Measurement for our planet
Measurement for our planet

As COP26 approaches, discover how NPL plays a key role in enabling climate action through the delivery of accurate, reliable data that supports decision making and enables low carbon innovation.

Temperature and humidity measurements

Primary acoustic gas thermometers

World-leading temperature measurements

NPL's world-leading primary acoustic gas thermometers are available to commission, ranging from individual resonators to complete systems. Our expertise in acoustic temperature measurement was developed during a ten-year project to make a low uncertainty measurement of the Boltzmann constant in support of the 2019 redefinition of the kelvin. Subsequently, the apparatus was used to make what are currently the world’s most accurate temperature measurements.

At the heart of the apparatus was the NPL-Cranfield acoustic resonator, which featured a diamond-turned quasi-spherical cavity manufactured to within 1.5 micrometres of its design form. The cavity includes a shape modification to enable measurement of individual triplet components of the microwave resonances. This allows the cavity dimensions to be tracked in operando with sub-nanometre resolution.

NPL has now developed that apparatus to create a Primary Acoustic Gas Thermometer (PAGT) suitable for use in world-leading calibration laboratories. The NPL PAGT is available either as a complete system, or as individual resonators with agreed peripherals: manufactured, built, tested and installed – as required.


Normal operational range is -60 °C to 100 °C, but measurements as low as 120 K and as high as 700 °C are possible.

The NPL PAGT can be operated in either relative or primary mode. In relative mode uncertainties are typically less than 1 mK across the operational range.

The PAGT features three SPRT wells. Careful thermal design allows convenient comparison of long-stem SPRTs against thermodynamic standards, and one well is modified to allow capsule SPRT calibration.

Read more about Primary thermometry

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