With just two protons in its nucleus and two electrons in its principal energy level, helium is the second lightest element. It is inert, electrically very stable, and has a high ionisation potential as well as a high capacity to absorb heat energy. With a boiling point of -269°C, just 4°C above absolute zero, liquid helium is the coldest substance on Earth. These properties combine to make helium the most versatile of the inert gases.
Leak Detection
The small size of the helium molecule means helium is an ideal gas for leak detection in vacuum and other gas-tight systems. By evacuating a pipe with a vacuum pump, and directing a stream of helium onto the areas where leaks are suspected, any helium entering the system can be detected by an appropriately tuned small mass spectrometer.
In order to help the refrigeration industry to comply with strict European rules governing the containment of fluorinated gases, gas companies are introducing oxygen-free mixes of nitrogen and helium to replace the oxygen-free nitrogen normally used for pressure testing refrigerant systems. This makes it much easier to detect very small leaks.
Welding Gases
Helium’s super inertness (and the fact that it releases more of its available energy in the arc than argon or carbon dioxide) makes it the shielding gas of choice for high speed welding of stainless steels, aluminium, copper and titanium. It is also used as a shielding gas in laser welding. In both these applications, the use of helium produces a superior weld penetration and a lower profile weld bead than can be achieved with other shielding gases. The higher ionisation potential of helium also serves to suppress the formation of a plasma, or charged cloud, of metallic ions that can spoil the weld.
Laser Gases
Helium also plays a role in the lasing gas for CO2 lasers – for example, the powerful, workhorse lasers used for very accurate cutting in industries such as the automotive sector. The lasing gas for CO2 lasers is made up of nitrogen, helium and CO2. In these lasers the nitrogen is first excited by an electrical discharge, and then transfers its energy to the CO2. As the CO2 molecules decay, they give off laser light. Helium is then used in this process to remove excess energy and bring the CO2 molecules down to their ground state, and back into the cycle.
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