The Importance of Traceability in Calibration

Calibration is the comparison of a measurement device or instrument against a standard. A standard in a measurement is considered the reference. Calibration reveals how far the unknown device is from the standard. The measurement result must be tied to a globally accepted standard to ensure uniformity and consistency of measurements results worldwide. Traceability in calibration ensures that your measurement results are linked through an unbroken chain of comparisons to national or international standards with known measurement uncertainties at every step.

What is Metrological Traceability?

It is possible to tie a measurement result to the SI through a series of calibrations in what is referred to as metrological traceability.

Traceability is the unbroken chains of comparisons linking your instrument back to a national or international standard such as NIST in US or NPL in UK.

Traceability in calibration is determined by ensuring the following:

• The measurand, the quantity to be measured, is clearly defined.
• A documented unbroken chain of calibrations is established between the measurement being made back to stated and appropriate references (these include national or international standards and intrinsic standards).
• Each step of the chain is implemented in accordance with the appropriate method, with the measurement results and with the associated, recorded measurement uncertainties.
• The organizations performing one or more steps in the chain are accredited i.e. provide evidence of their technical competence.

Traceability requires an established calibration hierarchy – a step-by-step transfer process.

Each step in the process builds a chain that clearly shows how each link in the chain relies on the links above it for traceability to be valid (all the way to the International System of Units (SI).

The Hierarchy of Standards

Traceability works in similar way to a pyramid, where accuracy increases as you move up.

• Working standards – the tools used daily on the factory floor to calibrate instruments.
• Secondary standards – higher accuracy tools kept in a controlled lab to calibrate the working standards.
• Primary/National Standards – These are maintained by National Metrology Institutes such as NIST.
• SI Units – The peak of the pyramid, defined by physical constants like the speed of light.

Related article: Measurement Standards

Traceability in Calibration Certificates

A traceable calibration exhibits an unbroken chain of measurements all the way to a national standard or measurement standard. An important feature of a calibration certificate is the traceability of measurement results and thus its international comparability.

A proper calibration certificate should include the following details:

• Unique identification
• Environmental conditions
• Measurement results
• Measurement uncertainty
• Statement of Traceability

Let’s delve into more details for each one of the above feature:

Unique Identification

It should have details such as:

• Certificate number
• Instrument ID/Serial number
• Calibration date
• Location of calibration

Environmental Conditions

Should contain environmental details such as:

• Temperature
• Humidity
• Pressure (if relevant)

Measurement Results

Measurement results that include:

• As-found data
• As-left data (if adjustment to the instrument was done)
• Tolerance limits
• Pass/Fail statement (if applicable)

Measurement Uncertainty

A compliant calibration certificate must show the following:

• Reported measurement uncertainty
• Coverage factor (k)
• Confidence level (usually 95%)

If measurement uncertainty is missing from the calibration certificate, the traceability is unbroken.

Statement of Traceability

The compliant calibration certificate should have a clear statement of traceability, linking the measurement results through unbroken chain of calibrations to the national standards or international standards. For example, a statement of traceability could be something like this:

“The measurements reported on this certificate are traceable to the international system of units (SI) through the unbroken chain of calibrations to National measurement standards maintained by the National Metrology Institute.

All the reference standards used in this calibration are calibrated and maintained with valid calibration certificates, and their measurement uncertainties are included in the reported expanded uncertainty

This laboratory is accredited in accordance with ISO/IEC 17025 by an ILAC-MRA Signatory accreditation body”.

What Makes a Proper Traceability Statement?

A compliant traceability statement must:

• Reference the SI Units.
• State an unbroken chain of calibrations.
• Identify a National Metrology Institute (NMI) such as NIST in US, NPL in UK, etc.).  
• Confirm valid calibration status of the reference standards.
• Include the measurement uncertainty.
• Mention the accreditation (if applicable).

Typically auditors look for the following details during ISO 17025 or ISO 9001 audit:

• Clear SI traceability.
• Valid reference standards.
• Proper uncertainty declaration.
• Accreditation status verification.
• No ambiguous wording in the traceability statement.

You may also check out these related articles:

• What is Calibration of the Sensor?
• Absolute vs. Secondary Instruments
• Instrument Parameters
• How to calibrate a dc voltmeter