Helium Glossary and Units

Understand helium purity, isotopes, cryogenics, recovery and the units used to compare gas and liquid quantities.

The material

Helium (He): element 2, a noble gas. Chemically inert under ordinary conditions does not mean physically harmless.
Helium-3 / helium-4: isotopes with different neutron counts and important differences at very low temperatures. They are not interchangeable supply categories.
Cryogenics: the science and engineering of very low temperatures. Liquid helium is a refrigerant for some systems.
Superconductivity: a state in which certain materials carry electrical current without resistance under suitable conditions. A superconducting magnet may require cryogenic cooling.
Superfluidity: a distinct low-temperature state with unusual flow behavior; it is different from superconductivity.

Applications

MRI: magnetic resonance imaging, which produces images using magnetic resonance signals. Not every MRI system has the same magnet or helium inventory.
NMR: nuclear magnetic resonance. Spectroscopy uses resonance signals to study molecular structure and environments; many high-field instruments use superconducting magnets.
Tracer gas: gas used to help locate or measure a leak.
Carrier gas: gas that carries a sample through an analytical instrument.
Heliox / trimix: breathing-gas mixtures used in specialist diving. Their names are not instructions for mixing or use.

Purity: percentages, nines and parts per million

Four nines means 99.99% purity: a helium fraction of 0.9999. It is not 0.9999%. On the same stated basis, the remaining fraction is 0.01%, or 100 parts per million (ppm). A purity label alone does not specify every individual impurity limit.

Grades and certificates are supplier- and application-specific. Read What Is Balloon-Grade Helium? before treating a trade label as a universal specification.

Compare quantities on the same basis

A liter of liquid and a liter of gas contain very different amounts of helium. A gas volume changes with temperature and pressure. Record the actual reference conditions whenever a supplier uses standard cubic feet, normal cubic meters or similar terms.

Geometrically, 1 cubic foot = 0.028316846592 cubic meter. Converting that volume does not correct for different gas temperatures, pressures or compressibility. Use a common mass or documented reference-volume basis for an inventory balance; do not apply an unexplained fixed liquid-to-gas ratio.

Supply and recovery

Inventory: helium held inside a defined system at a stated time; it is not automatically annual consumption.
Production: material recovered from a source and supplied to a market; it is different from an estimate of geological resources.
Collection / capture: gas gathered from an outlet or process. Collected gas may still need purification, verification and reliquefaction.
Reuse: verified material returned to an appropriate use. Internal circulation must not be counted repeatedly as new supply.
Recovery rate: a ratio that needs a stated numerator, denominator, boundary and period. A high collection rate is not necessarily the same as a low net loss.

Use the conservation worksheets to record these distinctions.

Reference: Royal Society of Chemistry: helium [S01]; NIST phase-change data [S13]; and NIST refrigeration review [S119].