Why are ISO Standards and NIST Traceability Important?

A sensor’s role is to measure environmental conditions - either the state of these or any changes, for example, in moisture or temperature.

The sensor will convert this change into an electrical or mechanical response, which is generally converted into a signal that can be interpreted by a device or read by a human via a display or computer interface.

A sensor’s output must be adjusted and calibrated to provide consistently accurate information on the change or environmental condition. This adjustment involves mapping the sensor output to the measured physical entity using ab-initio calculations or a reference measurement device.

The calibration process, meanwhile, involves comparing the previously adjusted sensor to a certified and traceable reference. This is generally performed in line with a standardized procedure, ensuring a sufficient degree of certainty in the calibrated sensor.

Standards and Traceability

Reference standards are trusted to provide reproducible, reliable comparisons or feedback and a measurement of the actual, expected value with minimal, but preferably no, deviation.

Many international, national and regional bodies have historically defined their own standards, but contemporary reference standards are generally based on fundamental definitions and units and managed by objective institutions.

Measurement standards are “characterized by a metrologically valid procedure for one or more specified properties and are accompanied by a reference material certificate that provides the value of the specified property, its associated uncertainty, and a statement of metrological traceability.”1

Now, a series of interconnected national and international bodies oversee measurement standards.

The US National Institute of Standards and Technology (NIST) is one of the most widely recognized of these bodies, with a diverse array of references or measurement devices now referred to as “NIST-traceable,” indicating that the devices’ characterization can be traced back to the use of NIST measurements.

Despite this, this pedigree is widely misunderstood, most notably because it does not include any implication of the method of characterization in use.

ISO Standards

The International Standards Organization (ISO) has established a set of standards with which compliance is certified – a critical factor in streamlining and standardizing a wide range of procedures. This is typically referred to as “ISO accreditation.”

Sensor calibration laboratories, for example, can be accredited according to ISO17025. This means that sensors calibrated in these labs can be considered certified devices due to the labs’ accreditation.

A standard NIST-traceable reference is generally calibrated with a device that is either NIST-traceable or calibrated by NIST. This approach allowed an unbroken chain of calibrations to be documented.

It is important to note, however, that this procedure can follow internal non-supervised guidelines with no assessment of measurement uncertainty. The skill level of the operator performing the calibration cannot be confirmed.

The same principle applies to standards traceable to two NIST equivalents: PTB (Physikalisch-Technische Bundesanstalt) in Germany and METAS (Swiss Federal Institute of Metrology) in Switzerland.

ISO17025 accreditation adds an additional level of certainty to NIST traceability because the certification also accounts for the “competence of the calibration laboratory.”

For example, an ISO17025 accredited laboratory is required to perform quality assurance regarding calibration data, validate the methodology and confirm its approaches to staff training and equipment maintenance.

Sensirion Temperature Sensors

Sensirion understands the value of working towards and adopting the highest levels of ISO standards, as well as the risks and inaccuracies that stem from the use of non-calibrated instruments.

The company’s top-of-the-line STS32/33 and SHT33 sensors are already ISO certified, and Sensirion has leveraged this expertise to develop the new, highly affordable STS40 temperature sensor – a state-of-the-art, cost-effective sensor offering guaranteed NIST traceability.2

The STS40 boasts a small footprint of just 1.5 × 1.5 mm2 and a height of 0.5 mm. This allows it to be installed onto circuit boards for a range of different applications, including consumer electronics, mobile phones and other portable devices.

The temperature sensor can accommodate a wide range of supply voltages, from 1.08 to 3.6 V.

Its low current consumption of 0.4 µA for a typical average current at one measurement per second makes it ideal for use in battery-powered devices. This portability is also ensured by its exceptionally low current consumption in an idle state - just 80 nA.  

Measuring Temperature

The STS40 has a wide operating range of -40 to +125 °C. It boasts an accuracy of up to ±0.2 °C and repeatability as high as 0.04 °C. Sensirion’s rigorous calibration process ensures that any reading taken with the STS40 will accurately reflect the environmental conditions.

The measurement duration can vary in line with desired measurement repeatability, providing users an additional opportunity to reduce measurement times and device power consumption.

A genuinely international sensor company based in Stäfa, Switzerland, Sensirion is at the forefront of its field.

Its reputation for quality is recognized by ISO/TS 16949 standards, and the company takes pride in offering traceable, calibrated measuring instruments that guarantee optimal measurement qualities in a range of high-performance applications.

Sensirion’s sensors see routine use in highly regulated areas, including medical devices. The new STS40 is an ideal choice for HVAC, smart home applications and industrial monitoring solutions.

References

  1. International Standards Organization (ISO) (2016) General requirements for the competence of reference material producers. ISO Guide 17034.
  2. Sensirion, (2021), STS4X, STS40-DIS - ±0.2°C Ultra-low-power temperature sensor (sensirion.com), accessed December 2021

This information has been sourced, reviewed and adapted from materials provided by Sensirion.

For more information on this source, please visit Sensirion.

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