Arkansas HVACR NewsMagazine November 2024

HVACR NewsMagazine November 2024

Tech News

So, why don't we measure the true amount of moisture as it is instead of as a ratio to the current temperature? Absolute humidity may tell us precisely how much water vapor is in the air, but it doesn't provide the critical context that RH does. RH lets us know how easy or difficult it is to sweat (which is why humid heat is so much more dangerous than dry heat). It also gives us an idea of how close we are to the dew point, which is when water vapor condenses and becomes a liquid. When our ducts and vents sweat, those surfaces are below the dew point. That doesn't even scratch the surface of the role of RH in IAQ. (The chart below speaks for itself; it's precisely why we aim for an indoor RH of 30 – 60%.)

So, why does the air in the supply airstream often have an RH of around 100%?

Air is Squishy!

I like to think of air as a sponge, and the water is the humidity. If you drop a sponge in a bucket, it'll be dripping when you take it out. If you put it in a sink and run the water, that water will run right off and not be absorbed. That's because the sponge is holding all the water it possibly can. That's essentially what it's like when the air is at 100% RH, also known as the dew point. If you squeeze the sponge, you'll wring the water right out because the sponge can't hold as much water when it's smaller. The same is true of colder air; hotter air can hold more moisture than cold air. Think of that sponge as our air mass.