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Name: Steve
Status: educator
Grade: other
Location: WI
Country: USA
Date: Spring 2012


Question:
Which is better for purging optics against moisture, leakage and best for light transmission; nitrogen gas or argon gas? Nitrogen (diatomic) gas has been used historically, but recently argon has turned up in higher end binoculars and spotting scopes with conflicting "arguments" as to why argon is better. These arguments have included that argon does not affect color as much, is a bigger atom and will not leak as easily with most comparisons not taking the fact that nitrogen is diatomic and finally, argon allows better light transmission. The web does not clarify any of these ideas reasonably.



Replies:
I do not see much difference in small optics like binoculars or spotting scopes. You are not asking for “high performance” in either case. For large instruments – for example infrared spectrometers – nitrogen has several advantages: 1. Cost 2. Nitrogen is transparent in the visible and infrared range comparable to Ar. Neither one absorbs significant visible light. 3. While it does absorb (in principle) in a Raman spectrometers. This is seldom an issue.

The purpose of purging gases is to remove water vapor, and to a lesser extent CO2. Using liquid N2 boil-off through a simple copper coil at room temperature you will obtain very pure N2 (it boils at 77 K) and there is no residual water vapor. You cannot beat that with any drying agent for Ar. And you do not even have to worry about traces of moisture. There may be reasons to choose Ar, but I do not see how you can beat the price. How many cylinders of Ar is equivalent to a 5 liters of liquid nitrogen??? The optical properties are essentially equivalent.

Vince Calder


I am not sure about the point you made regarding argon affecting color less than nitrogen. This implies that nitrogen does affect color transmission. Air is roughly 80% nitrogen, and yet one can see objects many miles away without apparent color change.

As for leakage, diatomic nitrogen molecule and monomolecular argon are not sufficiently different in size to significantly affect their diffusion rates through permeable media. If the comparison were made between nitrogen and helium, that would be a very different story!

But remember that diffusion through a permeable medium works both ways, and obeys the law of partial pressure. That means that irrespective of what gas is inside your sealed optics, atmospheric gases (nitrogen, oxygen and water vapor) will tend to be driven to diffuse inwards, since the partial pressure of these gases outside is significant, whereas inside it is zero. Water vapor diffusion inwards is particularly notable since the water molecule is smaller (and its diffusion rate is faster) than that of either nitrogen or oxygen.

Importantly, much depends on the optics' enclosure. Molded plastic enclosures are not hermetic and diffusion can be problematic. Any elastomeric materials that may be used as sealants (particularly silicones) are even more permeable than plastics. Diffusion rates through metals and glass, are essentially nonexistent. Housings made of these materials are considered hermetic. So, a gunsight (for example) with a plastic housing will tend to present a much greater diffusion potential than one which has a metal enclosure and is designed to use a minimum of polymeric sealants. In practice, the small difference in diffusion rates between nitrogen filled and argon filled optics is relatively insignificant, compared to diffusion due to the design of the housing itself.

Hope that is useful. Regards, Bob Wilson



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