A Density-Threshold Pressure Standard: Pressure Realisation from a Polarised-Gas Density Threshold

Primary pressure standards are few: the mercury column, the piston gauge, and ultrasonic interferometry exhaust the usual list, and each carries artefacts of its own. This paper proposes a standard anchored to a threshold rather than to an artefact: the density at which the spin-relaxation rate of a polarised helium-3 sample changes regime, a kink whose location is fixed by the spin-density physics of the sample rather than by any machined dimension. At a threshold density of one amagat class and a temperature of 293.15 kelvin, the realised pressure is 108.7 kilopascals with a computed combined relative uncertainty of 3.30e-05, or 3.6 pascals. A three-stage expansion cascade extends the realisation downward to 108.69 kilopascals, and compression to five threshold densities extends it upward to 543 kilopascals. SIMULATED, NOT MEASURED: the standard is specified and computed, not built or intercompared.