NIST’s unique photonic manometer relies on the fact that the refractive index (n) of a gas varies directly with its density, which when known along with temperature will define pressure in terms of energy density with the units of joules/m^{3}. So if the temperature is known and held constant, n can serve as a sensitive measure of pressure. n, in turn, can be determined to high precision by measuring how it affects light passing through the gas. One recently devised instrument operating on that principle is called the Fixed-Length Optical Cavity, or FLOC.
The NIST device works by sending identical beams of laser light through two cavities of exactly the same length. One cavity is held in vacuum; the other contains the gas whose pressure is to be measured. The difference in the speeds with which the beams pass through each cavity – as measured by picometer-scale interferometry – reveals the pressure of a gas, assuming that the relationship between refractive index and pressure is accurately known. Already a FLOC prototype the size of a travel mug has demonstrated a resolution of 0.1 millipascals (mPa), 36 times better than NIST’S official U.S. pressure standard, and makes measurements 100 times faster.*
Concurrently, NIST scientists are developing a key complementary device: the Variable-Length Optical Cavity, or VLOC, which will be able to determine n for any gas (e.g., nitrogen) whose pressure is to be measured in the FLOC. The experimental model, which has an adjustable cavity length that can vary from 15 cm to 30 cm, consists of four Fabry-Perot cavities mounted such that they can be extended or shortened as needed at constant pressure. The three outer cavities are in vacuum; the central cavity contains ultra-high purity helium gas. The relationship between pressure and refractive index of the gas is known to high precision. In coming months, the NIST team expects to achieve measurements of n with uncertainties in the range of 3 parts in 10^{11}. **
* Head-to-head comparison tests show that agreement between the FLOCs and UIM is within the combined uncertainty of both instruments. Further, the comparison tests between the two NIST-built FLOCs show that they agree to 2 µPa/Pa for all pressures in the range 100 Pa to 140 kPa, and are linear to 1 µPa/Pa. The expanded uncertainty for the FLOC was analyzed to be [ (2.0 mPa)^{2} + (8.8 x 10^{-6 }* p)^{2} ]^{1/2}, where p is the pressure in pascals. This means that for pressures below 5 kPa, the measure of pressure using the photonic pressure standard is more accurate than a primary realization of the pascal using the UIM.
** Variations in the device configuration and parameters will ultimately make it useful for measuring length in air and the Boltzmann constant.