Some years ago I used to maintain a 20 x 120 naval binocular of 1940s vintage. It had two small mesh-lined inserts into which a desiccant, presumably anhydrous calcium chloride, could be placed. This would reduce the likelihood of condensation on the internal optical surfaces. Today we do not use internal desiccants, but rather fill the binocular with dry nitrogen and make it gas-tight. Nevertheless, desiccants still have their place. The currently preferred desiccant is silica gel, an amorphous form of silicon dioxide that can adsorb up to a third of its own weight of water onto its surface (of which it has about 700 m2 per gram!). It can be regenerated by heating it in an oven to between 125° and 200°C (250° and 400°F).

Sachets of silica gel are included with a multitude of modern electronic devices, as well as with binoculars. I tape a sachet to the inside of each of my small binocular cases and also to the inside of each objective lens cap of my 100-mm binoculars. This is possibly a bit of overkill, but it seems to be a small effort to eliminate an inconvenience that may necessitate a far greater effort to remedy.

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