Why are binoculars filled with nitrogen?

Why are binoculars filled with nitrogen?

If you take a look at binoculars across all brands, you'll see that many of them come filled with nitrogen. But why? Well, worry not- read on, and we'll explain all!

Firstly, it's worth noting that only waterproof binoculars can be nitrogen-filled. After all, if the binoculars could let in water, then they would let out the nitrogen as soon as it was put in. They aren't just waterproof to let you use them in the rain, though. If any moisture can get in, then the inside of the lenses can fog up if the temperature of the binoculars changes. This phenomenon is imaginatively called "fogging". Obviously, one can't wipe down the inside of the lenses as easily as the outside of them, so the only option is to wait for them to clear up by themselves before the binoculars can be used again.

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Ideally, then all waterproof binoculars would also come vacuum-sealed. Unfortunately, though, this would be very expensive to do, even on a large scale, and all binoculars would therefore be more expensive. Leaving the binoculars full of air isn't an option either, as this air could well contain some moisture, as well as bacteria or fungi which can damage the internal components. Even without these, the actual oxygen in the air can react with those components in warmer temperatures and slowly wear them down.

Nitrogen, on the other hand, is far less reactive. While it will react with other elements at very high temperatures, it would have to get past the melting point of the actual binocular casing before that happens. In addition, pure nitrogen is denser than air, so when it is pumped into the interior of the binoculars, it forces the air out. The binoculars are then sealed watertight, and hey presto- you've got a completely fogging-proofed set of binoculars!

You might find some recent binocular models come filled with argon instead of nitrogen. That's because some people think that argon does a better job of keeping the interior components dry than nitrogen. Argon molecules are larger than nitrogen molecules, so in theory, they will take more time to leak out of the binoculars and therefore offer anti-fogging protection for a longer period of time. However, there haven't been any conclusive tests which prove this is the case, and either way, it would be take decades for the nitrogen to leak out. What's more, argon is a truly inert gas, meaning it won't react with other elements at any temperature. Again, though, nitrogen-filled binoculars would have to get far hotter than the melting point of the casing before this was an issue. In practical terms, it doesn't really make much of a difference to the average user- both will do an equally good job of keeping the lenses free from fogging.