Sapphire Whispering Gallery Thermometer

An innovative sapphire whispering gallery thermometer (SWGT) is being explored at the National Institute of Standards and Technology (NIST) as a potential replacement for a standard platinum resistance thermometer (SPRT) for industrial applications that require measurement uncertainties of ≤10mK. The NIST SWGT uses a synthetic sapphire monocrystalline disk configured as a uniaxial, dielectric resonator with whispering gallery modes between 14GHz and 20GHz and with Q-factors as large as 90,000. The prototypeSWGTstability at the ice melting point (0 ◦C) is≤1mK with a frequency resolution equivalent to 0.05mK. The prototype SWGT measurement uncertainty (k= 1) is 10mK from 0◦C to 100 ◦C for all five resonance modes studied. These results for the SWGT approach the capabilities of industrial resistance thermometers. The SWGT promises greatly increased resistance to mechanical shock relative to SPRTs, over the range from −196 ◦C to 500 ◦C while retaining the low uncertainties needed by secondary calibration laboratories. The temperature sensitivity of the SWGTdepends upon a well-defined property (the refractive index atmicrowave frequencies) and the thermal expansion of a pure material. Therefore, it is expected that SWGTs can be calibrated over a wide temperature range using a reference function, along with deviations measured at a few fixed points. This article reports the prototype SWGT stability, resolution, repeatability, and the temperature dependence of five whispering gallery resonance frequencies in the range from 0 ◦C to 100 ◦C.