NIST traceability, ISO17025 certification and commodity temperature sensing

Author: Product Manager Matthias Scharfe

Sensors measure the state of or a change in an environmental condition, be that light, temperature, or moisture, and typically convert that change into a mechanical or electrical response. That response is then usually transformed into a device-interpretable or human-readable signal using a display or computer interface.

To provide accurate information on the environmental condition or change, a sensor relies on adjustment and calibration of the sensor output. During adjustment, the sensor output is mapped to the measured physical entity by use of a reference measurement device or ab-initio calculations. In contrast, the process of calibration entails the comparison of the already adjusted sensor to another, usually certified and traceable reference, which is often done according to a standardized procedure, to obtain a degree of certainty of the calibrated sensor.

Standards and traceability

Reference standards are trusted to give a reliable, reproducible comparison or feedback that provides a measurement of the true, expected value with minimal or ideally without deviation. While kingdoms, counties, or even cities have defined their own standards in the past, today’s reference standards are usually based on fundamental definitions and units and are controlled by impartial institutions. Therefore, 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.”

In contrast to historic individual definitions of standards, the national and international bodies that oversee measurement standards with meteorological traceability are interconnected by numerous agreements. As the US National Institute of Standards and Technology (NIST) is one of the most well-known, many references or measurement devices are referred to as “NIST-traceable” indicating that their characterization can be traced back to the use of NIST measurements. Nevertheless, this pedigree is often misunderstood as it does not include any implication of the actual characterization method.

ISO standards

To simplify and standardize many procedures, the International Standards Organization (ISO) has established a set of standards with which compliance is certified. This in turn leads to the so-called “ISO accreditation”. Regarding sensors, one example of this are the calibration laboratories accredited according to ISO17025, which are able to deliver certified devices due to their accreditation.

A standard NIST-traceable reference will have been calibrated with a device that is either NIST traceable or calibrated by NIST, creating a documented unbroken chain of calibrations. However, this procedure can follow internal non-supervised guidelines with no evaluation of measurement uncertainty. There is no guarantee of the operator's skill level performing the calibration. The same applies to standards traceable to two NIST equivalents, namely METAS (Swiss Federal Institute of Metrology) in Switzerland and PTB (Physikalisch-Technische Bundesanstalt) in Germany.

The ISO17025 accreditation offers something additional to NIST traceability as the certification covers the “competence of the calibration laboratory”. This includes performing quality assurance measures for the calibration data, validating the methodology as well as checking equipment maintenance and staff training.

Sensirion temperature sensors

Sensirion has recognized the value of adopting the highest levels of ISO standards and the dangers and inaccuracies posed by non-calibrated instruments. Their top-of-the line STS32/33 and SHT33 sensors are ISO certified. Now, Sensirion is transferring this expertise to their new low-cost STS40 temperature sensor – a cost-effective, state-of-the-art sensor with the guarantee of NIST traceability.

The STS40 has a footprint of just 1.5 × 1.5 mm2 and a height of 0.5 mm, making it easy to fit onto circuit boards for several different applications, including portable devices such as consumer electronics and mobile phones. A wide range of supply voltages can be used to power the temperature sensor of 1.08 to 3.6 V. The very low current consumption of 0.4 µA for a typical average current at one measurement per second and the exceptionally low current consumption in idle state of only 80 nA also make it well-suited for battery-driven portable devices.     

Measuring temperature

The STS40 has an operating range of -40 to +125°C with an accuracy of up to ±0.2 °C. The repeatability can be as high as 0.04°C, and with Sensirion's rigorous calibration process, you can be sure that all your readings accurately reflect the environmental conditions. The measurement duration can be varied depending on the desired repeatability of the measurements, providing another opportunity to reduce measurement times and power consumption of the device.

Sensirion is a leading international sensor company based in Stäfa, Switzerland, and its quality reputation is also recognized by ISO/TS 16949 standards. Sensirion takes pride in offering traceable, calibrated measuring instruments to guarantee their customers the best measurement qualities for a range of high-performance applications. Sensirion's sensors can be widely found in strictly regulated areas such as medical devices. The new STS40 is ideal for smart home applications, HVAC, and industrial monitoring solutions.