# Myth of Condensation in Fuel Tanks



## Martinini (Jun 18, 2008)

What does everyone think about this?

The Myth of Condensation in Fuel Tanks
by David Pascoe

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Frequently we hear it said that the cause of water in fuel tanks is due to condensation. I have long doubted this assertion but the issue has come up so frequently that I was finally motivated to try prove to the point. The basis of my belief or assumption is that:

There isn't enough air volume within a tank to hold much vapor.

On average, tanks are half full, further reducing volume

The amount of water vapor in air is very small, even at 100% humidity

Conditions aren't right to cause condensation in a fuel tank

Research produced the following values for the maximum amount of liquid water in air at the following temperatures:

30C/86F 30 grams/cubic meter 
20C/50F 17 grams/cubic meter 
10C/13F 9 grams/cubic meter

There are 28 grams per ounce, so 30 grams = 1.07 oz; 17 grams = 0.6428 oz.

A cubic meter equals 264 gallons of liquid volume, therefore:

A 200 gallon tank = 0.76 cubic meter.

At 86F, an empty 200 gallon tank could contain 22.8 grams of water vapor, or 0.81 oz.

At 50F, an empty 200 gallon tank could contain 12.92 grams of water vapor, or 0.46 oz.

Note that this is the maximum amount of water vapor that a completely empty tank could contain, in neither case a full ounce of water.

In order to condense water out of the atmosphere a surface must be much colder than the air. The problem for the condensation in tank theory is; how do we end up with a fuel tank that is much colder than the air? One way would be to have a very cold day that suddenly warms up dramatically, but when does this ever happen? The weather can turn cold very fast, but does not suddenly get very warm.

Aluminum is second only to copper for rapid heat transfer properties; it will therefore adjust to atmospheric temperature changes quickly. Gasoline and diesel fuel, like water absorb [sic] heat and cold slowly. Thus one might expect to see sweating on the outside of a tank as the day warms up from cold mornings, but do we? Well, I can say that after 35 years of inspecting boats, I've rarely seen tanks sweating. Note: Sweating may be likely to occur with boats in very cold waters when warm days are encountered.

This issue first came up a number of years ago over a question of whether internal engine rusting could be due to condensation caused by sudden temperature changes as from day to night and vice versa. Since that time, inspection of hundreds of engines showed that rust only occurs on the underside of valve covers due to water contamination of the oil. Very few engines have rusty undersides of valve covers, thereby proving the point that ice cold engine blocks in the morning don't sweat at it warms up during the day. If that is true, then how could it be true that fuel tanks sweat?

My answer is that they don't and these calculations prove it. My initial assumptions were correct. You do not need to store or lay up your boat with full fuel tanks. If you are getting water in your fuel, it is getting there some other way.

Contaminated Fuel

Years ago we had serious fuel contamination problems due to underground steel storage tanks that rusted and leaked. Today all tanks are fiberglass, so this no longer happens (that I know of). However, those underground tanks do have fill plates on the ground surface (usually the parking lot) that can leak just like your boat deck plate. As the marina pumps its tanks nearly dry before the next fuel delivery, those who buy fuel from the near empty tank are the ones that are going to get the water (because it's pumped from the bottom of the tank). This despite the fact that the dock fuel pump has a water separating filter. I've opened the panels on occasion and have found the sight bowls completely filled with water, so at this point the water is being passed on to the customer. Next time you buy diesel, ask to see the filter at the pump! You have to remove the lower pump panel to see it.

However, it is important to note if you're getting water from your fuel retailer, chances are that it won't be a small amount. Most likely it will be a lot and your filters will fill up and engines crap out post-haste.

Leaking Fill Caps

By far the most common cause of contaminated boat tanks are deck fill caps that leak. Most of these things are stupidly mounted flat on decks which may puddle with water. The cap has a tiny little O-ring that is supposed to seal and keep water out. DOES IT? I wouldn't depend on one of these things unless I could prove that it doesn't leak. Check the condition of the o-ring and weather it is sealing.

One way to check positively is to clean the o-ring seat thoroughly; next apply some black or any color paint to the o-ring and screw the cap in place, tight. Then remove it and see if the paint has been completely transferred to the ring seat. If not, you now know where the problem is.

Another problem is the simple failure to seat the cap fully after refueling. This actually happens a lot, so check to see if the cap is loose.

The Tank Vent

Improperly located fuel tank vent fittings are one of the top causes of water getting into tanks. When this is the cause, if you are a salt water boater, then it will be salt water in your tank. A fuel tank vent fitting on the side of the hull should be angled down and aftward. If angled in any other direction, you've got a problem that needs fixing. Watch out for deteriorated plastic and zinc alloy fittings; some of these things deteriorate incredibly fast.

