Developing new techniques to measure thermodynamic temperature
Primary thermometry is the measurement of thermodynamic temperature directly, without calibration against a more accurate thermometer. These advanced measurements are not yet used directly in industry, but instead are incorporated into international agreements that are used to calibrate more practical secondary thermometers, such as platinum resistance thermometers and thermocouples. NPL is carrying out world-leading research in acoustic thermometry, practical Johnson noise thermometry and indirect primary radiometry.
Acoustic thermometry exploits the fact that the speed of sound in a gas is directly related to the average speed of molecular motion. So, by accurately measuring the speed of sound in a gas of known molecular mass it is possible to work out the average speed of molecular motion and hence the temperature of the gas.
NPL has built an apparatus for ultra-precision measurements of speed of sound in a gas. Up until 2013, the apparatus was used at the temperature of the triple point of water (273.16 K, 0.01 °C) to make very low uncertainty measurements of the Boltzmann constant. This work contributed significantly to the re-definition of the kelvin.
Since then the apparatus has been used to check the accuracy of the International Temperature Scale of 1990 (ITS-90) over range 118 K to 330 K (-155 °C to 50 °C). This thermometer is currently the most accurate thermometer ever made. Current research is directed at extending the measurements to 700 °C using a cylindrical resonator.
The expertise we have developed has enabled several more immediately practical applications. For example, by combining speed of sound measurements in free space with measurements of water vapour concentration using a laser absorption technique, we have created a new technology for measurement of air temperature and humidity. This technique requires no direct physical contact with air, allows up to 50 measurements per second and is immune to many of the sources of error in air temperature measurements using contact sensors. It has applications in both meteorology and precision metrology.
Johnson noise thermometry
Johnson noise thermometry exploits the fact that although the average voltage across a resistor is zero, the voltage fluctuates and the size of these fluctuations is directly proportional to absolute temperature.
Historically, the small size of the voltage fluctuations (typically microvolts) and the need to average for a long time (typically hours) has restricted applications of Johnson noise thermometry. However, by exploiting developments in signal processing, a measurement uncertainty of 1 °C in is now achievable in just a few seconds. NPL is working with Metrosol to create a product which would be suited to long-term use in harsh environments such as nuclear reactors, nuclear waste management and storage and severe accident monitoring.
Indirect primary radiometry
Absolute radiometers can determine temperature directly by measuring the intensity of the light emitted by hot objects through measurements traceable to a primary spectral responsivity scale. This offers an alternative temperature measurement technique, enabling calibrations to be traceable to thermodynamic temperature instead of to ITS-90.
NPL is using a primary radiometer to measure thermodynamic temperatures of ITS-90 fixed point cells over the range from ~232 °C up to ~1085 °C. These will provide T – T90 values which will feed into the redefinition of the kelvin.