Traceable to the SI

Traceable to the SI

In a recent ISO 17025 audit Teltherm was asked to update their Test Procedure statement to say that they are traceable to the SI. Depending on whether the certificate is for temperature, pressure or gas this statement shall be included on any IANZ certificate we produce.
For example:

Measurements are traceable to the SI via national standards of measurements as held by MSL NZ via Teltherm Instruments Ltd.

The results of the tests, calibrations and/or measurements included in this document are traceable to the SI via National and International Standards.

Our traceability procedures have not changed but our traceability statement has.

What is the SI

The International System of Units (abbreviated SI from systeme internationale , the French version of the name) is a scientific method of expressing the magnitudes or quantities of important natural phenomena. There are seven base units in the system, from which other units are derived.

The Base Units of the SI

The SI base units and their physical quantities and symbols are the
metre for measurement of length - m
kilogram for mass - kg
second for time - s
ampere for electric current - A
kelvin for temperature - K
candela for luminous intensity - Cd
mole for amount of substance - mol

The SI and Calibration Labs

For calibration laboratories, the program for calibration of equipment shall be designed and operated so as to ensure that calibrations and measurements made by the laboratory are traceable to the International System of Units (SI).

How is Teltherm traceable to the SI

The traceability chain is the sequence of measurement standards and calibrations that is used to relate a measurement result to a reference.

The farther your measurements are down in the traceability chain, the higher the uncertainty of the measurement. For this reason Teltherm seeks to have its reference equipment certified by National Measurement Institutes where available.

Teltherm Pressure;

• International Prototype of the Kilogram
• Primary and Secondary National Standards as held by National Measurement Institutes for example MSL in NZ or NPL in UK
• Teltherm Deadweight Testers
• Calibration of Customer instrument

Teltherm Temperature;

• Kelvin Thermodynamic Scale
• International Temperature Scale of 1990 (ITS-90)
• Primary and Secondary National Standard as held by for example MSL in NZ or NPL in UK
• Teltherm Reference thermometers
• Calibration of Customer instrument

Teltherm Gas;

• Mole
• Primary and Secondary National Standards as held for example by NATA in Australia, NIST in USA, NPL in UK
• ISO 34 Certified Reference Material (CRM gas)
• ISO 17025 Calibration Gas
• Calibration of customer instrument

Traceability

The integrity of a calibration is established by the traceability and level of confidence of the measurement results. The traceability refers to the measurement’s relation to the SI achieved through an unbroken sequence of calibration steps or comparisons all of which have stated uncertainties along with their level of confidence.

Traceability is so important in modern metrology that there have been specific elements assigned to the concept that must be satisfied to be able to successfully claim that the measurement result is traceable to the SI.

These properties are as follows:

• The unbroken chain of comparisons
• The measurement uncertainty must be calculated according to defined methods (true for every step in the chain)
• Each step in the chain must be performed according to documented and generally acknowledged procedures; the results must be equally documented
• Laboratories in the chain must supply evidence for their technical competence (e.g. they are IANZ accredited)
• The appropriate standards must be primary standards for the realization of the SI units
• Calibrations must be repeated at appropriate intervals (traceability may be lost by the passage of time)

During the calibration of a customer instrument at Teltherm we establish a link between the measurement of the customer’s instrument and the SI.

Teltherm also document the calibration, calculate the combined measurement uncertainty of all influential items in the chain of comparisons going back to the SI and the level of confidence of this measurement uncertainty.

The results of this process are documented in a calibration report which the customer receives along with the instrument when it is returned.