April 09, 2010
Waterproofing rugged computing equipment
During the course of testing in the RuggedPCReview.com lab, we examine ruggedness specifications and claims. For the most part, while we report and comment on those specs, we do not put them to the test. That's because ruggedness testing is pretty involved business, and checking how much punishment a device can take before it fails makes about as much sense as a car magazine running a test vehicle into a concrete wall to see if it is indeed as safe as the manufacturer says.
There are, however, exceptions. If a manufacturer claims their product can be dropped from four feet without damage, we may try that. And if a product is advertised as being waterproof, we may check that claim out as well. And this is where it gets interesting.
Most rugged products have an ingress protection rating in their specs. If the IP code system is used, as defined by international standard IEC 60529, then the second number in the code indicates protection against water. An IP67, for example, means that the product is totally protected against dust (that's the "6"), and also protected against immersion into water down to one meter (3.3 feet) for up to an hour. IP68 means protection against continuous immersion, as specified by the manufacturer.
So are there mobile computers that are waterproof? The answer is yes. There is a small, but not insignificant number of systems, primarily handhelds, that carry IP67 ratings. And the marketing for those systems often includes pictures or videos of full immersion. At trade shows you sometimes see waterproof handhelds or tablets sitting in tanks, running video to show that they are, indeed, alive and unharmed.
Now it is abundantly clear that even machines that carry IP67 ratings are not dive computers and that few will ever even be immersed in water. However, given their intended use, they MAY fall INTO water, just as they may fall off a speeding pickup truck and get stepped on. Hence our occasional testing of the stated design limits and a bit beyond.
That said, as a certified scuba diver with a good degree of experience, I've come across some pretty fascinating underwater electronics that are sealed. Diving is really interesting in that pressure plays a huge role. Each 33 feet of sea water (or 34 feet of fresh water) adds one atmosphere, or 14.7 psi, of pressure. You'd think that divers get crushed down at 100 feet, but that's not so because the human body is mostly water anyway (60-80%, depending on the individual), so all we have to worry about are the air spaces inside of us (lungs, sinuses, ears, mask mostly). We equalize pressure by breathing in pressurized air that perfectly counterbalances the water pressure. The result is that even at substantial depth, your dive mask doesn't leak at all; the flimsiest of seals will keep water out as long as there is no pressure difference and as long as there is indeed a seal that keeps air and water apart.
This means that, theoretically, if there were a way to dynamically pressurize the inside of a rugged computing device, even very delicate seals (like the very thin silicone skirt of a dive mask) would be enough to keep water out even at great depth. Now obviously, no one is about to put automated compressed air pressure equalization systems into a handheld computer; that is not what such devices are for. It's interesting, though, to examine how underwater electronics ARE sealed:
- Most underwater cameras use special housings that still allow access to the camera's controls. They usually have one big O-ring seal for the housing clamshell, and then individually sealed pushbuttons.
- Recently, an exceedingly simple waterproofing method for cameras has come on the market. It simply consists of a sealed bag of clear plastic with a lens in it. It isn't protecting against pressure, but it sure keeps the water out.
- Dive computers (the ones that compute nitrogen loading, depth, dive time, remaining time, etc.) are sometimes oil-filled. Since oil cannot be compressed, there are no pressure issues.
- There are a number of waterproof cameras now that can handle up to 33 feet of water. Examples are the Olympus Tough series, the Canon D10, the Panasonic TS2 and more. We've tested most of those down to 50 feet, and had one down to 77 feet. Those are regular cameras with LCDs, battery and I/O compartments, and numerous controls. So it might be interesting for rugged computer engineers to take one of those cameras apart and see how they do it. (Btw, LCDs sometimes get compressed so that the image is temporarily impacted, and sometimes buttons are pushed in from the water pressure).
What does all this mean for the waterproofing of rugged mobile systems? Mostly that a good understanding of pressure and sealing is required to design reliable waterproofing. Apart from the fairly complex issues of pressure, there's also a good deal of common sense. Keeping things as simple as possible is key. In a setting where ANY failure can be fatal to the equipment, it only makes sense to keep the potential points of failure as few as possible, and as simple as possible. It is not surprising that NASA has always been big on the concept of "fail-safe," i.e. systems that if they failed they failed so as not to jeopardize the larger purpose, such as survival of astronauts. Likewise, scuba regulators are designed so that if they fail, they free-flow rather than shutting off air, thus giving the diver a chance at survival.
The conclusion is that the key to waterproofing of rugged computing systems is keeping things as simple as possible. This means keeping openings to the inside at a minimum, providing double protection whenever possible, and designing things to be as fail-safe as possible. Whatever seals there are must be totally reliable; resistant to twisting, ripping or falling out; durable; and easy to procure and replace. Seals should also be noticeable so users can see if something is amiss (we once failed to notice that a black O-ring in a black housing was missing, with nasty consequences). Rule #1 though is that the less there is to seal, the better.
Posted by conradb212 at April 9, 2010 08:52 PM