The metrology revolution is gathering pace. Hot on the heels of an overhaul of the most fundamental ‘base’ units of measure in the International System of Units (SI) last month, some metrologists are focusing on their next target: pressure. US researchers have now 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, holds one of a handful of the world’s most precise manometers, known as primary standards — huge instruments that serve as the benchmarks against which all other pressure sensors are calibrated. But NIST 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. The team hopes to show in the next year that its apparatus can rival the manometer — and to encourage other metrology labs to use it as their primary standard.
If widely accepted by the metrology community, the method would do away with the need for mercury, which is toxic and faces international bans. Moreover, the technique measures pressure directly, using a fundamental constant of nature, meaning metrologists can derive the pascal without relying on previous measurements of other quantities, such as density, which the manometer depends on. 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 metrologist at the Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences in Beijing, who works on a related method to measure very low temperatures.
Room for improvement
Metrologists have long been striving to replace manometers, the principles of which date back to the mercury pressure gauge invented by Italian physicist Evangelista Torricelli in 1643. Modern manometers have two tall columns of mercury, and measure the force exerted on a surface due to a pressure by balancing it against the force generated by the weight of mercury. Pressure is derived by using gravity and a measurement of the density of that particular sample of mercury — but determining this density value is an intense effort that is done only every few decades. Manometers have also reached their limit of precision, whereas there is plenty of room for improvement in the new method’s uncertainties, says Jay Hendricks, a metrologist at NIST who is leading the project.
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.
The method is “neat”, says Hendricks, because it measures pressure by counting the number of gas particles in the cavity, using just quantum calculations and a fundamental constant of nature. “We have, in essence, ‘quantized’ pressure,” he says. This is in the spirit of the new SI base units, which are now all defined by their relationships to fundamental constants, rather than through arbitrary references or objects. “This is fantastic from a metrology point of view,” he adds.
The technique has a lot of potential, says Gao. But some issues, such as understanding how impurities in the gas affect measurements and how the cavity deforms during experiments, need ironing out. The team is working on these issues to reduce uncertainties, but Hendricks says that, at certain pressures, the FLOC will be ready to use as a primary standard — against which NIST calibrates sensors used in industry — within a year.
The team says that FLOC measurements have an uncertainty of 6 parts per million at atmospheric pressure — almost on par with the mercury method, the uncertainty of which can be as low as 3 parts per million. When measuring lower pressures, the team says its uncertainties are one-third of those of the mercury manometer. The results are “impressive”, says Stuart Davidson, a metrologist at the National Physical Laboratory (NPL) in Teddington, UK.
A force for change
But metrologists in other countries still need convincing that the FLOC is ready for the prime time. NIST will first aim to make the FLOC its primary standard. To do so, it will publish comparisons of the laser method against its manometer, a standard already internationally validated through comparisons overseen by a subsidiary of the International Committee for Weights and Measures, and carry out an internal review. Then, to gain official recognition that its claims about the FLOC are accurate, NIST must apply to the same body to oversee a comparison against conventional equipment at Germany’s metrology institute, the PTB, in Braunschweig.
To really embrace the new method, metrologists might need to see a second FLOC, created in another lab, that achieves the same results. Other national metrology labs, including the PTB, are developing equivalent devices, but none is yet ready. “For new experiments to be validated and for people to have confidence in them, it’s going to take a long time,” adds Davidson.
In theory, because the SI base units are now defined in terms of constants, any suitably rigorous technique that links a quantity to these constants is an acceptable way to derive a unit. Under these rules, the FLOC automatically qualifies as a way of defining pressure. But, in practice, no one knows how much evidence the international community will need before it accepts the FLOC as a valid, or superior, method. “I think this is probably a test case,” says Davidson.
Meanwhile, the NIST team is developing a prototype portable version of the FLOC, which could enable more precise measurements in industry. Used in altimeters, for example, which measure height using pressure, that would allow planes to fly closer together and reduce fuel consumption. The team is also adapting the method to work at higher pressures, which are currently most accurately measured by piston gauges.
The advantages of NIST’s approach are not only practical, says Michael de Podesta, another metrologist at the NPL. Increasing the precision of pressure measurements could fundamentally advance science, in the same way as increases in image resolution do, he says. “When you can measure pressure more precisely, you see the world more clearly. And exactly like with an image, you don’t know what you’re going to see until you build the instrument to look.”
Nature 570, 424-425 (2019)