The vent line should have a riser loop on the inside. That is, it travels upward first, then downward. If not, that is another potential problem.

Check the Fuel Gauge Sender

One final possibility is the fuel gauge sender plate on top of the fuel tank. These are often made of steel or have steel screws that can rust away, a situation I've seen several times. Is water puddling on the tank top? Test all screws with a screw driver to make sure they are securely seated.

Posted February 21, 2004


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## eMKay (Aug 18, 2007)

LOL @ "myth" It's not a myth, it's basic physics.


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## Stillraining (Jan 11, 2008)

Very interesting and informative...I disagree to some extent...OK the air volume is small but how many times does that volume change over through venting? also where does he think all that white smoke ( Steam ) come from every morning we all start our cars?...yep...condensation...lastly every valve cover I have ever pulled off was pretty well coated with a film of oil...not very conducive to rusting period..Just some thoughts off the top of my head..


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## Maine Sail (Jan 6, 2003)

> In order to condense water out of the atmosphere a surface must be much colder than the air. The problem for the condensation in tank theory is; how do we end up with a fuel tank that is much colder than the air? One way would be to have a very cold day that suddenly warms up dramatically, but when does this ever happen? The weather can turn cold very fast, but does not suddenly get very warm.


In Maine in the winter where we can see temp swings inside boats of more than 40-50 degrees from night time lows in the single digits to the am sun hitting the shrink wrap, or dark colored Awlgriped top sides, and warming the boats interior to 40 degrees in a mater of an hour or two.... That said I am not a real believer that physical water appears in tanks when they are left half full etc... I leave mine empty...


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## sailingdog (Mar 19, 2006)

Also, if the boat is stored on the water, the level of water vapor available for condensing goes way up.


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## SteveInMD (May 11, 2007)

Stillraining...actually water is a product of burning gasoline. It condenses in the cold exhaust system before the exhaust system heats up. As the system heats we see it exit as steam.


I believe it's fairly common for fuel to have some water in it when we buy it. I've doubted to condensation theory for a while.


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## SailKing1 (Feb 20, 2002)

As for having water in fuel when you buy it, you are correct. Back in the late 80s I worked for an auto dealership service dept. We did a test of exon, mobile, shell and 7-11 gas. Filled a shot glass with fuel from each (no we did not drink it) and poured it out on the concrete and set on fire. The idea or bet i should say was to see which would leave the largest wet spot when finished burning. Supposidly this was from water in the fuel.


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## Classic30 (Aug 29, 2007)

Fuel (all hydrocarbon fuels - even LPG) contain water in differing amounts. I dunno what it is for Diesel, but for Aviation Kerosene the limit is 30ppm. The refiners don't worry about it too much because, in small quantities water can actually aid combustion - it's when there is too much it can be a problem. Two things...

1. Entrained water doesn't mix with oil and so settles out over time (several hours), causing the "condensation" you see in the bottom of the tank. ie. The water has "condensed" out of the fuel.

2. Most hydrocarbon fuels significantly change in volume with changes in temperature, so if you buy fuel at, say 70degF and fill your tank on a 60degF day, the fuel volume will decrease gradually drawing a small amount of air in through the vent. Any moisture in the air will then settle on the tank walls and add to the "condensation" at the bottom of the tank.

The way to avoid issues with water in your fuel?? You have two options: Either pump it out before you go or recirculate it though your fuel system on a regular basis.. 



halekai36 said:


> *Yes or .81oz EVERY DAY THE CONDITIONS ARE RIGHT FOR CONDENSATION!!!!!!!! Someone remind him there are 365 days per year and tanks in boats should be expected to last well into the 30 year mark..?? Even if your tank could only hold .01 oz of water vapor over time it ADDS UP!!!!!*


Halekai, there's a small problem with your premise - an empty tank doesn't have any fuel in it.. If what you're saying is correct, over time an empty tank would magically fill itself with water - in practise it doesn't.


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## camaraderie (May 22, 2002)

Cameron...the empty tank allows the moisture to evaporate. Under a nice layer of diesel it does not evaporate. 
Pascoe has other interesting theories that contradict every other expert. 
In this case, he is simply dead wrong as actual experience affirms for many of us.


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## Maine Sail (Jan 6, 2003)

> Halekai, there's a small problem with your premise - an empty tank doesn't have any fuel in it.. If what you're saying is correct, over time an empty tank would magically fill itself with water - in practise it doesn't.


 Of course it won't condensate with no fuel to _slow the temperature change_..

FWIW I drain my tank in the winter. I also remove my wet exhaust hose and plug the exhaust and air intake.. Overkill? Maybe but I have yet to ever rebuild or even repair the internals, fuel injection pump or injectors of a diesel engine and never had a single drop of physical water in my tanks, so, I think I'll keep doing it..


