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DIY Engine dehumidifier purge pump

Steve Watkins

Active Member
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I found this air mattress pump on Amazon that’s snaps perfectly into the blue oil fill snorkel that comes with Phillips oil. I run it for about 5 minutes after flying to displace all the steam that outgasses from the oil with ambient air. It only make half a psi but moves a lot of air that blows out the breather hose. Before I leave the hangar I switch it out for a home brew dehumidifier that uses a fish tank pump and a container full of desiccant.
 
Tell us more about the home brew dehumidifier. I've been thinking of building some sort of dehumidifier but it sounds like you already figured it out.
 
Great idea, Steve! They also sell portable units that could be used after every landing. Currently I just loosen the dipstick, but this looks like it would be quicker and more effective.
 
I do exactly the same things as Steve. Blow down the crankcase after flying to remove water-saturated hot air and replace it with ambient drier air for 5-10 minutes, then plug in my home brew dehumidifier system which is on a timer for 2 hours per day. This keeps the crankcase around 25% humidity, which will not support rusting on steel parts (camshaft and lifters). I have an extension cord that drops down from the ceiling and hangs a couple of feet above the engine so I don't have to trip over cords on the floor. Simple, clean setup that works well and makes me feel good about my engine. :)
 
My dehumidifier is nothing fancy. It is an aquarium air pump, a plastic mixed nut jar filled with desiccant and a #6 rubber stopper. It runs on a timer for 4 hours after the 5 minute ambient air purge.
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I also made my own dehydrator. Similar to yours except I have a tube in the breather to return the air to the air pump. That way I am slowly driving down the humidity to the limits of the dehydrator (usually 24-28% here in FL). I've noticed that after I land the humidity in the dehydrator goes up to almost 100% so I tried your suggestion of a purge pump. I left it on for 5-10 min today and the max humidity only went up to 44%. I consider that a major improvement because it will increase the longevity of my desiccant before needing to be dried. Thanks for the suggestion.
 
I mean no disrespect. But I am not sure what is to be gained with this setup. I have never seen anyone ever with this setup. I do understand about rust potential on crank and lifters. I like the concept, but is it overkill. ? Airplanes that sit long term perhaps?
 
I mean no disrespect. But I am not sure what is to be gained with this setup. I have never seen anyone ever with this setup. I do understand about rust potential on crank and lifters. I like the concept, but is it overkill. ? Airplanes that sit long term perhaps?
I doubt anyone can tell you conclusively if this is overkill or not. I hope it is. My thinking on this is that avoiding rust is worth a little bit of hassle and cost to try to keep dry air in the engine. Plus it's kind of fun to muck around with these engine dryer systems, at least for me.
 
I mean no disrespect. But I am not sure what is to be gained with this setup. I have never seen anyone ever with this setup. I do understand about rust potential on crank and lifters. I like the concept, but is it overkill. ? Airplanes that sit long term perhaps?
I have used a dehydrator for more than 10 years. My last three oil analysis 9/22, 4/23, and 9/23 report 0% water. I am awaiting the results for the latest oil change.
 
i can't use the power outlets in the hangar continuously because my RV sits on a carousel. but i have an battery powered air pump for my stand up paddle board. seeing this thread me thinks this air pump could provide dual use. tested the pump yesterday, plugged it to the oil filler tube after flight and felt a steady stream of warm/wet air coming out the breather tube. the air exiting the breather was not extreme or violent but was very noticable on my finger.

but could that do more harm than good? if the air stream possibly "wipes off" the oil covering surfaces...? or maybe this hot/humid air is pushed into places inside the engine where it wasn't before?
 
I also built a dehydrator, or more accurately for the one I built, a purge system since it doesn't use dessicant (based in Arizona). The objective is primarily to clear the water formed during combustion. Posted the simple, inexpensive design a few months ago--pic re-attached.

