Helium technical data: density boiling point, compressibility, purity grades and the gas in balloons

Helium is the second-lightest element and the most inert: a single atom, molar mass 4.0026 g/mol, a gas density of 0.1785 kg/m³ at 0 °C and a boiling point of 4.22 K that no other substance comes near. Those properties explain all of its uses, from the balloon at a wedding to the liquid that keeps an MRI magnet superconducting. This page sets out helium's physical data from the NIST Chemistry WebBook, how it behaves in a cylinder, how its purity grades are named, and where it comes from. Helium is not produced in India; KIGL supplies it from licensed producers.

Read three rows together. The density, one-seventh that of air, is why helium lifts and why it collects under ceilings when it leaks. The speed of sound, about 1,008 m/s at 20 °C against 343 m/s in air, is why the voice changes when someone breathes it; that is a sign the lungs hold no oxygen, as helium safety data explained sets out. The ratio of specific heats, 1.67, is the textbook value for a monatomic gas; helium has no molecular rotation or vibration to store heat, which makes it a near-ideal gas at room temperature.

The lift of a gas is the mass of air it displaces minus its own mass. Hydrogen lifts about 8 % more than helium for the same volume, but it burns over a 4 to 75 % range in air, so party, display and promotional balloons use helium. The gas sold for balloons is usually a lower purity than laboratory helium, because a few per cent of air makes little difference to lift. It is still an asphyxiant: inhaling it, from a balloon or a cylinder, replaces the oxygen in the lungs. The lift figures above match those on uses of helium gas.

Helium is delivered as a compressed gas, normally in seamless steel cylinders, because its critical temperature of 5.2 K means it can never be liquefied by pressure at ambient temperature. Like hydrogen, it is slightly less compressible than an ideal gas at cylinder pressures. NIST gives these densities at 15 °C:

So multiplying water capacity by gauge pressure in bar (150 or 200 litres per litre) overstates helium contents by about 9–11 %, and even the ideal-gas figure overstates them by 7–10 %. A cylinder's contents are best stated in cubic metres at a named temperature and pressure, or by mass. The method, and the corrections for temperature, are worked through in gas laws and cylinder pressure.

Helium is sold by purity, written either as a percentage or in the N-notation used across the gas industry. The first digit is the number of nines; the digit after the point is the last figure.

The N-notation purities are definitions; the uses are typical practice. The Indian product standard is IS 15583, Compressed helium gas — Specification, issued in 2005 and revised in 2026; its grade limits are not quoted here, so ask for the grade by its stated purity and impurity limits. For any grade the certificate should list the impurities that matter for the job, typically moisture, oxygen, nitrogen and total hydrocarbons in ppm. The general system is set out in gas purity grades explained.

Liquid helium boils at 4.22 K and has a density of only about 125 kg/m³, so one litre expands to roughly 750 litres of gas at 20 °C. Its main use is cooling the superconducting magnets of MRI scanners and NMR spectrometers, whose niobium-titanium windings superconduct only a few kelvin above absolute zero. A loss of superconductivity, called a quench, boils off a large part of the liquid very quickly, and the gas must be vented outside the building. Liquid helium is a cryogen as well as an asphyxiant; any surface at 4 K condenses air, and the liquid air that forms is enriched in oxygen. Helium does not freeze at atmospheric pressure at any temperature, which is one reason it is the coolant of choice for low-temperature physics.

Helium is recovered from certain natural-gas fields where radioactive decay has built it up to a fraction of a per cent or more. Air contains only about 5 ppm, too little to recover economically, so air separation plants do not produce it. India has no producing field, and all helium used in India is imported, as liquid in large vacuum-insulated containers or as compressed gas. KIGL does not produce helium; it supplies helium sourced from licensed producers through the helium gas page. Because the supply is finite and the price volatile, recovery and conservation matter, as discussed under helium supply and conservation.