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## sailingdog (Mar 19, 2006)

Cameron-

Also, once a certain amount of water was in the tank, the partial pressure of the water evaporating would prevent more water from entering the tank and condensing out.


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## Classic30 (Aug 29, 2007)

halekai36 said:


> FWIW I drain my tank in the winter and plug the vent to prevent any air turn over. I also remove my wet exhaust hose and plug the exhaust and air intake.. Overkill? Maybe but I have yet to ever rebuild or even repair the internals, fuel injection pump or injectors of a diesel engine so I think I'll keep doing it..


Draining the tank for the winter is an excellent idea, but I'd suggest leaving a small amount of fuel around (just the dregs) and cleaning this out with a rag before you refill. Fuel vapour will minimise the chance of rust forming and contain any condensed water.

As for plugging the vent: So long as you remember to unplug it when you refill the tank, you'll be fine, but I wouldn't suggest this if you've still got fuel in the system and you're likely to have extremes of temperature. I have a few photos here someplace of a large storage tank that collapsed because of a cold snap whilst the vents plugged for maintenance (painting) - highly embarassing for the owners!


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## sailaway21 (Sep 4, 2006)

Mr. Pascoe spent a great deal of effort to get it exactly wrong.

The first fundamental error he made was in associating fuel tank temperature with air temperature. Fuel tank temperature is much more likely to be associated with sea water temperature than ambient conditions. the air in the tank though is much more likely to be closer to ambient temperature and so will go through a warming and cooling phase each day.

This can easily be observed in the bone dry cargo tanks of a tanker. Dry the night before and an inch of water on the double bottoms in the morning. Condensation.

It is not entrained water that produces water during combustion. Water is a natural by-product of combustion. Those of you with 80-90% efficient furnaces know this because your furnace has a combustion chamber drain on it! That's the one that is usually PVC and goes to a little pump in a box which pumps it to your drain line or, alternatively it goes to your floor drain.

All fuels expand and contract with temperature change. If you take bunkers in Maine and press your tanks up, and then sail to Florida without using any fuel you can expect to see fuel coming out your tank vent.

Refineries routinely strip water out of their fuel oil tanks. It is not regarded as a desirable aid to combustion in their view. Likewise motorists are cautioned to avoid service stations with a history of water in their fuel tanks. There is technology for injecting water during the combustion process to provide cleaner burning of less refined products but the water is injected at the burner tip and is not carried in the fuel tanks. Water in the fuel tanks of any combustion type power plant is undesirable. Steam turbine engines, specifically oil-fired boilers, have fuel water separators that the yachtsman would envy. Slugs of water have a nasty habit of first extinguishing the flame at the burner tip and then, when fuel returns, the combustion process is reignited by the heat off the brick-works of the boiler in an uncontrolled fashion generally resulting in catastrophic failure of the firebox. The USS John F. Kennedy got an unexpected three month shipyard visit from ignoring such matters.

And yes, if you were to place an empty tank in 60F water, half submerged, with an ambient daytime temperature of 80F, and vented it, you might be quite surprised at the amount of water that would magically appear in it's bottom. Filling it half full with fuel oil, of any type, would not significantly alter the results.

Lastly, the data cited on rusting within internal combustion engines due to condensation is poppycock, particularly for engines in a marine (read humid) environment. Diesel lifeboat motors have block heaters that are left plugged in year 'round regardless of temperature. Aside from keeping the lubricating oil warm in cold climes they reduce condensation, particularly in the tropics. and one of the prime reasons that biocides are used within diesel storage tanks that do not get drained and refilled routinely is that the condensed water vapour is a major ingredient to the algae that grows in diesel fuel.

There might have been something in Mr. Pascoe's conclusions that was correct but, if there was, I've either overlooked it or it's nonexistent.


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## sailingdog (Mar 19, 2006)

Actually, if condensation did not occur, there would probably be no need for a biocide additive to diesel fuels, as the algae that grows in the tanks only grows along the diesel and water interface. No water--->no algae.


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## peikenberry (Apr 26, 2000)

I have known David Pascoe for years and admire his expertise on many things, but this is something we disagree on. Sailaway's analysis is right on. Sailingdogs comment about biocides is too. 

Condensation does occur in tanks, especially metal tanks when there are extreme temperature swings.


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## noreault (May 14, 2008)

Its not the case that it condensation doesn't occur, its just that it is to little to be concerned about. Also the tank is not filled with air, it is air and fuel vapors.

By the way for all the water to condense out of the air, it has to go to zero percent humidity.


