A new way to measure pressure
Last month, the new definition of four basic units-the ampere, the kilogram, the kelvin, and the mole-took effect. Now researchers focused on the next target of the metrology evolution: pressure. Conventionally, pressure is defined as force per unit area and has unit newton per meter squared. For nearly 400 years, the value of pressure was measured by mercury-based instruments called manometers. But now, researchers from NIST (the US National Institute of Standards and Technology) developed a new and high precise method of measuring pressure by treating pressure as energy density, which is an equivalent physical description to force per unit area because it is derived from the same combination of the SI base units.
NIST’s new pressure sensor, called a fixed-length optical cavity (FLOC), compares the speed of a laser traveling through a gas-filled cavity with that of an identical beam in a vacuum. The speed of light varies with the density of the gas in a way that quantum chemists can calculate based on the properties of atoms. For a steady-temperature system, metrologists can combine these density measurements — effectively the number of particles in the cavity — with the Boltzmann constant, which relates temperature to kinetic energy. This calculates the gas’s ‘energy density’, which is equivalent to pressure.
This new method not only can help us get rid of mercury but also provides a way to measure pressure directly, using a fundamental constant of nature, without relying on previous measurements of other quantities, such as the density of the mercury in conventional manometers. In theory, it could also allow anyone to measure pressure from first principles without “the tedious work of” a chain of calibrations to a primary standard that is currently required, says Bo Gao, a Chinese metrologist.