The Tanis folks have come up with an interesting idea: a small, electric air-recirculation unit that passes crankcase air through dessicant crystals to keep your engine's innards dry (and cut down rust formation on steel parts, presumably). Tanis has found that after a flight, humidity inside the crankcase can be from 85% to 98% (which makes sense, because I'd expect even a small amount of ring blowby to force an enormous amount of water vapor into the case). After hooking the dehydrator unit up, the humidity can be drawn down to around 10%, according to Tanis. (You "hook it up" by attaching one hose to the oil filler neck and the other to the crankcase breather line.)
Interestingly, Tanis claims: "We have taken up to one cup of water out of a hot simulated crankcase without reactivating the desiccant. At that point it would not pull the relative humidity below 27% and the desiccant was pink indicating it was time to reactivate." (The desiccant crystals can be reactivated by heating them for four hours. You can do this right in the unit: Just open a little door and flip a switch. An LCD screen shows humidity and temperature in real time.)
Tanis has a FAQ document that makes interesting reading, and the User's Guide is available online as well.
Sporty's is selling the Tanis Dehydrator for $649.
Is it worth it? Clearly, if it makes your engine last longer, it is worth it. So the real question is, of course, whether it will extend the life of a real engine under real conditions. (And clearly, I don't have the answer to that!) We know that a steel part, put in a humidity closet, will rust quickly. And your crankcase is indeed a humidity closet, of sorts. But your camshaft (and most other steel parts in the engine) is coated with engine oil after a flight. Oil is a pretty good barrier to oxidative attack. Most engine parts also have a thin carbon coat from oil being "cooked down" (but I don't think there's enough of a carbon film on cam lobes or lifter faces to provide any help there).
So the question that lingers in my mind is: How long does it take for enough oil to drain off of, say, an O-320 camshaft to allow oxidation to start accelerating? I don't have the answer to that, although I'm sure this is something that could be adequately simulated in a test lab. I'm not worried about crankcase humidity being 98% after engine shutdown, because in the first minutes after shutdown, parts are pretty well covered in oil. But a day later? That's another matter.
Where does this leave us, then?
I don't have a lot of science to back me up on this one. But I think most people would agree that being based in a coastal area (where it's humid year-round) doesn't do anything good to the inside of an engine between flights. (And it's well-accepted that inactivity is bad for an engine.) So if I were based in a humid part of the world and my plane wasn't stored in a humidity-controlled hangar, I'd be strongly inclined to invest in the Tanis device.
But let's be clear on what a "humid part of the country" is. It's not just coastal areas. At http://ggweather.com/ccd/avgrh.htm, I found a chart of average humidity values (night and day, for each month of the year) for 280 U.S. cities. I ran a quick analysis of the data and found that there are only 5 cities where humidity doesn't exceed 50% at least one month out of the year. Amazingly, 80% of the cities experience an average humidity of 80% (or more) at least one month out of the year. I have to admit this comes as a shock to me.
So unless you live in the desert (and never leave it), you're going to encounter a lot of humidity, at least part of the time.
More reason to consider the Tanis device.
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