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## Classic30 (Aug 29, 2007)

Martinini said:


> Contaminated Fuel
> 
> Years ago we had serious fuel contamination problems due to underground steel storage tanks that rusted and leaked. Today all tanks are fiberglass, so this no longer happens (that I know of). However, those underground tanks do have fill plates on the ground surface (usually the parking lot) that can leak just like your boat deck plate. As the marina pumps its tanks nearly dry before the next fuel delivery, those who buy fuel from the near empty tank are the ones that are going to get the water (because it's pumped from the bottom of the tank). This despite the fact that the dock fuel pump has a water separating filter. I've opened the panels on occasion and have found the sight bowls completely filled with water, so at this point the water is being passed on to the customer. Next time you buy diesel, ask to see the filter at the pump! You have to remove the lower pump panel to see it.
> 
> However, it is important to note if you're getting water from your fuel retailer, chances are that it won't be a small amount. Most likely it will be a lot and your filters will fill up and engines crap out post-haste.


Just thought I'd comment on this bit and try to keep it non-technical (although I really should get some work done!):

Underground fuel storage tanks at marinas and service stations are typically double-skinned fibreglass; although aboveground ones can be fibreglass or steel.

When installed, the tanks are not level - one end is *deliberately* lower than the other. Fuel is either pumped from just below the surface (called "floating suction") or from near the bottom of the tank at the high end. At the low end is a sump with a sample drain. Over here (Down Under), for the Big 4 Oil Co's, the water drains are checked *daily* and any water pumped out - dunno what the regime is over there, but I assume it's the same.

The fuel bowser is fitted with a water-separating filter designed so that any entrained water is trapped at the bottom whilst the fuel flows out the side. The sight glass on these things shows the amount of water separated out of the fuel since last emptied. Some have an automatic cistern-style dump-valve that automatically drains the water away to a small holding tank when the sight-glass is full. Remember: The sight-glass is *not* an indication that water is being pumped in to your tank!

Pascoe is correct that if you get water from your fuel retailer, it won't be a small amount. If you're worried, check your tank each time you fill up and before you start the engine - but I'd think that the retailer would be more worried than you are to allow this to happen - he could get sued! - and would make sure his sample points are checked regularly.

Hope you find this helpful..


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## Classic30 (Aug 29, 2007)

noreault said:


> Its not the case that it condensation doesn't occur, its just that it is to little to be concerned about. Also the tank is not filled with air, it is air and fuel vapors.
> 
> By the way for all the water to condense out of the air, it has to go to zero percent humidity.


Noreault, the water is mostly in the fuel itself and not so much in the air - and a significant amount can settle out over time.

It doesn't take much of a slug of water in your fuel system to block the injectors and stop your engine mid-stroke.


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## Stillraining (Jan 11, 2008)

SteveInMD said:


> Stillraining...actually water is a product of burning gasoline. It condenses in the cold exhaust system before the exhaust system heats up. As the system heats we see it exit as steam.


I did not know that...I guess that explains why my hands always feel wet if I use the exhaust to warm them....Thanks...
Sillearning..

One of life's little mysteries solved.


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## Plumper (Nov 21, 2007)

I get more than .81oz of condensate on my glasses when I come in from the cold. Any water is bad. It all accumulates at the bottom of the tank providing a great spot for those anaerobic microbes.


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## cardiacpaul (Jun 20, 2006)

I'll let Mr. Pascoe buy the draft beer at the local sports bar.
The glass (or fuel tank) sweats like a pig looking at the knife. And thats just a 12 ouncer. Now, multiply that by oh, 40- 50 gallons of fuel stored in cool underground tanks, plus the air exchange cause by usage, and general re. humidity, and his math goes flying out the window.

And he obviously hasn't seen thecubans hair on a west palm beach monday morning... 
current conditions here,
ambiant temp, 73, dew point 70, rel. humidity, 90%

conditions in Las Vegas?
ambiant temp, 89, dew point 29, rel. humidity, 12%

myth? my skinny white butt.


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## chucklesR (Sep 17, 2007)

Pascoe's entire premise is incorrect because he talks / calculates as if the fuel tank was a closed system; i.e., he completely ignores the FACT that the tank is vented and outside air (latent with humidity) will circulate through.

That's the problem when the chalk board meets the real world, facts keep getting in the way.


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## TSOJOURNER (Dec 16, 1999)

not a likely source for water in the fuel but: aluminium can be porous


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## Maine Sail (Jan 6, 2003)

Bullofbarney said:


> not a likely source for water in the fuel but: aluminium can be porous


Quick someone let Anheuser-Busch know that their cans are letting in water! Hence the watery flavor I guess..


