# Helium question

1. Jan 15, 2006

### Facial

I have a question about helium :

If it is the most inert substance, why isn't it used more than krypton or argon for high-temperature incandescent bulbs?

I don't see neon in light bulbs too often either.

2. Jan 15, 2006

### rachmaninoff

Helium can diffuse through a thin glass wall, so it would escape from a light bulb (not very useful).

3. Jan 15, 2006

### GCT

What would the color be with helium?

4. Jan 15, 2006

### rachmaninoff

Would it matter? The OP was about incandescent, not fluorescent bulbs.

5. Jan 15, 2006

### kawikdx225

Helium is also much harder to ionize.

6. Jan 15, 2006

### rachmaninoff

7. Jan 16, 2006

### ShawnD

It might have something to do with heat capacity. High temperature instruments like ICPs use argon as the cooling gas because it can hold more heat than helium can (and take the heat away).
Might also have something to do with cost. Does helium cost more than argon or krypton?

8. Jan 16, 2006

### Facial

I suppose, then, that it can probably diffuse through a thick glass wall, but I'll have to look that up sometime with regards as to how much slower.

9. Jan 16, 2006

### ShawnD

I did an experiment testing this last year. It turns out that effusion rates are proportional to molar mass (Graham's law?). If helium has a molar mass of something like 4 and krypton is way way up there in terms of mass, that's a considerable difference.

10. Jan 17, 2006

### Facial

Graham's law explains the relation between molar masses on the rate of diffusion. It should be the the inverse square root of the molar masses if I remember correctly, meaning Helium diffuses roughly 3.2 times as fast as argon (turns out that it has around 40 daltons of mass).

However, I need a reference on how 1) composition 2) thickness affect diffusion rates as well.

11. Jan 17, 2006

### Facial

For argon the answer must be yes. The atmosphere is about 0.7% argon (which is quite a bit if you think about it) by volume, meaning you can distill this gas right out of the air. The cost definitely makes sense here.