(09-16-2015, 04:31 PM)zonk Wrote: ...I fill the tank with tap water..., just below the top line, use about 1/3, and dump the rest of the water.
Water use depend on bedroom temperature, humidifier and pressure settings, mouth breathing and mouth leaks uses more water too.
The humidifier methodology is passive-evaporative, meaning that the amount of evaporation is based entirely
on the ability of the air in contact with the surface of the water to take up water molecules.
That is modulated by five factors; including water temp, air temp, surface area, and the amount of water already in the air (ambient humidity). It is also modulated by how regularly the air in contact with the water surface is evacuated and replaced by new air.
[As a side issue, that last phenomenon is also the reason we humidify the air in the first place, because more air moves in your nose and sinus area while on xPAP pressure than when breathing still ambient air, and the turbulence and movement of the moving air will act to remove moisture from your tissues if that air is not already humidified. The humidifier fixes that problem by raising the humidity of the breathed air to ~ the same humidity that your nasal/sinus area has naturally, meaning that the moving air has less or no ability to remove that moisture.
Close inspection of the H5i tank reveals that air enters in a little funnel area designed to speed up the air and cause more Venturi turbulence, along with a little staging void just above the tank design with circular ends which preserves the speed and turbulence and creates a little vortex effect so the the air is moving as quickly and turbulently as is possible as it hits the water surface, increasing the evaporative function based on how quickly the air in contact with the surface is changed out; if the air in contact with a surface molecule accepts that molecule, then that speck of air has to be moved out of the way before a new molecule can evaporate into "new, dry" air.
This means that the airflow is cleverly engineered to create maximum air movement in the tank, at every pressure, which enhances the efficiency of the H process.]
Back on topic, while pressure will modulate how often air is replaced a tiny bit, at every pressure air is quickly and regularly replaced with new air, so how far the tank goes down will be nearly the same at 4 cm as it is at 20 cm (with all else held equal). The turbulence process (see above) also means that air moves quickly at all pressures, so that humidification is relatively the same for high pressures as well as low pressures.
And that means that leaks, which only cause the pressure to change on APAP-capable machines and only for a short time, and which change the pressure marginally, will have very little effect on how much water you have left in the morning. Same for changing pressure a couple of cm.
The temp of the water is constantly the same for a given H setting, and H %-age is based on the setting. IOW, a higher humidity setting is achieved by heating the water to a higher temp.
Since water temp is essentially constant, surface area is constant, and since air flow quickly replaces humidified air with ambient air at every pressure setting, those three of the five factors are basically insignificant.
And of the two remaining significant factors, while ambient room temp might mean the air temp varies a bit as air enters the humidifier, far and away the largest factor is how humidified the air is as it enters the system, which is based on ambient humidity, because that is what determines chiefly the ability of the air to take on more water molecules.
In AZ the humidity indoors is 30% part of the year, and under 10% the rest of the year, and my tank reflects this appropriately; in the spring and fall with the windows open I go through nearly a whole tank, while with windows closed (winter and summer) I go through about half a tank.
I recommend Zonk's procedure; fill it all the way, and dump, rinse (I even soak it) and fill with fresh. If you do this, it will keep the mineral precip onto the sides of the tank to a minimum (which is the only problem with using tap water). If you have a nearly empty tank, this means that the remaining water left in there is fully super-mineralized, so just adding more water adds to that problem, as does not filling it all the way.