How Well Do You Understand Psychrometrics?

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1. What is denser: dry air or humid air? Assume that the dry-bulb temperature and atmospheric pressure are constant.

   1. Oh, that’s a good one. I should have put that in there, too.

      1. Since no one was willing to take a stab at this question, I will give the answer. Humid air is less dense than dry air. The difference isn’t a lot, but most people I know think that humid air is “heavier”.

         1. See you saying that air at the same temperature that is more humid is lighter?

            1. Yep

2. When air passes through a furnace, the RH decreases. Does that mean the furnace is “drying” the air, i.e., removing water vapor from it?

   1. Roy: That’s another good one. I covered that in an article in 2014.

      Does a Gas Furnace Dry Out the Air in Your Home?

3. I was correct on numbers 1, 2 5,8 and 10.

   I am giving myself partial credit on 3 and 4: I did not read 3 completely…missed the “what climate are you in?” part of the question. On 4 – I said to just warm up the inside of the windshield, forgot the part of AC and setting the system to non-recirculating air.

   I was wrong on 6 and 7.

4. A good set of questions! I initially interpreted the first one to be about the resultant humidity in the indoor space, rather than being strictly about the infiltrating air’s relative humidity, since the latter cannot be separated from the other air in the house. If the outside air were relatively cold vs. the inside (say 55F) and near the dew point (as we sometimes experienced in the Atlanta area when we lived there…fog over the area), leakage into the house would bring in a significant amount of moisture, while on the other side of the house (the leeward side, or maybe the recessed light fixture cans) the leaving air would presumably be at a the 70F interior temperature, and much lower RH and dew point than the incoming leakage air. So wouldn’t the answer to this question, if considering the RH of the living space, and not just the intruding air, be that the moisture in the space (dew point) and thus the RH have risen (slightly), not fallen, since the indoor temperature stays essentially the same (assuming a normal ratio of intruding air to total house air volume)?

5. “ Yep. If there’s nothing for the water vapor to condense onto, you can get supersaturated air at higher than 100% relative humidity.”

   What is an example of this? Fog? As I recall fog is water entrained in 100% RH air.

6. “ 1. On a cold rainy day, outdoor air leaks into a house and warms up to room temperature. What happens to the indoor relative humidity?”

   On a similar note, if the HVAC system is bringing in a lot of OA during the winter on say a commercial application with an economizer and high internal load, the specific humidity inside will approach the specific humidity of the OA.

   An exception would be if you have a high latent load inside the space such as a relatively large number of people.

7. I consider myself, a thoughtful HVAC contractor and local (northeast Florida) BS guru, pretty good on this psychrometric stuff. You got me fair and square on #7… I’ve seen similar behavior with liquid water heated above its boiling point without being stirred or agitated, and same with liquid water cooled below freezing. In both situations there needs to be some sort of physical starting point for vapor bubbles and ice crystal formation.

   I agree that the temperature and humidity conditions in #10 are so extreme as to have yet to be recorded (Though my news feed recently reported 158*F heat index in Iran…not sure what the underlying drybulb and humidity conditions went into that 158*F heat index).

   That said, I wonder if we in the southeast and southwest US do very occasionally experience an unrecorded micro-climate at or near 100*F drybulb and 90%RH…stay with me a minute or so:

   Scenario – Hot sunny day with area weather stations reporting drybulb mid 90s or higher. Picture a typical dark asphalt shadeless parking lot outside a mall or Walmart supercenter – acres of asphalt with a surface temperature easily above 125*F…way too hot to touch. Air temp right above that parking lot is easily 5-10*F above local weather station figures. An afternoon thundershower passes nearby, close enough to drop, say, a tenth of an inch of rain on that asphalt…not enough to substantially cool the pavement, but definitely enough to be rapidly vaporized by the hot pavement and temporarily increase the humidity of the air just above the pavement – the air we walk through enroute to / from the store.

   While feelings aren’t data, I feel like I’ve experienced 100*F DB 90% RH while walking across parking lots right after the above sequence of events…classic microclimate situation.

   Thoughts?

8. Like Marty R and JayW, I too answered #1 as “it increases”, because I was considering the air in the entire space, not just the air that leaked into the house. Is this thought process correct?

