- Without air in the bag, there is no excess pressure that pushes water into the bag when the bag is submerged.
- The vacuum causes the plastic from both sides of the bag to stick together. They remain stuck together even when you open the bag - so if the bag developed a hole, water would not fill the entire bag.
Up until recently, I had always used Aquapac armbands to hold my GT-31 (either a medium or a large bag). However, I always noticed that the insides of the bags where getting wet after just a couple of uses, even if the bags were clean and did not show any cracks or holes. After using bags for a few months, they would often develop cracks which let a lot of water in. Since the GT-31 is waterproof for immersion (but not for high-speed windsurfing crashes), that was not a problem. I just put some clear tape over the crack, and the combination has worked fine for me. Eventually, I had to replace a bag or two, though, when the cracks got to big.
However, our fellow Fogland Speedsurfer Sabah has not been so lucky. After a recent session, he noticed that his entire Aquapac bag had filled up with water, and that the GPS turned off. He could not get it to turn back on even after drying it, and trying to recharge it. I took his GPS apart a few weeks later, and this is what I saw:
You can see a lot of corrosion on top of the battery pack and on different parts of the circuit board - apparently, a lot of salt water made it into the case. A close look at the housing reveals problems at the three top screws that hold the upper and lower part of the housing together: one mount is broken off completely, one partially, and the third on is cracked. Most likely, the GPS housing was damaged either during a bad crash where the arm hit the mast or board, or maybe when the GPS was dropped. That alone would not have been a problem - the GPS was still working fine at the beginning of the session. But together with the tendency of the Aquapac bags to develop leaks, it eventually killed the GPS. I tried to rescue it by rinsing with water, but I was way too late. By now, the corrosion as so bad that some parts of the electronics just fell off.
I think the GPS would still be working if it had been in a Dry Case bag instead. I found the Dry Case bag through luck when I needed a waterproof armband in Hatteras, and the only one I could find was a Dry Case armband at WindNC. I have since used the Dry Case several times to hold an Android phone so I could have GPS Speed Talker tell me my speed while sailing, and it has remained perfectly dry so far (I am sure that an Aquapac case would have let water in by now). The Dry Case has a microphone connector which is not needed for the GT-31, but the connector in the bag is actually useful: if you put it on top of the GPS close to the push buttons, you can prevent accidental pushing the buttons while sailing.
It remains to be seen how long the Dry Case really will remain dry. But as I said, I think it is already doing better than the Aquapac armbands. The plastic material seems sturdier and less likely to develop cracks; and using vacuum to get the air out of the bag and to make the sides of the bag cling together is a great idea. When I am using it with my phone, I'll keep double-bagging the phone in a little clear plastic bag. That has been overkill so far, but why risk a $130 piece of electronics that is not waterproof? I also added the little piece of brightly colored plastic. With the phone, the bag may sink in water, even though the armband itself is floaty. That should not be problem with the GT-31, since the GT-31 is "floatable". The armband on the Dry Case also is sturdier than on the Aquapac cases, but all armbands can slip off, especially when using dry suits or no suit. The brightly colored plastic may just make it easier to find the GPS when it's floating in the water. Of course, it would be even better if Dry Case would come with brightly colored (or at least white) arm bands in the first place...
