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Rkitech78
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Posted: Tue Jul 22nd, 2008 09:55 pm |
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To those of you studying for the ME test...
here's a website touching on the use of the psychrometric chart...
http://www.vivoscuola.it/US/RSIGPP3202/umidita/copie/g626.htm
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bigapple
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| Location: | New York, New York USA |
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Posted: Wed Jul 23rd, 2008 12:20 am |
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Rkitech78 wrote: To those of you studying for the ME test...
here's a website touching on the use of the psychrometric chart...
http://www.vivoscuola.it/US/RSIGPP3202/umidita/copie/g626.htm
Anyone can check the link avove and see Example 3? The last part of the answer says "This air would also have a relative humidity of approximately 85%."
Anyone can explain what does that mean?
Thanks,
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Rkitech78
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Posted: Wed Jul 23rd, 2008 12:32 am |
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bigapple....
"This producer can probably expect to obtain air at about 73°F from the evaporative cooler under these conditions. This air would also have a relative humidity of approximately 85%. "
i wonder if that is the relative humidity of the 73 Degree (wet bulb temp)..based on the chart.
not sure.. just a guess
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Far Galaxy
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Posted: Wed Jul 23rd, 2008 12:43 am |
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I swear I will curse out loud real bad if they have this kind of question in the exam. I posted a similar question several days ago and I couldn't get anybody to help me understand what exactly is really up with this psycho metric chart reading and how they came up with some weird guessing numbers like that...
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bigapple
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Posted: Wed Jul 23rd, 2008 12:49 am |
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Far Galaxy wrote: I swear I will curse out loud real bad if they have this kind of question in the exam. I posted a similar question several days ago and I couldn't get anybody to help me understand what exactly is really up with this psycho metric chart reading and how they came up with some weird guessing numbers like that...
It will be my third time around for MEP and I can say that the examples in the link are not higher than real questions at all. We should be prepared to solve those level of questions. Anyway, I still cannot understand the last part of Answer #2. 
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LSU
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Posted: Wed Jul 23rd, 2008 02:03 pm |
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big apple,
in example 2, at 80 deg F, the air is at 70% of its capacity to hold water vapor. The dew point temp of 69 deg (the horizontal reading at the left side) is the temp at which relative humidity becomes 100% (maxed out in its ability to hold water). In other words, if the original 80 deg air would come in contact with a cold surface and fall to 69 deg or below, condensation will occur.
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bigapple
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Posted: Thu Jul 24th, 2008 01:07 am |
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LSU wrote: big apple,
in example 2, at 80 deg F, the air is at 70% of its capacity to hold water vapor. The dew point temp of 69 deg (the horizontal reading at the left side) is the temp at which relative humidity becomes 100% (maxed out in its ability to hold water). In other words, if the original 80 deg air would come in contact with a cold surface and fall to 69 deg or below, condensation will occur.
Thanks for the explanation although my question was Example #3. Sorry.
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weezer
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Posted: Fri Jul 25th, 2008 05:40 am |
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Far Galaxy
You may want to bring something to muffle your cursing at the test center. They will most likely warn you the first time. But repeated outbursts may get you tossed.
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Punchlist
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Posted: Fri Jul 25th, 2008 04:40 pm |
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My take on the second part in example #3 is to take another thermometer and to now measure the temperature (dry bulb) of this room.
What they're saying, and correct me if I'm wrong, is if you have hit the 69-degree wet bulb temp, your dry bulb temp will be 4-degrees (roughly) above this. So, connect the dots where the 73-degree dry bulb vertical stripe hits the 69-degree wet bulb sloping line and you'll hit the 85% relative humidity curve.
I guess we have to look at it like there are two thermometers in this room: one that's wet and hovering at 69-degrees and one near it that's dry and because of the supposed inefficiency of the evaporation-cooling method, the dry thermometer will read 3-4 degrees above its wet little brother. Now, once we have this new number (73-degrees), it's time to play connect-the-dots again by using the two thermometers' temperature readings: 69 and 73-degrees respectively.
Did that make sense? I'm barely hanging on by a thread myself, so bear with me here.  It's all about playing "Battleship" when looking at charts like this. The hard part is knowing when to shift gears. And the writer of this article does a terrible job of using the clutch. "Oh, I thought you all followed along with my internal monologue that asked you to picture a dry bulb thermometer near the wet bulb one. Did you not catch that?"
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