Quantum for Pressure

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measure for measure Quantum for pressure Jay Hendricks tells about ongoing work to change the realization and dissemination of the pascal, which will lead to the elimination of mercury-barometer pressure standards.

ressure is one of the most widely leads to a determination of density, which measured quantities in everyday life provides a determination of pressure. Pand is critical to processes from simply The FLOC further enabled us to taking a breath to weather forecasting or determine nitrogen’s refractive index flying an airplane.Yet the way we meausre with a precision of 1×10–6 at atmospheric pressure today is essentially unchanged pressure2, exceeding the performance of since the 1640s, when Evangelista Torricelli mercury manometers3. The NIST team also invented the mercury barometer. used helium’s theoretical refractive index to Indeed, primary pressure standards, make an independent measurement of the as curated at the National Institute of Boltzmann constant4. The FLOC has smaller Standards and Technology (NIST) and other uncertainties than a mercury manometer national metrology institutes (NMIs), still at low pressures and will achieve parts-per- deliver pressure measurements by reading a NIST BY IMAGE million uncertainties at pressures above the mercury column’s height, along with liquid range of manometers. density and gravity. The instruments are The equivalence of pressurep and The NIST team is currently working large, heavy and complex, and mercury energy density provides an alternative on a next-generation variable-length is neurotoxic. Although digital pressure measurement method. For an ideal gas, the optical cavity to enable experimental

transducers (which rely on changes in the relation p = (N/V)kBT holds (the ideal-gas measurements of nitrogen’s refractive index resistance, capacitance or frequency of law), where N is the number of particles independent of the mercury manometer.

mechanically strained diaphragms) have occupying volume V, kB is the Boltzmann By combining this measurement with the replaced manometers for most applications constant and T is the temperature. The theoretically obtained value for helium, the in science and industry, they must be number density N/V of a gas can be FLOC provides a new way of realizing and calibrated by a chain of traceability to a obtained via gas refractivity; the equation for disseminating pressure.

mercury manometer. pressure then becomes p (n – 1)kBT, where The world metrology community is now This situation will change on 20 May 2019 n is the refractive index of∝ the gas. moving toward the quantum realization with the impending redefinition of the Researchers realized that first-principles of the pascal, and other NMIs are joining International System of Units (SI), when calculations of the refractive index of the search for primary methods of each of the seven base units will be tied to helium could be a new route to realizing pressure realization by optical techniques5. the exact value of a particular fundamental the pascal. The NIST team worked with Continued progress will not only make physical constant. the University of Delaware to determine all quantum-based pressure realizations In that context, if a new technique for a high-quality theoretical value based on and measurements directly traceable realizing a unit relies on a quantum property, quantum-chemical calculations1. This way, to the SI, but will finally end the four- theoretical calculation or invariant of nature, any measurement of the refractive index of century-long dominance of mercury in then that technique could itself serve as a helium can be a primary measurement of pressure metrology. ❐ primary standard with direct traceability to pressure and the pascal need no longer be the SI. It would not need calibration, and defined by a mercury manometer. JAY HENDRICKS is at the Thermodynamic would serve as both a sensor and a standard. The NIST team designed, built and Metrology Group at the National Institute of This will be the case for the new pressure tested a fixed-length optical cavity (FLOC, Standards and Technology (NIST), Physical standard being developed by a team at NIST pictured) for making precise determinations Measurement Laboratory, Gaithersburg, (Patrick Egan, Jacob Ricker, Jack Stone, of the refractive index. The FLOC consists Maryland 20899, USA. Greg Scace, Kevin O. Douglass, Greg Strouse of a pair of Fabry–Pérot cavities within a e-mail: [email protected] and me). single block of ultralow-expansion glass. Pressure is the amount of force per unit The top cavity is filled with gas; the lower is References area, and hence the pascal (Pa) — the SI unit maintained at vacuum. The change in path 1. Puchalski, M., Piszczatowski, K., Komasa, J., Jeziorski, B. & Szalewicz, K. Phys. Rev. A 93, 032515 (2016). of pressure — is 1 newton per square meter length between the two cavities depends 2. Egan, P. F. et al. Opt. Lett. 40, 3945–3948 (2015). –2 (N m ). But pressure can also be expressed on the gas’s refractive index, density, and 3. Egan, P. F., Stone, J. A., Ricker, J. E. & Hendricks, J. H. as the amount of energy per unit volume atomic or molecular properties. Helium’s Rev. Sci. Instrum. 87, 053113 (2016). –3 4. Egan, P. F., Stone, J. A., Ricker, J. E., Hendricks, J. H. (J m ), because the SI unit of energy, the atomic properties were calculated from first & Strouse, G. F. Opt. Lett. 42, 2944–2947 (2017). joule (J), is defined as 1 newton meter. principles, so the refractivity measurement 5. Jousten, K. et al. Metrologia 54, S146–S161 (2017).