Here are the densities at STP:
Air (ρair) = 1.292 kg/m3.
Helium (ρHe) = 0.178 kg/m3
so you'd need to compress Helium about 7x to achieve the same density as air.
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Helium loses its buoyancy when it reaches a compression ratio that increases its density to be equal to or greater than the density of air. This usually occurs at a compression ratio of around 1.7 to 2.0 times the original volume.
The minimum hydrogen-to-helium ratio observed is approximately 25% hydrogen to 75% helium. This ratio is consistent with predictions based on the conditions of the early universe during the Big Bang.
To calculate the air pressure at the end of the compression process in an ideal Otto cycle, you can use the formula for isentropic compression: P2 = P1 * (V1/V2)^gamma, where gamma is the specific heat ratio (1.4 for air). Given the compression ratio is 7, the volume ratio V1/V2 is 7. So, P2 = 98 kPa * (1/7)^1.4 ≈ 26.03 kPa.
Compression ratio is a ratio of the volume of gas in an engine's cylinder before and after combustion. To convert compression ratio to psi (pounds per square inch), you would need additional information such as the specific engine's cylinder volume, operating conditions, and fuel type to calculate the pressure generated during combustion. It is not a direct conversion and depends on various factors specific to the engine.
You can't convert a RATIO (which is a pure number) to a MEASUREMENT such as psi. You get such a ratio by dividing two different pressures, for example.
You need the gas mixture to be mostly helium, so that helium atoms can be excited. The excited helium atoms collide with neon atoms, exciting some of them to the state that radiates 632.8 nm. Without helium, the neon atoms would be excited mostly to lower excited states responsible for non-laser lines.A neon laser with no helium can be constructed but it is much more difficult without this means of energy coupling. Therefore, a HeNe laser that has lost enough of its helium (e.g., due to diffusion through the seals or glass) will most likely not lase at all since the pumping efficiency will be too low.