Rainout.....why?

[mamazipp]

I’ve never quite understood rainout and hoping someone can explain it to me.  
This has not happened to me before but after sleeping for about 3 hours last night I was awakened by a gurgling sound, and ended up pouring quite a bit of water out of my heated hose.
My heated hose is set to Manual, 74* and humidity at 5.  My room is around 70*. The settings were chosen by trying to reduce feeling hot at night so were basically random choices that have been working for many weeks.
What am i doing to have produced this wet tube?  Any suggestions appreciated.  Somehow,  the science of it eludes me.
Thanks for any suggestions.

[Melman]

Rainout is the same phenomenon that causes dew on the grass in the morning and condensation on a can of cold drink. The temperature of the inside surface of the hose is low enough to cause condensation of the water vapor from your humidifier. You need to reduce the level of humidity or increase the temp of the hose. I can't explain why this is suddenly happening if your humidity setting, hose temp, and room temp have not changed. I suspect one of those has changed.

[elwoodpdowd]

I was just thinking that what if you set the humidifier to auto and decrease the temperature of the hose to reduce the heat setting for the hose. For me personally I'm happy with 27 Celsius and on auto humidifier.

[Stom]

My heated hose is set to Manual, 74* and humidity at 5.  My room is around 70*.

Clearly, since you are getting rainout now and didn't before, something has changed. The trick is finding out what has changed. One possibility is that your heated hose is broken.

[mesenteria]

I’ve never quite understood rainout and hoping someone can explain it to me.  
...

I'll give it a shot.

The air at a given temperature has a limited ability to 'hold' moisture.  As the temperature of the air rises, so can the relative humidity content.  Warmer air holds more moisture as a general rule, but not always, not everywhere.  It depends on what moisture is available for the air to 'take up.'

You can have warmish air with low moisture content, so low humidity.  It may be below its upper limit.  So, warm and drier air running over a surface with water will take up more water as it travels further and further over the water until it can't hold any more moisture/humidity.  As long as nothing else changes, the air should retain that level of humidity. 

Now, run that warm, moisture-laden air over a cool jug of milk just set upon the kitchen counter.  What do we know happens?  The air loses temperature nearest the milk because that's the way heat flows....away from itself.  It 'radiates'.  Energy always tries to find its lowest state, and it does that by attempting to warm up the cold milk jug.  Problem is, it's also laden with that maximum load of water content.  As the air loses heat, it must give up some of its moisture.  The moisture will either fall as precipitation or condense, with the latter preferred in this case.  The milk gets warmer even FASTER because, not only is it being warmed by the air flowing around it, but it's helping to draw the moisture away from the air flowing nearest the jug.

Now, imagine the room temperature air moving over the water surface in the reservoir, and it flows through a warmed hose.  No problem, and it doesn't need to give up its newly acquired water content.  But, if the hose is cooler than the air that was warmed briefly as it moved over the heated water (there's a small heater plate at the bottom of the reservoir), the flowing air cools and must give up some of that water.  It condenses just as it would flowing around the milk jug.  After a while, you have enough water to cause the hose to sag under the new weight, which encourages even more to move toward the sag, and soon you have a puddle through which the air cannot move, or the flow is sufficiently powerful that it bubbles its way through.

[SarcasticDave94]

Quite a good explanation of this rainout thing.

As several have mentioned, 1 of these 3, tube or room temperature or the humidifier setting have changed. So to reduce rainout, you'll need to modify these settings until you have the desired effect.

Touching on the heated tube possible failure: how old is your tube? Any obvious signs of damage? If you've got a second one on hand, try to swap it out and see if this helps.

[kilowatt]

In the past I sometimes had this problem, I hung my hose above my head, took away the loop so any water drained back into humidifier.

[Big Guy]

In the past I sometimes had this problem, I hung my hose above my head, took away the loop so any water drained back into humidifier.

I've never had a rain out issue as of yet. My CPAP machine sits on a short file cabinet alongside our bed. The hose runs up to the top of our bed's headboard and then to a hose hanger. 

No way any water can run up the hose that way. 

[DaveL]

Hi!
ahh....thermodynamics. When the temperature of the air in the hose is below the dew point of the air, you get condensation in the hose.

I was born in Victoria; it's humid where you are. you may not even need humidification in Vancouver!

Let the machine do the thinking....Try the auto setting on your hose first. I've set temperature higher than you have. I've set it as high as I can. Frankly, I have trouble determining whether mine is working at all. I don't have rainout and that's good!

Next, if you have a defective heated hose, I bought a ResMed heated hose from a Vancouver supplier. Their price was 1/2 what i would pay locally. (Toronto). And shipping was free. You may even be using that supplier...try asking them.

[sonicboom]

Cutting to the chase - most find that setting the tube to Auto solves most rainout issues.