Fogging is a real problem with GoPro cameras, particularly in diving where humidity is so often an issue. Closing the camera in a damp tropical environment, then submerging the camera in cooler water to dive a tropical reef is a sure recipe to fog the inside of the optical window.
I understand the desire to keep the case as small as possible. particularly for an action cam. But why? Why could GoPro not include enough room for a small desiccant package, a few millimeters would have been enough. Yes, there are the little pieces of blotter paper sold by GoPro and others, but these are not nearly as effective at removing moisture from the case as true silica gel desiccant products that absorb far more moisture per volume. I have seen GoPro cameras fog up even when properly used with the Anti-Fog inserts.
There is plenty of room in my other camera cases for desiccant packs. I keep a good pile of desiccant on hand in nice little packets just perfect for loading into the camera cases. I put the used packs in my toaster oven for an hour at 150°F to bake the moisture out and reuse the packs over and over again. Simply store in a tightly sealed, airtight container for use.
As I sat at my kitchen table loading the camera for the next days diving an idea occurred to me. In front of me was a can of Dust-Off I was using to blow the optics clean of the usual specks of cruft. From the dim recesses of my memory I recalled a posting from more than a decade back, where someone was using a similar product to keep the CCD in an cooled astronomy camera from condensing and forming ice on the detector. The idea was simple… Displace the air, and the associated moisture from the interior of the case. I have done this same sort of thing with professional CCD cameras, using dry nitrogen to displace moist air, this should work!
Note: An air temperature of 82°F and humidity of 80%, not unusual on the Kona coast, gives a dewpoint of 75°F. Thus the camera case will condense at 75°F or lower. Water temperature is typically about 75°F in Hawaii, condensation is quite likely.
Dust-Off is simply the most common product found in retail stores used to blow optical surfaces clean of particulate matter. Available in a small aerosol can the product is actually 1,1-Difluoroethane, a light fluorocarbon molecule that is a colorless gas at room temperature. Supplied from the aerosol can there should be no moisture in the gas and should not interfere with the operation of the camera. The can is also supplied with a small red tube that can be easily used to inject gas into an almost closed case.
Difluoroethane is heavier than air, thus positioning the case with the opening upwards will tend to collect the gas and force out the lighter air. I simply placed the GoPro case lens down, inserted the little red tube down the side of the case and closed the lid as far as I could with the tube in place. I lightly pulled the trigger on the can to create a slow and gentle flow of gas for about ten seconds, then slid the tube out and closed the case. Now to go diving!
Result? Perfect, not a speck of fog all day long. I checked the camera both underwater and on the boat after returning to the surface. Given that the case was closed in fairly humid air (>50% RH) at about 80°F it should have been fairly moist. The water temperature was the usual 75°F water found in Hawaiʻi when diving… The case would have fogged if there was just air inside, it always has in the past without any other precaution.
I will be using this method in the future, it is easy, cheap, and apparently reliable. Of course this does not allow for opening or re-closing the case without a can available. Not usually a problem, I tend to setup the cameras the evening before where I can inspect and close the cameras in a relatively clean environment.