One concern I'd have with the two systems shown above would be that the air pumped in is unfiltered. It's dusty here. If the pump in the initial post is really pumping 100+ gallons/min, and it's running for quite some time, that would be a lot of silica being pumped into the oil sump.
 

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I built this pretty cheaply, I like the desiccant in a vertical tube for a couple reasons. Can watch it progress saturation up the tube and gauge its useful life remaining. In my mind, like a cascade O2 system, I’m maximizing the use of the desiccant by passing through more saturated beads first. Not sure if desiccant really works this way. But feel guaranteed I’m getting dry air as long as top of the tube is orange.
BTW, 7/8 OD tube fits snugly into the oil fill on my IO 360, simple, cheap and secure.
 

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I mean no disrespect. But I am not sure what is to be gained with this setup. I have never seen anyone ever with this setup. I do understand about rust potential on crank and lifters. I like the concept, but is it overkill. ? Airplanes that sit long term perhaps?
I had a C-140 once upon a time. I got concerned about moisture in the crank case one cold morning when I saw frost covering the inside the oil filler tube. I guess frost was all over the inside of the case. so, thats a data point to consider.
 
Internal engine moisture is definitely an issue in most parts of the country. If you scan the matrix on the photo, it will take you to a page with graphical data gathered while using this particular dehydrator. It's real data showing an example of what is going on in your engine. The monitoring probes were inside the engine to collect the data. This particular unit is similar to many of the homebrew units but there are refined features that make it safer and more convenient to use.


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I also built a dehydrator, or more accurately for the one I built, a purge system since it doesn't use dessicant (based in Arizona). The objective is primarily to clear the water formed during combustion. Posted the simple, inexpensive design a few months ago--pic re-attached.

One concern I'd have with the two systems shown above would be that the air pumped in is unfiltered. It's dusty here. If the pump in the initial post is really pumping 100+ gallons/min, and it's running for quite some time, that would be a lot of silica being pumped into the oil sump.
The mattress air pump is unfiltered air, but it only runs for a few minutes to purge the bulk of the humidity out of the crankcase. The small aquarium pump pushes air through a filter, then the desiccant, then another filter. I will send an oil sample to avlab at the next change and compare results to previous samples.
 
i can't use the power outlets in the hangar continuously because my RV sits on a carousel. but i have an battery powered air pump for my stand up paddle board. seeing this thread me thinks this air pump could provide dual use. tested the pump yesterday, plugged it to the oil filler tube after flight and felt a steady stream of warm/wet air coming out the breather tube. the air exiting the breather was not extreme or violent but was very noticable on my finger.

but could that do more harm than good? if the air stream possibly "wipes off" the oil covering surfaces...? or maybe this hot/humid air is pushed into places inside the engine where it wasn't before?
I have a 12volt sealed battery to power a mattress inflator. I suggest you use a HEPA filter for it. If using 3/8" fuel tubing it should it will be around 20-25 liters/min and that is an easy breeze but will purge a 4 cylinder in 3 min, but I leave mine on for 5.

The purge is useful (required IMO) to keep the hot engine blow- by from condensing inside a freshly shut down engine. Condensed exhaust gas/blow-by is corrosive from sulfur in the fuel. The additional drying via desiccant can be lower flow and timed.

Everyone should know that the little rectangular humidor humidity meters don't measure below 20%. I recently used the EliTech logger on a PSA style dryer and found it measures down to 0.6%, and the little humidor one stayed at 22% in the same flow stream.

I'll update all when my breathing study is complete. and validated. A one hour run every 4-7 days will keep the humidity stable and low as your unit produces.

On another note - - I ran quick test of air in and air out humidity. Ignoring a ton of details, the EliTech (X2) measured 3.7% in and 17.7% out after and hour and 37 min of running. I have a 1/2 Raven and very sure this is real. I am highly skeptical that the 17% measured is water. There are temporary contamination in some sensors as they read VOC's. More from the rabbit hole in a dedicated thread.
 
If you use silica desisicant, you need to use a filter on the outlet side. I did not for a while, and Blackstone found silica in my oil analysis. Asked if I had been flying in a desert...
 
