Scientists Discover Air Is Heavier Than We Thought
Scientists have discovered that the air in the atmosphere around us is heavier (more dense) than they had previously thought. Knowing this will enable scientists to measure the mass of objects more accurately than ever before.
Writing in the Institute of Physics journal Metrologia, a team from the Korea Research Institute of Standards and Science (KRISS) and the International Bureau of Weights and Measures (BIPM) in France, report a new determination of the content of argon in air, the first since 1969.
If asked to name the major chemical components of air, most of us would list oxygen (about 21%), carbon dioxide (about 0.04%) and water vapour (typically about 1%). The principal component of air is nitrogen and the only other major component is argon (about 0.9 %). Argon is chemically inert and its presence in the atmosphere poses no problem to human well-being. Old measurements dating from as early as 1903 gave the content (moles of argon/mole of dry air) as 0.934 %. The most recent value available until now was lower (0.917 %) and was thought to supersede the previous result. The work reported in Metrologia gives a new figure (0.9332 ± 0.0006)%, very close to the measurement results of 100 years ago. The uncertainty in the new measurement is given at the 95% confidence limit and is of unprecedented accuracy.
The analysis was performed at KRISS using high precision mass spectrometry. A set of air-like calibration gas mixtures was prepared by very careful weighing of pure gases into high pressure cylinders. Analysis of these synthetic air mixtures along with samples of natural air contained in other high pressure cylinders yielded the result reported in Metrologia.
Argon content is important to a small community of scientists working on precision mass measurements. To understand why, think of the old puzzle: which weighs more, a kilogram of feathers or a kilogram of lead? If it were possible to do the weighing on a very precise balance, we would see that the balance readings are identical for the feathers and the lead if the weighing is carried out in a vacuum. But the feathers would produce a considerably lower balance reading for measurements in air. This is because feathers are more buoyant in air than is lead (Achimedes’ Principle).
Mass metrologists use an equation to correct for the effect of air buoyancy. The equation includes the air density which, in turn, includes a parameter for the content of argon in the atmosphere. The different historical values for argon content lead to a difference in air density of just under 0.01%, or about 15 micrograms in the apparent mass of one kilogram made of stainless steel (15 parts in 109). The higher the argon content, the denser the air.
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