What is calibration？

Calibration involves comparing a device under test (DUT) with an unknown value against a reference standard with a known value. This process is typically performed to identify the error in the DUT or to verify its accuracy.

For instance, consider calibrating a thermometer by measuring its temperature reading in water at the known boiling point of 212 degrees Fahrenheit. To enhance precision, you might use a calibrated reference thermometer with an exact known value to verify the DUT thermometer, as visually determining the exact boiling point can be imprecise.

Following calibration, the next logical step might be to make corrective adjustments to the instrument to minimize measurement errors, although technically, this step is distinct from calibration.

International System of Units (SI) – The Top Level of Known Measurement Standards

How do we arrive at measurement standards of known values against which we calibrate our devices under test? For the answer, we turn to the International System of Units, abbreviated “SI”, which is derived from “Le Système International d'Unités” in French. The SI consists of seven base units which are the second, meter, kilogram, ampere, kelvin, mole and candela.

The seven base SI units are derived from quantities in nature that do not change, such as the speed of light.

Calibration Interoperability

A key benefit of having the BIPM manage the SI on a worldwide basis is calibration interoperability. This means that all around the world, we are using the same measurement system and definitions. This allows someone in the U.S. to purchase a 1-ohm resistor in Australia and be confident that it will be 1 ohm as measured by U.S. Standards, and vice versa. In order to have interoperability, we need to have all of our measurements traceable to the same definition.

Calibration Accreditation

When calibrations are conducted, ensuring the reliability of the process is crucial. Calibration accreditation instills this trust by giving instrument owners confidence that the calibration was performed correctly. Accreditation signifies that a calibration process has been reviewed and found to comply with internationally accepted technical and quality metrology standards. The global metrology quality standard to which calibration laboratories are accredited is ISO/IEC 17025.

Calibration Certificates

A calibration laboratory often provides a certificate with the calibration of an instrument. The calibration certificate provides important information to give the instrument’s owner confidence that the device was calibrated correctly and to help show proof of the calibration.

A calibration certificate might include a statement of traceability or a list of the calibration standards used for the calibration, any data resulting from the calibration, the calibration date, and possibly pass or fail statements for each measurement result.

Calibration certificates vary because not all calibration laboratories follow the same industry standards, and they also can vary depending on where the calibration fits within the calibration pyramid or hierarchy. For example, the calibration certificate required for a grocery store scale may be very simple, while the calibration certificate for a precision balance in a calibration laboratory may have a lot more technical content. Calibration certificates coming from an accredited calibration process have some very particular requirements which can be found in the international standard ISO/IEC 17025.

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