What material is used as the desiccant on the Mohave and how does it get recharged? I cant find that on the website or web search. The graph indicates the relative humidity approaches zero. Pretty hard to believe, actually.
 
Oh, I have an aquarium air stone in my desiccant as the filter.
I don't know what the filtering capability of aquarium air stones is, but they are designed primarily as diffusers, not filters. To be effective at filtering silica, you'd need at least a HEPA filter (particles down to 0.3 µm) on the outlet hose of the desiccant chamber.
 
What material is used as the desiccant on the Mohave and how does it get recharged? I cant find that on the website or web search. The graph indicates the relative humidity approaches zero. Pretty hard to believe, actually.
My homemade dehydrator uses 7.5 pounds of silica gel and typically runs between 3 to 5% relative humidity in the desiccant chamber...much more than enough to keep air being pumped into the engine well above the dew point
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My homemade dehydrator uses 7.5 pounds of silica gel and typically runs between 3 to 5% relative humidity in the desiccant chamber...much more than enough to keep air being pumped into the engine well above the dew point
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how long between dry outs does 7 pounds last? What’s your process to dry that much out?
 
What material is used as the desiccant on the Mohave and how does it get recharged? I cant find that on the website or web search. The graph indicates the relative humidity approaches zero. Pretty hard to believe, actually.
The Mojave Dehydrator uses orange indicating silica gel desiccant without colbalt chloride. It saturates with moisture to 15% by weight. I run my dehydrator on my O-360 (RV-7) 100% of the time. I regenerate the desiccant about every 2 months by 'baking' it in the oven at 225F for an hour or two. The system uses 2 lbs that are contained in vented bags for less mess in handling. The air is filtered twice before entering the breather on my engine.

If the graph showed near zero humidity it would be hard to believe. It does not show that.

The small hygrometers included with the Mojave will go down to 10% relative humidity, they are not a scientific instrument only a reference.
 
how long between dry outs does 7 pounds last? What’s your process to dry that much out?
Excuse me, I misspoke. I actually use 15 lbs of desiccant in the chamber. They come in 7.5 lb containers so I use two at time. Depending on ambient relative humidity, they last about 2-3 months. Longer in the winter, shorter in the summer. The stuff turns pink when is reaches saturation, and as it turns pinker I see the humidity in the desiccant chamber go up. I have two additional containers of dessicant, so when the 15 lbs in the chamber turns pink, I pour it out, replace it with the other 15 lbs, and take the pink stuff home to cook in the oven. I put it in a roasting pan at 250°F for about two hours, stir it every so often, and it turns it back to blue. The desiccant I have now has held up fine in this routine for the last 3 years. I use that much (15lbs at a time) so that I don't have to change it out as often. My wife has actually taken over that cooking job, based on her observation that I tend to spill silica beads around the kitchen. Those little suckers do escape quite readily and then bounce a long way on the tile floor of the kitchen. (I've also tried to demonstrate that I don't know how a vacuum cleaner works, but she isn't falling for that one).

I do note that when I was using a higher volume aquarium pump (350 gph) as air source, I had to change the silica more frequently. The one I use now is about 250 gph and is adjustable for output. I don't think it takes a high rate of airflow to do the job.

Mine pumps air in from the bottom through an aquarium stone as filter to keep beads out of the tubing (which is a pain). I don't have a HEPA filter on the output tubing...good idea but I haven't gotten around to it. I see there are several such inline HEPA filters on Amazon, so I guess I'll get some. I will say that in the five Blackstone reports I have since using this thing, no report of silica has come up despite the fact that it is pumping this air directly in through the oil dipstick tube.


