A MMT Division - Military Stock - Optical Equipment & Services

Nitrogen purged and filled binoculars

Over the years i have been faced with the subject of nitrogen purged and filled optical instruments, particularly concerning binoculars, so i would like to mention some successive notations. Primarily for this case of strange speculations about nitrogen filling, going to the point of the assumption that binoculars would only be waterproof if they would be nitrogen filled. Using the example of the Zeiss/Hensoldt binoculars Fero-D 16 8x30 M, Fero-D 17 7x50 M, Fero-D 18 7x50 M, Fero-D 19 10x50 M i would like to say something in general. But basically this would be apply for any optical instrument.
Binoculars can only be filled with nitrogen or any other gas if they are watertight sealed, because these are also gastight - a splash water protection is not sufficient. There is no over pressure inside the binoculars because of these filling, but the same pressure as the ambient pressure.
Basically the idea of nitrogen filling comes out of the requirement specifications of the military awarding authorities, which made high demands of the environmental Mil-Specifications and then has been taken over from the civilian sector, as so many things. But that is a different kattle of fish if this also would be made any sense in the civilian sector.
The aim of nitrogen filling is firstly to eliminate the hydrogen portion of the air bounded as water vapor from inside the binoculars to avoid condensation (known as fogging on prisms and lenses) as a result of fast and extreme changing of temperature. Secondly to eliminate the oxygen portion of the air as an agressive reactant leading to corrosion. The chemical composition as percent by volume are 78,08% nitrogen , 20,95% oxygen,  0,93 % argon and other trace gases, whereof the hydrogen portion of the air bounded as water vapor is about 0.1-4 percent by volume depending on ambient temperature.
It is actually impossible to completely avoid diffusion across housing, sealing, etc. at binoculars, which leads to a balance of the atmosphere inside the binocular and the ambient atmosphere after a certain period of time. But theres no comment regarding the period of time by zeiss. Which means in reverse, that if the binoculars should be persistent nitrogen filled, this filling has to be periodically checked.

For the above discussed Zeiss-/Hensoldt models the following applies:
Nitrogen filled are the models which have been delivered since 1990. These one with the ´bended shoulders´ Fero-D 16 8x30 M, Fero-D 17 7x50 M, Fero-D 18 7x50 M, Fero-D 19 10x50 M. All the former binoculars are not nitrogen filled.


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Some ex factory and new in box Hensoldt/Zeiss 8x30 M Fero-D 16 - still originally packed and not opened yet. Color is "Bronzegrün" (RAL 6031).
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Some unissued, mint conditioned Carl Zeiss Jena "Einheitsdoppelfernrohr" EDF 7x40 out of former GDR Authorities NVA and MDI.
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