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## sailingdog (Mar 19, 2006)

Not only is the fuel system vented, it is also designed to act as a pump...as the air heats up, it will pick up more moisture, since the relative humidity drops....then as it cools down, it will dump the moisture out...where ever it can... repeat this over the heating and cooling cycle of day, especially up north, where the night temperatures can be 20-30 degrees cooler than the daytime temps... you're gonna get water collecting. Of course, as pointed out previously, with a nice layer of diesel sitting on top of the collected water, it ain't gonna get a chance to evaporate and go back into the air.


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## Rockter (Sep 11, 2006)

I wish it didn't condense, but it does. 
When I open the top of the tank, you can see it on the walls.

Don't forget to factor in radiated heat. If the water is cold, and with it the hull, the tank will radiate heat to the cooler body. Even in Scotland, where humidity is very rarely high, you still see it on the walls of the tank.

It is an unwelcome mystery.


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## chris_gee (May 31, 2006)

I think Pascoe gets the basic premise wrong when he writes "In order to condense water out of the atmosphere a surface must be much colder than the air. The problem for the condensation in tank theory is; how do we end up with a fuel tank that is much colder than the air? One way would be to have a very cold day that suddenly warms up dramatically, but when does this ever happen? The weather can turn cold very fast, but does not suddenly get very warm."
The problem is at the interface. It is the outside air that gets cold and this chills the top of the tank and thus lowers the water the air in the tank can hold giving condensation which in due course settles to the bottom of the tank.
You see this with fog where the sea or land is warmer than the air but the air is cold enough that it cannot hold the moisture it previously did. It persists through lack of air movement a condition which applies in a boat giving condensation.
In essence he fails to distinguish the outside air and inside air when he asks how the metal can be colder than the air particularly as it contains fuel. 
The air in the tank may be warmer than the outside air but colder than it was so there is some temperature gradient, with the metal being colder than the inside air and condensation from the inside air having reduced water holding capacity. The less full the tank the greater the temperature drop and the greater the air volume able to condense water. Over time the few cc of water will add up.


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## Stillraining (Jan 11, 2008)

chris_gee said:


> I think Pascoe gets the basic premise wrong when he writes "In order to condense water out of the atmosphere a surface must be much colder than the air. The problem for the condensation in tank theory is; how do we end up with a fuel tank that is much colder than the air? One way would be to have a very cold day that suddenly warms up dramatically, but when does this ever happen? The weather can turn cold very fast, but does not suddenly get very warm."
> The problem is at the interface. It is the outside air that gets cold and this chills the top of the tank and thus lowers the water the air in the tank can hold giving condensation which in due course settles to the bottom of the tank.
> You see this with fog where the sea or land is warmer than the air but the air is cold enough that it cannot hold the moisture it previously did. It persists through lack of air movement a condition which applies in a boat giving condensation.
> In essence he fails to distinguish the outside air and inside air when he asks how the metal can be colder than the air particularly as it contains fuel.
> The air in the tank may be warmer than the outside air but colder than it was so there is some temperature gradient, with the metal being colder than the inside air and condensation from the inside air having reduced water holding capacity. The less full the tank the greater the temperature drop and the greater the air volume able to condense water. Over time the few cc of water will add up.


Good points and Its not just that...Have you ever put your hand on your tank after motoring for say one hour...All diesels have a return line from the injector pump that is returning very warm fuel back to the tank all the while your running your engine...This fuel is at least 80 to 90 degrees from my experience..

It dosent take long to bring up your tank to this temperature..


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## Boasun (Feb 10, 2007)

You fellas mean to tell me that you don't have the water vapor filter on your fuel vents?? You really need to get one for your fuel tanks. 
You can get those WV filters at the same place you can get the 'Cloud spliter' for your sextant that is used on heavily overcasted days.
Any quesions?


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## bubb2 (Nov 9, 2002)

cardiacpaul said:


> I'll let Mr. Pascoe buy the draft beer at the local sports bar.
> The glass (or fuel tank) sweats like a pig looking at the knife. And thats just a 12 ouncer. Now, multiply that by oh, 40- 50 gallons of fuel stored in cool underground tanks, plus the air exchange cause by usage, and general re. humidity, and his math goes flying out the window.
> 
> And he obviously hasn't seen thecubans hair on a west palm beach monday morning...
> ...


