# Thread: Psychrometrics Quiz Question

1. Dry air consists of oxygen O2 with a molecular weight of 32 and nitrogen N2 with a molecular weight of 28.

Water is obviously H20 meaning two hydrogens ( molecular weight of 2) and one oxygen molecular weight of 16. Therefore a water molecule 'weighs' 18.

Steam with a weight of 18 is lighter than something that is roughly 0.2x32 + 0.8x28 = 28.8. Air is actually a bit heavier however the math is simpler with 20% oxygen and 80% nitrogen.

2. Originally posted by NormChris
Let's see now, the clouds are a collection of water vapor and they are located where?

We always seem to come back to clouds Norm. They are water droplets. Vapour is invisible. But what the hell keeps the droplets up there?

3. Professional Member
Join Date
Apr 2003
Location
Iowa
Posts
2,652
Post Likes
Please enlighten us with the answer Norm. Inquiring minds want to know.

4. Regular Guest
Join Date
Jan 2003
Posts
506
Post Likes
Trick question Norm? Temp and pressure of air dictates water vapor.
Warm air holds more water than cold air.
clouds,
When air rises, it expands and gets colder.
The colder air cannot hold as much water as warm air.
As the temp and air pressure continue to drop, tiny water droplets grow together into clumps called cloud droplets.
At this point,the air becomes a visible cloud.
So it would seem to me,temp diff. Fog and cloud are same.Colder surface,fog. Warmer surface,cloud.

But i'm probobly way off!

5. Professional Member
Join Date
Sep 2002
Location
South Dakota
Posts
6,580
Post Likes

No trick, just common sense and good reasoning. Give it more thought.

6. Professional Member
Join Date
Apr 2003
Location
Iowa
Posts
2,652
Post Likes
I guess this illustrates the value of further education. However, how many techs REALLY need to know this on a daily basis?, just curious is all.

7. Clouds which consist of water droplets are a bad anaolgy.

Have to look at molecular weights, daltons law of partial pressures, vapour pressure of water and ideal gas law to try to work things out

8. Professional Member
Join Date
Sep 2002
Location
South Dakota
Posts
6,580
Post Likes
Carnak, what is the density of a cloud? In the English system, not the metric system.

9. Professional Member
Join Date
Sep 2002
Location
South Dakota
Posts
6,580
Post Likes
How about if I use steam leaving a tea kettle? Or will you have a problem with the fact that it is at too high a temperature?

Let's go back to the fog. The reason fog is on the ground is because it (water vapor) was formed on the ground. When we see it, it is actually rising because it is lighter than the air around it. We see it because it is being replaced as fast as it rises and disappears. So fog as water vapor is a good example of water vapor being lighter than dry air.

When teaching people it is more useful to provide practical examples they can relate to than spend a great deal of time getting highly technical. Getting technical is fine but it all comes together faster when you can provide an example people can immediately identify with.

Richard Feynmen (sp) one of the greatest physics professors was well know for just that. When he gave lectures to his students the auditorium was also packed out with liberal arts students. That is how plain and simple he made physics.

10. If you collected water droplets it would be a little shy of 62.36 pounds per cubic foot.

Cloud is droplets, what keeps them up there I don't know, but it is not density lighter than air.

What keeps dust in air?

[Edited by Carnak on 07-23-2004 at 11:42 PM]

11. Professional Member
Join Date
Oct 2003
Posts
566
Post Likes
Its comparing Apples and Oranges really...
Liquids is Specific Gravity.
Gases is Vapor Density.
Water has a specific gravity reading of 1. Liquids less than 1 are lighter than water, vice versa for heavier
Air has a vapor density reading of 1. Gases less than 1 are lighter than air, vice versa for heavier as well

12. I always thought fog meant air temp dropped below its previous dewpoint meaning water condenses out. Back to what keeps droplets suspended.

You are seeing droplets, a liquid, not the invisible vapour.

THe steam off the kettle is saturated and is in equilibrium condensing/ re-evaporating. Saturated steam comes out of kettle, contact with surrounding air mometarily condenses some, however the presence of surrounding air means the droplets are free to revaporate as long as surronding air is not saturated. Sort of like a Dri Steem Humidifier.

When you breathe in the winter you can see your breath because the vapour is condensing, if it dissapears it has re-evaporated.

Just as much vapour in breath in the summer but you don't see it because it is not condensing.

My real point to this whole matter is that vapour is invisible.

With clouds, there is a temperature drop with elevation, however there is also a barometric pressure drop with elevation. At a higher elevation, with the lower barometric pressure, more water can evaporate at a given temperature.

Its all to do with Daltons Law of partial pressures

[Edited by Carnak on 07-24-2004 at 12:08 AM]

13. Professional Member
Join Date
Apr 2003
Location
Iowa
Posts
2,652
Post Likes
Well, then I thought right.

But thanks LMTD for the little lesson, that's why I like this site. I work on residential mostly, the biggest I really see is 10 tons but it's neat to see what else needs to be considered on a bigger scale of things. Just once I'd like to work on a huge chiller, just to get familiar with a little of everything.

Page 2 of 3 First 123 Last

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•

## Related Forums

The place where Electrical professionals meet.