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Posted by Kiwi G II on December 18, 2002 at 18:56:31:In Reply to: Theoretic question about humidifier posted by CluelessandTired on December 17, 2002 at 20:04:42:
Quite apart from flow rates and pressures, one factor is the slight temperature rise caused by the CPAP, due to the work done by the fan to increase the pressure?
If you look at a psychometric chart for air, you'll see that even a small increase in the "dry bulb" temperature (i.e. the temperature that would be measured by an ordinary thermometer) will generate a rather large drop in relative humidity of the air (i.e. make it much drier). This effect becomes greater at lower room air temperatures.
When outside air is heated as it is brought indoors, the same effect occurs. The air becomes much drier, the relative humidity drops. I noticed it especially on a recent trip when my humidifer was stolen. I was fine in France, with temperatures around 12 - 20 C. A few days later in Norway and Sweden where the outdoor temperature dropped to as low as -10 C I was really suffering. I eventually had to replace the humidifier.
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