influence of the environment to which it is exposed, wear and tear, overload or improper use. The accuracy of the measurement and test equipment should be checked before use and regularly calibrated or after exposure to influence factors. Recalibration is not necessary for certain simple types of measuring instruments made of glass such as measuring cylinders, pipettes, burettes, or certain thermometers, if used within the working conditions they were designed for.
During calibration, the value of a quantity measured by the equipment is compared with the value of the same quantity provided by a measurement standard. If you have instruments of different accuracy classes for the same quantity and the same measuring range, at least the instrument with the highest accuracy – also known as ‘precision instrument’ − should be calibrated by a calibration laboratory, preferably by an accredited calibration laboratory.
The calibrated precision instruments can be used for in-house calibrations of instruments of lower accuracy. Details of the calibration such as a short description of the calibration method and/or a sketch, the standard used, the results obtained, the date and the name of the operator should be documented and stored together with the operation manuals and other documents relevant for the instruments.
Usually, the result of a calibration (or measurement) should include a calculation of the uncertainty. This is a requirement for professional calibration laboratories. Since the calculation requires a profound knowledge of the calibration process and of statistics, it might be too complicated and not absolutely necessary for calibrations requiring not too high accuracies (not too low uncertainties). Instead of using methods according to the ‘Guide of expression of uncertainty in measurement’ (2008), other statements may suffice. For instance, if the accuracy of the standard used is 10 times higher than that of the instrument to be calibrated, a detailed calculation would not be necessary for in-house purposes. But calibration certificates must always indicate the uncertainty.
A sticker should be attached to the instrument after successful calibration showing the date of the calibration and an indication of the person who carried it out. It is not recommended to mention the date for the next recalibration because this might lead to the assumption that no calibrations are necessary until this date. This is true only if the instrument has not been exposed to abnormal conditions or if it shows unexpected results. In case the calibration status is no longer valid or the calibration is doubtful, the instrument should be marked as such and not used until a recalibration has been carried out.
− Has not been calibrated;
− The time that has passed since the latest calibration is longer than the one recommended by the manufacturer of the instrument; or
− Your instrument has been exposed to overload, mechanical shocks or vibrations, wrong supply voltage or other unusual conditions.
Any of these conditions may cause wrong measurements and requires calibration.
6. Certification may be required by your customers and may raise your competitiveness. Even if certification is not required by your customers, think about the positive effect it may have on your competitiveness. However, you will first have to spend time and money on the certification process.
Source: Export quality management second edition (ITC) & (PTB)