Humidity Experiments Interlude: The Dew Point Mystery

When we think about humidity, we usually think in terms of %RH, relative humidity. Mostly that’s because that’s what’s easiest to measure, and what drives most physical phenomena that matter to us. But it’s not the whole story, especially when we talk about dehydrating stuff.

RH is a measure of how much moisture is in the air RELATIVE to how much moisture it can hold. Warmer air can hold more moisture, so therefore, if you take the same air and heat it without letting any gasses exchange, the relative humidity will drop.

If you want to know ABSOLUTE humidity, that is, how much moisture is in your air total, you can discuss that a couple different ways. One is in units of mass: grams of water per kg of air, or perhaps grams per liter of air. Another way is dew point, which is a single temperature representing the concentration of water vapor in air.

The dew point of a parcel of air is the temperature where the air will become saturated and water will start condensing, or dewing up. It depends only on the amount of water in the air, not its current temperature. When you wake up in the morning and your car has dew on it, that’s because the surface temperature of the car got low enough overnight that it was below the dew point of the air. This can often happen even when it doesn’t rain or get foggy, because surfaces outdoors will radiatively cool overnight in the dark, often to a temperature below the ambient air temp.

So how do we measure dew point? One way is to calculate it from relative humidity and temperature. It’s not trivial, but knowing RH and temp, you can calculate the absolute humidity of the air, and therefore the temperature at which it would saturate – the dew point. Another option is to measure it directly using a device called a chilled mirror hygrometer. This device works by shining a light at a mirror, measuring how much of that light is reflected, then cooling the mirror to the point where the reflected light amount starts decreasing. It starts decreasing when dew starts forming on the mirror, so by measuring this temperature precisely, we directly measure the dew point.

So armed with this knowledge, we can set up a simple experiment with a simple hypothesis: If we put a sensor in one of our moisture barrier bags, then we seal that bag up and heat it, RH should drop but dew point should stay perfectly the same at all temperatures. Let’s try it!

I put a bluetooth temp/humidity monitor in a moisture barrier bag, left it at room temp for a bit, then put it in a lab oven at 50C for a bit. I then let it cool down and heat back up, all the while logging.

The results did NOT show a consistent dew point!

I don’t know what formula the “bmLogger” app is using to calculate dewpoint, but the results agree reasonably with Magnus-Tetens which should be accurate to 0.35C in the range of -40 to 50c, which our experiment adheres to. Given (generous) temperature/humidity uncertainties of 1F and 3%RH, I calculated the possible range of the true dew point given the measured temp and RH:

So even with error bars the dew point increased by at least 8.7 degrees, when it should have stayed constant. Why?

I have two theories so far. One is moisture absorbed into the walls of the bag. We know the bag is made of plastic, and we know plastic tends to be hygroscopic, so the plastic could be absorbing moisture in the cool condition and releasing it into the air in the bag in the hot condition.

Another theory is similar: mositure adsorption. Most materials support some degree of moisture or other gas adsorption to the surface, which is why you typically need to “bake out” a vacuum chamber by heating the metal walls while pulling vacuum, to liberate (mainly water vapor) from them so it doesn’t later foul your experiment.

Trying to test the former theory, I tried using a glass jar instead of the plastic bag, figuring it should be similarly moisture impermeable but also wouldn’t itself ABSORB moisture. There was a second consideration here: with the plastic bag, the air mass can expand as it becomes less dense with heat. The glass should maintain the airmass density while pressure increases. Maybe.

One interesting thing about this part of the experiment: Previously with the bag, the temperature and relative humidity both settled to steady state after temp change smoothly without overshoot, however clearly temperature responded faster than RH in the oven since dew point overshot before settling. However with the glass, HUMIDITY overshot, while dewpoint had an underdamped response. That doesn’t make a lot of sense to me. The internal RH sensing element is temperature-dependent, but I don’t know how that could be related to this behavior. Once again, tabulating results:

This suggests at least a 20F rise in dew point. It surprises me that that’s even larger than before,

So I’m sort of out of theories for the time being. I think my next step is to actually build a chilled mirror hygrometer to ground the calculated dew points and verify that the temp/humidity sensor agrees with a direct measure of dew point.

If you’ve got any theories as to what I’m missing or how this all works, I’d love to hear them!