Page 41: of Marine Technology Magazine (March 2023)

Figure 1

No surprise that marine air is moisture laden making condensation inside an undersea housing a real problem. on diffusion to bring water vapor into contact with the desic- terior. cant, which can take a few hours. If you have the time, this The vacuum pump removes half the air for a second time. approach has a lot less steps involved. As half of the water vapor was removed the ? rst time, half the remainder, or ¼ of the original water vapor, is removed. The

MultiSorb Technologies , valve system allows external air to be drawn back in by the in-

DryPack Industries , McMaster- terior partial vacuum, passing through the desiccant chamber,

Carr removing all water vapor, further reducing the interior relative and others make these packages in a variety of sizes (Figure humidity.

3). The vacuum pump removes half the air for a third time, re-

The dynamic method involves the use of a vacuum pump as moving half of the remaining moisture vapor, this time just part of a purge system to remove interior air. Care must be 1/8 of the original amount of moisture vapor. At this point, exercised to not damage any interior components by pulling 7/8 of the original moisture vapor has been removed, and the too high a vacuum. A maximum of ½ atmosphere vacuum is interior air is as dry as a desert. recommended. I have successfully used this technique on deployments in all

With half the air volume removed, half the water vapor is the world’s oceans for decades, from inside the Arctic Circle removed, and a partial vacuum is created inside the housing. to the bottom of a number of ocean trenches.

A valve system allows the partial vacuum to draw external air A passive desiccant pack or two zip tied to the inside frame back through a desiccant chamber, forcing it in contact with are useful for capturing outgassing moisture from interior the desiccant granules, removing all the moisture. The dried components such as printed circuit boards and cardboard cov- air then enters the sealed housing, mixing with the residual ered battery packs. It’s a double-check approach to control- interior air, thereby reducing the relative humidity on the in- ling interior moisture.

U.S. Government, NASA, http://virtualskies.arc.nasa.gov

Photo courtesy of Multisorb

Figure 2 Figure 3

The relationship between relative humidity, temperature, Desiccant packs are available in many sizes, in either and dew point is shown in this graphic. As the temperature indicating or non-indicating absorbent, selected for the falls, the air’s ability to hold water vapor decreases. volumetric size of the housing.

At the dew point temperature, 37°F in this example, condensation forms. www.marinetechnologynews.com 41

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