- According to the study published in the Journal Nature, US researchers have developed a new way to define pressure and its derived SI unit, the pascal — one that they say will, within a year, begin to replace the mercury-based measurement methods that have been in use since 1643.
- Pressure is conventionally defined as force per unit area, and the pascal as a force of 1 newton per metre squared. For nearly 400 years, values at air pressure and below have been measured using mercury-based instruments called manometers.
- The US National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland scientists have developed a highly precise method for measuring pressure that is based on treating pressure as energy density — an equivalent physical description to force per unit area because it is derived from the same combination of the SI base units. Their method involves probing atoms of gas in a cavity directly with a laser to determine their pressure.
- NIST’s new pressure sensor, called a fixed-length optical cavity (FLOC), compares the speed of a laser travelling 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. (Nature)
