Saturation, Dewpoint, and Vapor Pressure

Tutorial on concepts of humidity, including depoint, saturation, vapor pressure, and relative humidity.

[@allsops3062]

This has enhanced my understanding. Your teaching style meets with my learning style. Thank you.

[@nancyhope2205]

That was very useful. I came here because I am trying to understand a recent SciAm paper about how the planet is browning due atmospheric water vapour pressure deficit. At the same time as warming adds 7% more moisture to the air per degree C of heating. I think I understood your discussion but I still want to be sure I understand. The saturation pressure or dew point is going up faster than the relative ambient vapour pressure goes up.

[@Lady_Katie]

Great video! Thank you!

[@quincylongman7469]

This makes more sense than what my prof has been teaching. Thanks for doing this!

[@BrianBlais]

no problem! anything else that you'd like to hear about?

[@quincylongman7469]

We'll see how my test goes tomorrow.. haha! thanks again!

[@allsops3062]

hi Brian i came across an exercise that requires the calculation of the RH that I am having trouble with...i'll use similar but fictitious numbers... the outdoor air temp is say -30C with an RH of 75% and is heated to indoor temp (room temp) at 22C. Do you have any tips that may help in approaching this type of question and remembering the formula, if any? any help is appreciated tnx

[@shotgundanl]

Thank you! Spent way too much time going in circles on these problems.

[@casanovaena]

Thanks, I always confused that svp should give me dew point, but now cleared..

[@danielpaul8003]

thanks, understandable and helpful to me.

[@BrianBlais]

This is the same thing as before. The actual equation for making the curve above is SVP=6.75 [mb] x 2^{T/11.5 [C]}. Thus, for T=-30 C this translates to about 1 [mb]. 75% of that is 0.75 [mb] and is the VP. The new saturated level, at 22C is around 25 [mb]. This the new RH would be 0.75 [mb]/25 [mb] x 100% = 3% RH - really dry! This is one of the reasons heating systems in the winter produce very dry air. Does that help?

[@Dont430]

Brian, in a compressed air system, lets say the air is compressed to 150PSI, moves through a dryer (lowering DP to 40F), what affect on PDT does sending it through a reducer to 120PSI? How does this affect DP if the air entering the compressor is saturated at 100F?

[@anaranjith]

Thanks a lot

[@gooddeedsleadto7499]

SVP= Maximum vapor air can hold Dew Point= The temperature at SVP. What is precipitation? Precipitation temperature is lower than condensation temperature & pressure? Could u please verify the temperature at which the fluid boils is below its dew point? Boiling temperature=condensation temperature ? Thanks

[@BrianBlais]

thanks! glad I can help!

[@hasansio]

Really Thanks for better clarification about the topic sir

[@jollyscaria1922]

Iam jolly,sir,,,,,How muchheat must beremoved from 500lb0f befto coolit from 94to34'F=ws(t2--t1)=500×077×(34'F-94'F)=385×60=-23100btuofheatAns,sinesthe Aneswerisa minus numberit indicates heattobere moved, ●●●¿vapres expls,,,,144btu perlb heatoffusuonof ice meltasexpls3cane, s,2ndis,bturaise,1b water,1'F970btuperlb=heatofvaporization, 3rd,,only about0.5btu raisllb steam 1'F,,,1can ice met,2,canwater boils,,3,can,superheatingdependinon the temperarure,

[@Jeka2008]

Thanks

[@zhichuangli1343]

where can I get the saturated vapor pressure curve of some other substances, like CO2?

[@BrianBlais]

There are some empirical equations described here: en.wikipedia.org/wiki/Antoine_equation and I found a large list of the fitted parameters here: www.eng.auburn.edu/~drmills/mans486/Diffusion%20Tube/Antoine_coefficient_table.PDF but I am sure there are other places.

[@anaranjith]

Is the wet bulb temperature and dew point temperature are same?

[@carultch]

No. At saturation, they are the same, and the same as the dry bulb temperature, but anywhere else on this chart, all three will be different. Wet bulb temperature is achieved in a swamp cooler, while dry bulb temperature is achieved in an air conditioner's evaporator duct. Wet bulb temperature is what you get if you evaporate water into the air to cool it. Heat lost by the air becomes heat gained by the vapor, and the end result is that you end with a lower temperature of the more humid mixture. Dew point by contrast, is what you get if you cool air at a constant absolute humidity, and is the point at which the absolute humidity can no longer remain constant while you are cooling it. The water vapor has to start condensing in order to continue cooling the air mass.

[@meteorockogist]

Thank you sooooo much. I am in my final year of meteorology school and was still so baffled that I couldn’t grasp svp. You made this all so easy, I wish you were my professor.

[@BrianBlais]

Glad I could help! Good luck with school.

[@Mrgabouful]

How do you find the dew point again?

[@carultch]

Locate the point on the chart, and trace horizontally to the left. Identify, where it intersects the saturation curve, and trace downward to the corresponding temperature. That is the dew point. The dew point is where the airmass will cool to saturation, if it doesn't gain or lose any water vapor along the way. It will need a sink for its heat, such as an Air conditioner's evaporator, or the grass over the nighttime collecting dew (the namesake for the dew point).

[@sarahkaveh7739]

Thank you🤩

[@rodrigox.8037]

excellent video, but i have a few questions (my thermodynamics teacher went through this very fast...): what does it mean when you say "air at X°C temperature cannot hold more water vapor, it will condensate"? what's happening phisically that makes the air unable to hold more vapor? why is it that cold air is more easily saturated (again, what is happening phisically)?

[@BrianBlais]

it actually is the equilibrium between condensation and evaporation - I was speaking a little roughly for convenience. at higher temperature, the water vapor in the air can be higher to be in equilibrium between the two processes of condensation and evaporation.

[@razzmatazzh]

Hi, I really liked the video, liked how you explain the concepts. I am working on designing a thermodynamic model for a air compressor and would like to get in touch with you in regards to the same, if it is at all possible? thanks Brian.

[@BrianBlais]

sure. you can contact me on twitter @bblais

[@Something22222]

Hey can you help me do my exam?? Geo? I can cover any expenses

[@BlaDe0ner]

Thank you this helped me out a lot, but just some criticism maybe you should use paint or photoshop (w/e) and type the words out and use the line tools. No offense but I had a hard time reading your hand writing. I hope you don't take any offense to this.