You aways give a answer I can understand. thank you, Paul


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## TSOJOURNER (Dec 16, 1999)

I think we all agree ( except possibly Mr Pascoe) that there is some water vapor condensing in our fuel tanks. It may be very small, but since it sinks under the fuel and therefore does not evaporate (like the morning dew) it will accumulate. However, I have recently read that one operator of a large, well known marina on Lake Michigan, suggests that there is more water in your diesel fuel fron deck fill leakage than any other source. Most of us top off our tanks while winterizing our boats, but how many of us carefully inspect the sealing o-ring on the deck plate? I just checked mine and found it was cracked badly enough that I really don't need the vent connection! My fuel fill plate is on a horizontal surface, and while it will not hold a puddle of water, I am sure there has been some rain that has gotten in there. Since the current flood conditions prevent me from sailing, next weekend will be spent replacing o-rings and pumping fuel from the bottom of tanks for water removal and filtration, on my boat and a couple others I care for! Have you checked your o-ring lately? By the way, my water tank filler also needs a new o-ring - the only good one was on the cap of the holding tank!


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## KeelHaulin (Mar 7, 2006)

I think the calculation is incorrect also; the problem is that a vented tank will always allow more water vapor in to balance with the humidity of the outside air. The condensation accumulates because as said before (condensed water drips and then sinks beneath the diesel or gas).

But I also agree with the premise of his article - much more water can get in via a leaking deck fill or vent that gets submerged while heeled. Check to be sure that your tank vent is not "going under" and check the o-rings on your deck fills to be sure they are properly sealing.


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## GaryHLucas (Mar 24, 2008)

I believe Mr. Pascoe is not correct. I used to build a keyboard box that was 6" x 3" x 9" for a machine used in greenhouses, which are hot and high humidity. It was connected to another electrical box by a 25 conductor cable. We had problems with water getting in all the time. We then went to a box with a submersible rating. To test it we put the boxes in a bucket of water with a brick holding them under for week at a time. They were always completely dry after this test. We then put them out to customers. Aftera month or so every single one had at least a teaspoon of water in it! The other enclosure though was completely dry. We finally figured it out. The other enclosure had some electronics in that produced a little heat. It was always 10F warmer than the greenhouse. The keyboard though had no heat source. During the day it got real hot in the sun. The hot air was forced out through the strands of the cable, and every nigh the cooler damp air got sucked back in, to condense in box!

Drilling a drain hole in the bottom of the submersible enclosure kept it MUCH dryer! A little heat would have completely soved the problem, but we had no spare wires to do that.


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## bobmcgov (Jul 19, 2007)

I put a new gas tank on my motorcycle -- old one rotted thru from (yes) condensation moisture. I fogged the new tank before I mounted it. But for one reason and another, the bike lived thus in storage for two years. When I opened the stopcock on the bottom after that two years, a half cup of water came out. This is a new tank, never seen gasoline, so the water didn't come from fuel. And we live in a desert -- typically less than 25% relative humidity. It is dry, dry, dry, and the dew point is a loooong way down there.

If not condensation, where did that water come from? Stored indoors, so it isn't rain. Very selective vandals? Mouse urine? Well, Mr. Pascoe?


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## rbarnes (May 4, 2009)

*Condensation*

I disagree.
Arizona is a relatively dry state (10% or less most of the year) and hot as well (115 degrees at times) and yet........... enough condensation accumulates in the sumps of airplane tanks to bring them down if not properly drained. If it is a myth, it is a killer of a myth !!!!!
My boat in the middle of the desert gets condensation in its 3 gallon tank.


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## motion300 (Feb 12, 2009)

I have rarely seen water in a tank that did not contain a large quanity og dirt and junk This tell me it came from the supplier and not handled properly


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## RichH (Jul 10, 2000)

As a physical chemist and mechanical engineer, who has for nearly 40 years worked deeply in fluid and thermal sciences, the ultrapurification of fluids (including exotic 'oils', dielectric oils, etc. etc. for the electric power generation, semiconductor, ultrapure chemical, bio-pharma industries, etc. ), etc. etc. etc. .... Mr. Pascoe is essentially correct, not totally correct but close. Sorry to state this; but, most of you who somehow 'intuitively' oppose Mr. Pascoes statement are not correct. 

1. Most oil made from cracking or distillation are almost completely dehydrated by the heat of the process - water content essentially nil.
2. When in CONTACT WITH **ATMOSPHERIC** AIR, water (as vapor and as free gaseous molecules) enters the mass of oils, etc. due to ***vapor pressure equilibrium drive (partial pressure of the water vs. the partial pressure of the oil)***. 
3. Water in oil exists in various 'states': 
a. molecular water (azeotropic mixture) ... insensible without using 'instrumentation', etc. 
b. emulsified water (water that is held 'in suspension') ...insensible without using instrumentation, etc. 
c. free water ... sensible water that can be observed *without* instrumentation, etc. In typical oil mixtures, free water is the only 'phase' of water able to 'gravimetrically settle out'. 

