Calibrating of Measuring Instruments

Basics and Definitions

One important requirement of the quality assurance system is the traceability of a measuring instrument to national standards. This demand is generally based on ISO 9000 requirements. The traceability is given, if a measuring instrument or measuring system was calibrated in an uninterrupted chain using a reference, which in turn is traceable to national standard.

Definitions

Testing

Determining, if a demand is fulfilled. Example: an air-operated shut-off screwdriver is torque tested against a torque transducer. If a deviation is detected, then the screwdriver must be readjusted.

Adjustment

Change made to the measuring instrument to eliminate a systematic measuring deviation.

Certification

Testing of a measuring system to agree with the calibration laws as required by the consumer protection agency. This includes checking whether the number of the measuring deviations of each instrument does not exceed the allowable error limits. Thereafter the measuring instrument will be certified by the testing agency or its representative. The calibration law clearly states which measuring instrument needs to be calibrated!

Calibration

Measuring means comparing. A measuring instrument compares something unknown with something known. A measuring instrument needs to be checked, using a calibration process that measures an object with known standard. Any shown deviation for this measurement is called a measurement uncertainty. The smaller the deviation, the more accurate the measuring instrument is measuring.

Standards

A measuring instrument needs to be regularly checked, using a calibration process. Such a calibration process measures an object with known proportions. An object with known proportions is called a "Standard". There are standards in different hierarchy steps. These are compared in accordance with a concrete schematic. If the measuring instrument indicates the same value or is within the allowable tolerance range, then the measuring instrument fulfills the specified requirements.

Calibration Hierarchy:

At the top of the calibration hierarchy stands the Federal Agency of Physical Technology (PTB), which defines the national standards and the national accreditation body (DAkkS) which advises in the accreditation of calibration laboratories.

These accredited calibration laboratories, such as the DEPRAG D-K-18255-01-00, calibrate measurement tools in accordance with fixed calibration procedures and using traceable reference measurement tools.

At the base of the calibration hierarchy are the tools used in production such as screwdriving tools, measurement platforms and measurement devices. These tools are checked at regular intervals using the company’s measurement tools.

Calibrating

Comparing a measuring instrument with a reference-measuring-system – under the same conditions – to determine a systematic deviation and to subsequently trace it to national standards. A calibration always acquires the currently IS-condition of an instrument.

Reference Measuring System

The applicable standards must be tested and certified by an accredited entity, such as the DEPRAG DAkkS calibration laboratory to allow complete traceability.

Measuring Chain

A measuring chain includes all components, from the transducer to the display instrument (transducer, cable, measuring instrument).


Calibration performed by DEPRAG

Basically, all components of the measuring chain have to be independently calibrated. Measuring instruments for the Piezo transducer are calibrated using a charge-calibrator and measuring instruments for the strain gauge transducer are calibrated using a stain gauge calibrator. Each one of these instruments is connected over a DAkkS-calibrator to the national standards of the Federal Agency of Physical Technology and therefore corresponds to the highest possible quality requirement. The transducers are calibrated in specialize calibration-device using traceable reference-measuring systems.

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For how long is the calibration valid?

In principle, a calibration is only valid at the time of its performance.

The determination of when to perform a calibration is solely the responsibility of the user. The application, which means the conditions of the workplace, the type of usage of the measuring unit, frequency of use and safety requirements of the product to be assembled, decisively influence the required number of calibration intervals.

If measuring systems are integrated into the moving assembly process, then it will certainly be more meaningful to select shorter calibration intervals compared to measuring systems used in a laboratory environment. Calibration intervals can be significant anywhere from 3 months to approximately 2 years.
In general we recommend checking our measuring systems at least once a year. If unacceptable deviations are found, then we recommend having the measuring instrument newly calibration at DEPRAG.