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Excuse me, I misspoke. I actually use 15 lbs of desiccant in the chamber. They come in 7.5 lb containers so I use two at time. Depending on ambient relative humidity, they last about 2-3 months. Longer in the winter, shorter in the summer. The stuff turns pink when is reaches saturation, and as it turns pinker I see the humidity in the desiccant chamber go up. I have two additional containers of dessicant, so when the 15 lbs in the chamber turns pink, I pour it out, replace it with the other 15 lbs, and take the pink stuff home to cook in the oven. I put it in a roasting pan at 250°F for about two hours, stir it every so often, and it turns it back to blue. The desiccant I have now has held up fine in this routine for the last 3 years. I use that much (15lbs at a time) so that I don't have to change it out as often. My wife has actually taken over that cooking job, based on her observation that I tend to spill silica beads around the kitchen. Those little suckers do escape quite readily and then bounce a long way on the tile floor of the kitchen. (I've also tried to demonstrate that I don't know how a vacuum cleaner works, but she isn't falling for that one).

I do note that when I was using a higher volume aquarium pump (350 gph) as air source, I had to change the silica more frequently. The one I use now is about 250 gph and is adjustable for output. I don't think it takes a high rate of airflow to do the job.

Mine pumps air in from the bottom through an aquarium stone as filter to keep beads out of the tubing (which is a pain). I don't have a HEPA filter on the output tubing...good idea but I haven't gotten around to it. I will say that in the five Blackstone reports I have, no report of silica has come up despite the fact that it is pumping this air directly in through the oil dipstick tube.


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Are you using a 'closed loop system' where you input into the oil filter neck and return from the breather tube? Seems like you are getting saturated beads as fast as some setups with quite a lot less. Are you running this 24/7 or on a timer for a couple or so hours a day?
 
Dumb questions I guess, but....

These systems appear to pull air out out of the crankcase via the dipstick tube. The inlet is the accessory case breather fitting? Seems like I've seen references to recirculating dryers. Does that mean a connection to the dipstick tube and the breather, or something else?

EDIT: Hmmm, looks like Lucky had the same question more or less.
 
Are you using a 'closed loop system' where you input into the oil filter neck and return from the breather tube? Seems like you are getting saturated beads as fast as some setups with quite a lot less. Are you running this 24/7 or on a timer for a couple or so hours a day?
It runs 24/7/365. Input through the oil dipstick tube. I was using a rubber stopper from the hardware store. It worked great, but I thought a 3D-printed screw-in from Scott Card would be more elegant. It is, but kind of a PITA to screw in. I'd guess that I could decrease amount of silica or decrease change frequency by decreasing air pump volume from 255 gph, but I don't know what the minimum effective air volume would be and this system as-is works to my satisfaction.

I did use a closed system for awhile. I have some larger diameter tubing that slips over the output tube of my ASA oil separator and returns air to the pump input. I stopped using it as getting down on the hangar floor to put it on the separator after each flight was more of a PITA than I wanted to endure for the small benefit that I perceived (less silica bead change). Now, it just pumps dry air in through the dipstick tube and out to the atmosphere from the oil separator/breather tube. I could hook the breather tube back up any time if I wanted, I just don't want to.
 
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Duhhhh on me...air in through the dipstick tube.

Pumping one way means drying atmospheric air prior to flow through the crankcase. Seems like that would mean a shorter bead cycle than recirculating just the engine air, in particular here in the steamy deep South. Am I missing something?
 
The money shot would be a randomized prospective large-N study looking at whether or not the use of an engine dehydrator resulted in a longer TBOs or objectively decreased cam/cylinder corrosion. In this application, that would be a completely impractical study so all we can say is that "crankcase moisture is necessary for corrosion, therefore decreasing crankcase moisture will decrease corrosion". Such inductive reasoning is not actual evidence but it's all we have, and a dehydrator use on the basis of "can't hurt/might help" is reasonable IMHO, especially when you factor in the small investment involved.
 
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Duhhhh on me...air in through the dipstick tube.