Water that is 'condensing' on tank walls, etc. is the result of the ~'end stage' of the equilibrium changes to the oil being SATURATED with water ---- the oil is now becoming **fully SATURATED** with water (molecular water, emulsified water, AND free water). The same physical laws of vapor pressure 'equilibrium' which is temperature dependent can reverse the equilibrium direction so that the relatively warm oil, etc. begins to lose it water content ... and the water (as vapor) begins to condense on adjacent cooler surfaces. The greater the water saturation of the oil the greater the equilibrium drive now in the 'opposite direction' if the adjacent surfaces, etc. are cooler (and according to the partial pressure of the water, etc.) ... and the water vapor in the oil NOW condenses on the cooler tank walls ... only to gravimetrically fall/slide back down into fluid until it settles on the bottom. 
If you took the same amount of oil and poured into an open 'pan' and exposed it to atmosphere .... it would 'pick-up' essentially the SAME amount of water as if it were in a 'tank' exposed to atmosphere ... and there would be NO evidence of 'condensation' !!!! 

The water that gravimetrically settles to the bottom no longer becomes a 'true' part of the mixture (a 'liquid-liquid phase boundary' forms between the oil and free water) .. and this influences to the sum of the partial pressures 'above' the phase boundary ... but leaving the oil (+water mixture) above the boundary free to re-equilibrate and accept vapor migration from additional (humid) atmospheric air in contact with the oil surface. .... and now what is occurring (again by equilibrium) could be described as a 'pump with out moving parts' being operated solely by physical & chemical 'equilibrium'. 

Water that is 'condensing' on the walls of a tank is primarily a SYMPTOM that the oil is SATURATED with water - chemical equilibrium (partial pressure equilibrium) moved the water vapor in the atmosphere into the oil. 

Dont want 'water' in your oil, .... keep the MINIMUM amount of oil in your tank, put a desiccant trap (silica gel, activated alumina, etc.) on the vent line .... or if you have a 'vacuum rated' tank simply close the vent valve when the engine is off; the minimum mass/volume of oil will 'pick-up' the minimum amount of water vapor ... so DONT keep your tank 'topped-off' especially if you're not using the oil quickly !!!!! Its the MASS of oil in the tank and the sum of partial pressures (equilibrium) of oil AND water vapor that is causing the 'transfer' .... not a tank wall !!!!! Condensation is a SYMPTOM that oil is ALREADY becoming fully saturated with water. 

In 'industry', ultrapure oils that have become saturated with water are typically placed into a vacuum chamber, a strong vacuum is applied (to change the partial pressures which reverses the equilibrium) ... and THEN, the oil is run through water absorbing 'filters' (filters that contain the starch hydroxymethylcellulose (the same stuff in "pampers") to achieve less than 1 ppm water content. If water saturation is your problem you can buy filters that contain a water absorbing starch (for gasoline fuels) from the 'typical' suppliers. 

Question? If your 'intuitive' condensation hypothesis are correct, then why don't EMPTY tanks (exposed to atmospheric air) automatically fill up with water ????? 

BTW- My 'fuel system' includes a constant recirculation filtration stage (including a free water 'knock-out' trap), the tank VENT line includes a bio-blocking filter to retard **fungal spores**, and the vent includes a desiccant chamber to remove incoming water vapor down to -40 deg. dewpoint .... 
I gots no water, gots no biological fouling, gets my 'filters' free but hardly ever change them, .... gots NO 'fuel problems' in a 100 gallon black iron fuel tank ... and I never have to down into the bilge during a heavy seastate to change out filters .... and 'power-puke' at the same time. My system also has a 2 gallon 'day tank' that stores filtered oil ... if there is ever a 'problem' I can simply open the 'weir valve' and vent valve on the day tank and let the oil 'drain by gravity' down to the engine .... for about ~2-3 hours run time - time enough to 'motor-away' from any danger, etc. 

Water enters oil/fuel by *chemical equilibrium* (the sum of partial vapor pressures of the 'constituents'); the MORE oil, the MORE water. Condensation is a *symptom* that liquid is now FULLY SATURATED with water. Empty tanks (with atmospheric vents) dont automatically fill with water.


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## bobmcgov (Jul 19, 2007)

RichH said:


> Question? If your 'intuitive' condensation hypothesis are correct, then why don't EMPTY tanks (exposed to atmospheric air) automatically fill up with water ?????
> 
> Empty tanks (with atmospheric vents) dont automatically fill with water.


Rich: read my post above. I admire your knowledge of oil/water phase boundary behaviors ... but tank side condensation can take place in parallel to what you describe.

Good science has to explain the observed phenomena. I removed better than half a cup of water from a small, vented motorcycle tank that had never contained gasoline or oil, stored indoors, in a desert. Pls explain how it got there.

