Specific Gravity of Helium

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The Specific Gravity (SG) of Helium is 0.138 at normal temperature and pressure (NTP). Helium is slightly more than 7 times lighter than air, which is why helium filled balloons rise into the atmosphere and float away.

-Characteristics of Helium-

Helium, which is abbreviated He on the Periodic Table (the standard table of chemical elements), is considered a “noble” or inert gas.  Like all of the other noble gasses, Helium is odorless, colorless, and tasteless and has a very low level of chemical reactivity. In other words, it remains chemically inert and essentially pure, not joining or bonding with other elements to form compounds.

Helium is the second most abundant element in the Universe (hydrogen gets top billing as the most plentiful element). Most of the helium in the Universe is found in stars where it forms as a result of the nuclear fusion of two hydrogen atoms into one helium atom.

On Earth, Helium is not one of the original or primordial elements that were present when our planet first formed. Helium found on our planet today has formed over time as a result of radioactive decay and much of the helium is coontained as a component of natural gas, from which it can be isolated.

Because helium is relatively pure and inert, it has many industrial uses and is also an exceptional coolant. When liquefied under pressure, helium is the coldest substance known. While other gasses dissolve in blood under pressure causing embolisms, because it has very low solubility in human blood, helium is mixed with oxygen to provide “air” with the proper percentage of oxygen for breathing while scuba diving.

Most people know helium as the gas that fills balloons to keep them floating in air. It’s also used in the extreme sport, cluster ballooning. In the past it has been used in combination with hot air for hot air ballooning. Helium is also the gas that allows even the richest baritone to speak like a duck.

A popular party trick involves inhaling a small amount of helium from a helium balloon. Because of its low Specific Gravity, helium resonates differently around the vocal cords, dramatically changing the pitch of the voice. Although theoretically safe to inhale in small amounts (as in from a balloon), inhaling helium from a pressurized tank can have disastrous consequences. Helium is not inherently toxic in and of itself, but when it’s inhaled under pressure, too much helium can interfere with air exchange and can also lead to brain embolisms.

-Understanding Specific Gravity -

Specific Gravity, abbreviated as SG in scientific circles, is a way of expressing the relative density of either liquids or gasses by comparing them to the density of water or air. Specific Gravity is dimensionless – it isn’t measured in ounces or grams or units of pressure, but rather in terms of how it ranks proportionally with the density (mass per unit of volume) water or air.

The value of knowing that the Specific Gravity of a liquid is lighter or heavier than water allows you to predict how it will behave - will it sink in solution or float to the top if mixed with water. With gasses, gasses with a density heavier than air will collect close to the ground while those with a density that is lighter than air will float upward. You can also get clues to the identity of an unknown gas or liquid by testing its specific gravity, since SG is consistent for a given element or compound.

Knowing how a gas will behave when released in air is helpful when there has been a gas leak and can be used to help people to avoid a risk in a contaminated environment. With a Specific Gravity of 0.138, releasing helium into the air is of little consequence. It is non-toxic and so much lighter than air, it rapidly rises and dissipates into the atmosphere.

Carbon monoxide, on the other hand, is only slightly lighter than air and while it will rise slightly, in a fire, it quickly accumulates and understanding its characteristics may mean the difference between life and death. Even bent over, running in a smoke filled room forces you to breathe air that is heavily contaminated with carbon monoxide. Crawling with your face close to the floor where carbon monoxide is less likely to collect will help buy you the precious seconds you need to escape from a very dangerous situation.  

Emergency managers rely on characteristics such as SG when determining how the accidental releases of gasses should be handled and may base their strategy in part on whether the gas is likely to sink to the ground and be very toxic (like chlorine) or slowly float up and dissipate (like ammonia).

-How Specific Gravity of Gasses is Measured -

In order to make a meaningful measurement, the values given need to be at the same temperature and atmospheric pressure. The measurement is made with a meter and the values are adjusted for a standardized temperature and atmospheric pressure of 68 degrees F /20 degrees C at sea level with a relative humidity of 20%., which are the standard, accepted values for one atmosphere of pressure. You might see this listed simply as one atmosphere or abbreviated as 1 atm.

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