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System one humidifier
Now you're cookin'. The higher you run the humidifier (like #5) the more likely you will need a hose heater. That's assuming you will have a pretty cool bedroom. If your bedroom is at 70° F. or above, you won't likely need a hose heater.

Condensation inside the mask is another problem all together and the usual cure is to apply some sort of insulating material to the outside of the mask housing so that it stays warm enough (from your body heat) to not dew point your exhaled breath, which is practically 100% RH. The dew point depression of your exhaled air is very small so a cool mask will give you what is called "rain out". Rain out can cause your mask to lose its seal and you'll get air leakage that will either wake you up or possibly render your therapy less effective.
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Ok, cool, I just wnated to make sure I had it right and that anyone else reading the thread got it too.
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(09-18-2014, 05:25 PM)surferdude2 Wrote: The numbers 1 through 5 are only arbitrary numbers to imply that a higher setting will give you higher humidity in the air being passed through the humidifier.

The coefficient of heat transfer between the small surface of water in the humidifier and a rapidly moving stream of air is quite low as compared to the latent heat that is being transferred via the law of positive pressures as is taking place due to a change of state of the liquid water into a vapor. What little specific heat that will be added to the air in the process we are analyzing will not be be easily measurable without a lab instrument that can split a single degree. A high coefficient heat exchanger requires a large surface area in order to achieve a high deltaT when the air is moving rapidly. The major part of the electrical energy we are inputting into the water is being given off as latent heat of vaporization and does so largely independent of the surface area size.

You could prove all that by placing a common thermometer at the outlet of the air hose and let it blow until it stabilizes and then read the temperature. Then turn on the humidifier to its maximum setting and wait to see if the thermometer reading changes. Not very likely and not likely very much.

First I have a question. What is the law of positive pressures?

Second, you said that the coefficient of heat transfer is small due to the small surface area and the rapid movement of air across it. The coefficient of heat transfer at the air/water interface is the same regardless of the velocity of the air. What the velocity of the air affects is the temperature rise of the air.

What we are actually doing by heating the water is increasing its partial pressure which results in faster evaporation.

I do agree that the rise in temperature of the air for someone like me who is using a full face mask and running 20 cm/H2O is probably insignificant but I am not sure that someone using a nasal pillows mask with an 8 cm/H2O would be unable to detect a temp increase.

Best Regards,


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As you no doubt know, I meant to say the law of partial pressures. Sometimes referred to as Dalton's Law. It states that the total pressure of a gas is a sum of partial pressures of each individual gas it contains. It's an empirical law so there can be deviations but that not generally of any concern in most calcs.

You're correct in saying the heat coefficient is a constant and I didn't want to imply that it changed but merely compare the amount of heat being added by it versus the latent heat being added by evaporation.

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(09-15-2014, 11:02 PM)Galactus Wrote:
(09-15-2014, 10:51 PM)LJones Wrote: I'm a little confused on the settings as well... Does the dial control the heat level or the volume of the humidity? Both? I'd like more moisture (desert), but I don't like breathing in hot air... (Though I am accused of frequently being full of it)

It is the desired humidity level, 1 being dry, 5 being most humidity.

Hey you two, let me bring this thread full circle, Akoona Mattata and all that. The question has been asked, and answered. Let the beating of the dead horses be done.
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Precious isn't it? Big Grin
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