Pumping one way means drying atmospheric air prior to flow through the crankcase. Seems like that would mean a shorter bead cycle than recirculating just the engine air, in particular here in the steamy deep South. Am I missing something?
No, I think that's exactly correct. Recirculating the dried air in a closed-loop would certainly result in less bead change, and I found that to be true back when I was using such a closed loop, but AFAIK that would be the only advantage to closed-loop. I opted to accomplish the same end by just buying more silica to make it less of a PITA to hook the thing up after a flight. That degree of perceived PITA will vary widely from pilot to pilot. I chose the easier but more expensive route, bearing in mind that I live in less-than-steamy Minnesota.
 
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The system I have sucks air from the dipstick and pumps in through a filter into the breather tube. I can visibly see the moisture coming out of the oil filler hose going into the silica box. My system runs 24-7 and it takes about a month or two to go from 5% to about 20% humidity to when I cook the beads.
 
The system I have sucks air from the dipstick and pumps in through a filter into the breather tube. I can visibly see the moisture coming out of the oil filler hose going into the silica box. My system runs 24-7 and it takes about a month or two to go from 5% to about 20% humidity to when I cook the beads.
My experience too, although I pump the air in the opposite direction. I'm assuming that the direction of the circuit wouldn't matter.
 
Dumb questions I guess, but....

These systems appear to pull air out out of the crankcase via the dipstick tube. The inlet is the accessory case breather fitting? Seems like I've seen references to recirculating dryers. Does that mean a connection to the dipstick tube and the breather, or something else?

EDIT: Hmmm, looks like Lucky had the same question more or less.
Dan, Mine pushed dry air into the dipstick location. It has a HEPA filter for both the high rate purge phase (5min) and dehydration phase(60min). When I started a recirculation system was used but it contaminated the desiccant with VOC's and they made a big stink when regenerated. Conclusion: bad idea to recirculate. Now, it runs on timers with purge.
 
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Dan, Mine pushed dry air into the dipstick location. It has a heap filter for both the high rate purge phase (5min) and dehydration phase(60min). When I started a recirculation system was used but it contaminated the desiccant with VOC's and they made a big stink when regenerated. Conclusion: bad idea to recirculate. Now, it runs on timers with purge.

Big stink! Now that's some practical information for sure. I have an oven in my shop, but stink is stink, even when the wife isn't involved.
 
Dan, Mine pushed dry air into the dipstick location. It has a heap filter for both the high rate purge phase (5min) and dehydration phase(60min). When I started a recirculation system was used but it contaminated the desiccant with VOC's and they made a big stink when regenerated. Conclusion: bad idea to recirculate. Now, it runs on timers with purge.

Oh jeez...there isn't a bloodhound alive that has a better sense of smell than my wife and she does the silica gel cooking for me. This tip is very important in maintaining my happy marriage.
 
I use a recirculating system on a timer. I extended the crankcase vent down and back next to one of the exhaust pipes. I have a silicone sleeve on the end of the flex tubing that fits good over the vent pipe. I had the oil contamination/stink issue until I started venting the crankcase with an air mattress infllator for 10 min. after each flight. This cools everything down and vents the water/oil saturated air right away. Sometimes I wait a day or so to hook up the dehydrator system. I run the system about two hours a day now, which seems plenty. I use 5 lb. of dessicant and recharge every 2-3 months in a small convection oven in my shop. Keep those little beads out of the kitchen!
 
Dan, Mine pushed dry air into the dipstick location. When I started a recirculation system was used but it contaminated the desiccant with VOC's and they made a big stink when regenerated. Conclusion: bad idea to recirculate.
+1. Stunk out the house when I tried recircing. Not great. I use a 1.75L bottle of silica one way in through the dipstick. I also put plastic pill bottles over the ends of the exhaust pipes to cap off air infiltration. The desiccant will last about 1 week in summer and 3 in winter, here in the frozen north, before drying. Used to dry in the oven but now I now use an old crock pot on low with the lid just cracked to dry the beads. Left on over night the beads are dry the next day and no worry about over heating and turning them black and useless, which has happened to a few on our field who use a microwave for drying.
 
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