Purging Gas and Optics
Date: Spring 2012
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.
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.
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
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.
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Update: June 2012