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Back to Press Release Index
1 October, 1998
Calibrating Measuring Tools and Indicating or Recording Instruments
Cost-effective solutions for owners and users
To guarantee your professional performance and increase customer confidence, you need equipment that produces accurate, repeatable results.
Measuring tools and indicating or recording instruments need to be calibrated on a regular basis to ensure that you obtain reliable results. This is not just a requirement for ISO 9000 series compliance - Australian/New Zealand standard 3760:1996 stipulates that the equipment required to carry out in-service safety inspection and testing of electrical equipment should be subjected to routine calibration to ensure its accuracy is maintained.
What is Calibration?
A calibration is simply a comparison between the actual value given by the instrument or tool and a known value. The known value, or reference, must be of a suitable type and should be of a better standard than the item calibrated against it.
For example, to calibrate a steel rule you could place it alongside a reference rule with known scale markings and observe by how much each set of scale markings is offset. (This is a very rough and ready form of calibration, but could be acceptable in some circumstances.) For your comparison to be useful, you would need to know that the reference rule was correct and of a better accuracy than the rule you are calibrating against it (generally, for dimensional measurement, the reference should be at least ten times more accurate than the test item). This means that the reference rule would need to have been previously calibrated against a better reference standard of length.
Standards
In most quality assurance situations, the item used as a reference needs to have been calibrated against a national or international standard. Standards are simply nominal physical quantities against which all other measurements are defined, for example the metre, the kilogram or the volt. There is usually a single master reference (such as the Prototype kilogram which is located in France) or a definition which can be recreated in a high-level laboratory (such as the metre).
Traceability
Traceability is a term describing how any measurement can be traced back to a national or international standard.
In the case of the steel rule, you are comparing (calibrating) against a reference rule that is itself calibrated. This reference rule has been calibrated against a more accurate length standard by a calibration laboratory. The laboratory's length standard has been calibrated against the national length standard. Finally, the national length standard has been calibrated against the international standard for length. This series of calibrations establishes what is known as the "traceability chain" for the original rule.
Calibration certificates document one step in the traceability chain. This enables you to prove that your calibrated item is accurate within the limits as given in the calibration certificate. You must be able to prove the validity of any calibrations performed if you are operating under a quality management system.
Why is Calibration Necessary?
Calibration proves, to a degree appropriate for your requirements, that your measurements or comparisons will be the same as those of any other person, company, or organisation in the world.
If your multimeter displays a reading of 230 volts - what is the actual voltage likely to be? If your multimeter is accurate to plus or minus five volts the actual voltage should be between 225 and 235 volts. If the accuracy is plus or minus five millivolts, the actual voltage should be between 229.995 and 230.005 volts. If you do not know the accuracy of the multimeter, you cannot know with certainty what limits the actual voltage will be within.
Accuracy Limits
The required accuracy of the calibration, or the uncertainties for each measurement, depends upon your needs.
For example, an electrician may only need to know that a multimeter reads 230 volts to an accuracy of plus or minus five volts, while a manufacturer may need to measure the same voltage to an accuracy of plus or minus five millivolts.
Because specific needs vary, calibration requirements vary. If you are unsure of your requirements you should discuss this with the laboratory prior to having your equipment calibrated.
The Cost of Accuracy
Instruments capable of measuring to a greater degree of accuracy cost more than instruments with a lesser degree of accuracy. The calibration costs are also usually greater as the accuracy of the instrument or gauge improves.
You can save yourself considerable operating costs by purchasing equipment that is appropriate to your requirements and by calibrating your instruments to the appropriate degree of accuracy. A calibration laboratory can give advice on what options would be best for your business requirements.
Standards, such as AS/NZS 3760:1996, stipulate the degree of accuracy required for the type of testing being performed. Determining realistic uncertainties - or the degree of accuracy - is a large part of the work that a calibration laboratory does, thereby ensuring that your equipment complies with your quality management system requirements.
Maintaining Accuracy
Manufacturers of instruments and gauges give a specification for the accuracy of their product. The better equipment will normally meet this specification - at least when they are new - but many products do not. To ensure that your requirements will be met, any equipment must be calibrated before use. It is worth noting that the manufacturer's certificates or declarations included with many instruments are not usually acceptable for ISO 9000 purposes. (This should be checked with your quality system auditors, as their requirements vary.)
Regular Calibration
Many things cause instruments and gauges to become inaccurate. These can range from incorrect use through to simple wear and tear. Calibration must be performed at appropriate intervals throughout the service life if you are to ensure that your measurements and readings are accurate and repeatable. An 'appropriate interval' can be dependent on many factors, such as the quality, age and usage of the equipment.
Dealing with a calibration laboratory on a regular basis will allow the laboratory to build up a history on your measuring equipment. Once the laboratory has calibrated an item they will be able to advise you when it should be brought in for its next calibration, based on the characteristics displayed by the instrument.
Underwriters Laboratories
Underwriters Laboratories are internationally accredited laboratories offering physical, dimensional and electrical calibration to consultants, end-users, designers and manufacturers. Rapid turn-around times (same-day in some cases) and very competitive charges are available for a wide variety of instruments and gauges. Our aim is to assist our customers in ensuring their business requirements (including ISO 9000 series quality management systems and relevant AS/NZS standards) in the area of calibration are met on a timely and economical basis.
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