Cyclical heating and cooling of a metal vessel in the presence of water-bearing air will result in the accumulation of water condensate in the tank. Solar stills work on this principle.


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## pdqaltair (Nov 14, 2008)

*Is it all about temperature swings, and airplanes see the greatest swings of all.*

E-10 exaggerates this, and the following link explains:

http://www.epa.gov/otaq/regs/fuels/rfg/waterphs.pdf

True condensation, without fuel - common sense tells us this is rare (tanks do not fill). Water accumulation due to saturation followed by temperature swing - common, though not as common as leaking filler caps.

I am a chemical engineer working around refineries, and the effect is real.


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## Guzzi (Oct 9, 2008)

Anyone ever had to drain the fuel tanks on an airplane of the water that condenses overnight?
They have drains in them just for that purpose. Evaporation of the fuel causes cooling. The sun warms the surface of the wing and you have cool inside/warm outside, voila! Condensation. Fill a glass with water and add ice. What happens? Condensation forms on the glass.

E-10 makes matters worse, alcohol is hydroscopic-it attracts water, making the condensation problem worse, as the water doesn't separate out as in pure gas. Water and alky mix, but don't burn well.

Ragtimedon-I was a guest aboard your beautiful Endeavor one afternoon when you and your wife gave us a tour.
I'm a friend of Tom, the guy down the dock with the Helms 27.
Might have a boat after next week...gotta find it first.

Jon


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## Capnblu (Mar 17, 2006)

Well said Rich.


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## pdqaltair (Nov 14, 2008)

*Which is why e-10 is not used in aircraft... of on the Hood River bar.*



Guzzi said:


> Anyone ever had to drain the fuel tanks on an airplane of the water that condenses overnight?
> They have drains in them just for that purpose. Evaporation of the fuel causes cooling. The sun warms the surface of the wing and you have cool inside/warm outside, voila! Condensation. Fill a glass with water and add ice. What happens? Condensation forms on the glass.
> 
> E-10 makes matters worse, alcohol is hydroscopic-it attracts water, making the condensation problem worse, as the water doesn't separate out as in pure gas. Water and alky mix, but don't burn well.
> ...


11111


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## badsanta (Oct 16, 2006)

I replaced all my deck fill O rings as they were all flat and cracked. Just in case.


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## RichH (Jul 10, 2000)

bobmcgov said:


> Good science has to explain the observed phenomena. I removed better than half a cup of water from a small, vented motorcycle tank that had never contained gasoline or oil, stored indoors, in a desert. Pls explain how it got there.


Please reread my post ... especially the part that states that when the FUEL becomes saturated with water, the symptom of the full saturation becomes the condensation effect. With dehydrated fuels (as normally delivered from a refinery) will not appreciably 'condensate' until the fuel becomes saturated with water.


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## RichH (Jul 10, 2000)

bobmcgov said:


> Good science has to explain the observed phenomena. I removed better than half a cup of water from a small, vented motorcycle tank that had never contained gasoline or oil, stored indoors, in a desert. Pls explain how it got there.


I did, its called partial vapor pressure equilibrium. What you had is 'old' fuel that became saturated with water, the free water gravimetrically settled and crossed the liquid-liquid phase boundary in the bottom of the tank and started 'pumping'.

Next time close the vent on your tank and see what happens. If you leave fuel in direct contact with atmospheric air it WILL become saturated with water - thats the whole point of my 'discertation'. Again, 'condensation' is the end stage symptom that the fuel HAS become saturated with water. 
Either close the vent (on tanks that can withstand the pressures) upon long term non-usage, or 'enjoy' water saturation.


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## camaraderie (May 22, 2002)

RichH said:


> What you had is 'old' fuel that became saturated with water, the free water gravimetrically settled and crossed the liquid-liquid phase boundary in the bottom of the tank and started 'pumping'.


Rich...he specifically states there was never ANY fuel in his tank.


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## wright1d (May 21, 2009)

So I'm still confused?

Why leave the tank filled in the winter if this is going to happen anyway? Sounds like the water settles to the bottom pushing the lighter fuel up?


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## RichH (Jul 10, 2000)

camaraderie said:


> Rich...he specifically states there was never ANY fuel in his tank.


Naaaaah, that didnt happen !!!!! In a desert with normal dewpoints at 30-40 deg. of less, it would take hundreds of years for a cupfull of water to accumulate in an EMPTY motorcycle tank (1-2ft. cu. ft). If there was thermally driven mass transfer (heat/cooling cycles of the tank), then the same mass transfer would equilibrate BACK to the atmosphere - net equilibrium =~0. The same reason that the WWII Saharan debris is still in the SAME condition as the day its was destroyed. --- very 'doubtful'.


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