   1. I also approached the question this way. The change in relative humidity within the space would depend on whether the dewpoint inside is higher or lower than the dewpoint outside. From a humidity control standpoint, a rainy day at 60F will result in a very muggy interior because the there is enough infiltration to meet the sensible load but the rh could be 70% or more. It all depends on what counts as “cold” for a rainy day.

9. You could ask what your thoughts are about plotting a point on the psychrometric chart using enthalpy and wet bulb as the two values.

   Those two lines are almost parallel.

10. On most psych charts, such as the one in this article, wet-bulb and enthalpy lines are shown as being the same, thus, you can’t plot a point by only knowing the wet-bulb and enthalpy. The ASHRAE psych charts show the wet-bulb and enthalpy lines separately, and they are not quite parallel. If you do try to define the state of moist air with wet-bulb and enthalpy as inputs, you better have very accurate values or you will be way off on determining dry-bulb, RH, humidity ratio, etc.

    1. RoyC

       Agreed that the two lines for Wet Bulb and Enthalpy are almost parallel and thus wouldn’t work for defining a point. If nothing else, a little inaccuracy in one or both would throw the point way off.

       As I recall if you have any other two properties of a moist air condition you can use them to find the exact point on the psych. chart.

       This would include specific humidity, enthalpy, DB Temp, WB Temp, RH, DP Temp.

       Actually for the above besides enthalpy and WB, you also can’t use specific humidity and DP temp together. Specific humidity and DP Temp lies are actually parallel.

11. This 9-variable topic is broken, no new charts* since this horrible psychrometric one came out. This article about the confusion, proves the need for a new way to simplify & organize air-wetness information for use by consumers. Nobody needs to be familiar with the chart shown because they should only care about what goes on between the coolest space they control and the warmest space they control, or between say 60°F and 85°F.

    This is the number one google result on Earth if you search “ Mold Chart”: https://energyhandyman.com/knowledge-library/mold-chart-for-temperature-and-humidity-monitors/

    The purpose, is for use with standard temp and RH sensors, and to help people not to open their windows maybe. The phrase “Mold Chart for Temperature and Relative Humidity Sensors” or something similar is searched on Google millions of times a month by people who have already found the chart provided in this article.

    Stay unconfused my friends.

    1. Adam

       That is an interesting chart in the link you shared. Do you know the source of this data?

       Interesting argument for possibly not needing dehumidification in a well sealed crawl space. I wouldn’t want to get too close to the point where there is a possibility of a mold issue.

       1. Hey Jay,
          100% of the information is from DPCALC.org and there is an older version with more columns and all of those figures also come from the same place. Their formulas are really useful but not when you are presented with the whole spectrum versus the relavant sections only, and also “MY CHART” had to have some rounding out of necessity and as simple as it is people refuse to read the accompanying write-up. The main benefit of both of “MY CHARTS” is the limited temperature range, and the explanations at the bottom. Lots of people who store baseball cards, books, boxes, library systems ACTUALLY USE THIS CHART. You might never find the standard charts in use by anyone other than the author of an article about how tricky it is.

12. Great article and good questions although I do have a quarrel with a couple of them. On #7, the definition of relative humidity is based on the ratio of actual vapor pressure to vapor pressure at saturation. The actual vapor pressure cannot be >saturation. You can have a 2-phase situation, which is what you have when there is fog, but the vapor pressure cannot be > saturation so relative humidity cannot be >100%. There is an analogy with Steam flow. You can have 2-phase steam flow but the pressure is still saturation pressure. The presence of liquid does not increase the “vapor” pressure. The vapor is still at saturation. The saturated vapor just happens to be mixed with some liquid. I get the point you are trying to make, that you can have more moisture than saturation level, but the vapor itself will be saturated.

    My other comment is a nit pick. The question was if I had experienced conditions greater than 100F/90%. My answer was yes, it was in a sauna in Finland, not an ambient condition, but I have experienced it.

    1. I believe you are incorrect, unless I’m misunderstanding your point. Air can indeed be ‘supersaturated’ in which vapor pressure is higher than saturation. RH greater than 100% is definitely possible.

       Supersaturation is not prolonged, but for cloud formation, supersaturation must be greater than 0.

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