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Vapor Pressure Deficit vs Relative Humidity

maine_tyler | Posted in General Questions on

It wasn’t until recently that I had heard of Vapor Pressure Deficit (VPD).

It is similar to RH, but is simply the difference between saturation and actual vapor pressure at a given temp– as opposed to a ratio.
https://www.youtube.com/watch?v=IqD8uTg9Fgw

But VPD is what directly affects evaporation rates, not RH.
It is well-known by horticulturists and my googling yields mostly documents focused on plants.

So my question is: does VPD serve any useful purpose for human comfort, or other non-horticultural applications? 

Building scientists like to talk about Dew Point, for good reason, but will bring up RH when talking about interior humidity levels in winter (for example) in regards to human comfort or ‘safe levels’. It seems that technically, VPD would be the more accurate measure for human comfort since it governs the rate that moisture will leave the skin.

Of course, something is only useful if it is widely understood and easily measured. Additionally, if we assume that interior temperatures for human dwellings are kept within a pretty narrow temperature band, RH will give useful comparisons. VPD gives useful comparisons over wider temperature swings (look at the diverging RH lines as you move right on the psychrometric chart to see this).

But there may be some niche times when understanding VPD will help in understanding our environment. If, for example, we compare a house kept at 60F and 35RH, vs a house at 72F and 35RH, the latter will ‘feel’ drier because there is a greater vapor pressure deficit, even though RH is the same for both and even though absolute humidity is higher in the latter case.

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Replies

  1. charlie_sullivan | | #1

    I think that's a good thing to consider in some cases. For example, if you are thinking about a building material that is exposed to cyclical conditions day and night, and you want to know whether the drying during part of the time will be fast enough to make up for the moisture gained other times. RH is the controlling factor for what equilibrium it will reach, but VPD affects how quickly it moves in that direction, so just considering RH could miss something for dynamic situations.

    It's interesting that there are so many different parameters that you can use to describe the 2D space shown on a psychrometric chart. It seems excessive, but there are real reasons for all of them.

  2. creativedestruction | | #2

    Tyler,

    All great points. I think you've pointed to the hitch here: "widely understood and easily measured." I barely trust my home thermostat and add a margin of +/-10% on RH measurements. Like most people I don't have good instruments. It could take a retraining of the masses if someday new humidistats start to include VPD but I'd vote in favor; your logic behind it is sound. Maybe some already do(?)

  3. Expert Member
    KYLE WINSTON BENTLEY | | #3

    That term gives me verbage for a concept I've been trying to think of for quite a bit.

    A low RH means there's room for water vapor to evaporate into the air, but it never really tells you how much, because of the temperature dependence. AH tells you how much water is in the air, but now how much room is left.

  4. maine_tyler | | #4

    Well lo and behold, I didn't realize this, but the data logger I already have (Sensor Push) has VPD as one of its outputs.
    It's a simple calc after all. Many greenhouse tenders prefer it I believe so many sensors feature it.

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