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Psychrometer questions- air velocity, etc
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Author:  Todd Rose [ Sun Dec 09, 2007 10:41 pm ]
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Thanks to helpful info from Mario and others, I'm about set up to calibrate my hygrometers. Obviously, I want to be sure my psychrometer readings are reasonably accurate, and there are still a couple things I'm not sure about.

First, does the air velocity over the wet bulb matter? It seems to me that it would, but the only reference I've seen to this anywhere was in a recent post by David Collins. Mario suggests simply holding the thermometer over the intake of a dust collector. I was thinking of using my shop vac, actually, because I can take it into my humidity controlled assembly room - the dust collector is in the machine room across the hall. Anyway, I keep thinking that a higher air velocity would cause a faster rate of evaporation and therefore drop the temp of the wet bulb more, and vice versa, giving perhaps widely differing readings. N'est-ce pas?

Second, how thickly should the bulb be wrapped in wet gauze? Should it be soaked or just damp, or does that matter?

Finally, just to double check that I'm not missing something here - I'm planning on using just one thermometer. I'll read the ambient air temperature on the thermometer first and record that, then put the wet gauze on it and hold it over the end of the shop vac hose till the temp stops dropping, read and record that, subtract the wet bulb reading from the original reading, then look at my chart. I don't see any reason to hold a second (dry) thermometer over the air stream. Or is there something I'm missing?    
Author:  J.R. Hunter [ Mon Dec 10, 2007 12:40 am ]
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I've got the same questions, thanks Todd!


Pardon my piggy-backing, but does it matter if the thermometer is mercury or non-mercury?


Thanks,


J.R.
Author:  J.R. Hunter [ Mon Dec 10, 2007 12:48 am ]
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Oooops hit the post reply button to soon!


The reason I ask is I tried it with a shop vac using a mercury thermometer and the temp only dropoped 4 degrees, which can't be right. maybe someting else I was doing wrong?


Thanks again,


J.R.
Author:  Cecil [ Mon Dec 10, 2007 1:38 am ]
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Todd,

The velocity does not matter. It might cool it quicker but can only cool it to the dew point,no lower.

When I was taking weather observations for the US Weather Service we just used a cotton jersey tube over the bulb, dipped it in a cup of water and placed it in front of a fan. Just be sure it is thick enough to reach the lowest temperature it can before it gets dry.

One thermometer will work it is just a pain to wrap it every time you take a reading.

J.R.,I believe mercury thermometers are more accurate than non-mercury. Whether it is enough to make a difference, I don't know.

Cecil
Author:  David Collins [ Mon Dec 10, 2007 2:08 am ]
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First question, your shop vac should be just fine if you place the bulbs
right in front of the hose. The 600 linear feet per minute I think is a
number I pulled from literature of one of my psychrometers. A simple rule
would be that you just need enough air flow to bring the wet bulb temp
as low as it will go, which a vacuum with a small hose should easily do.
The temperature should reach a point and stop, not going any further.
That is the point where the air is absorbing as much water as it can from
the bulb, and the temperature drop stops. It essentially hits a ceiling at a
certain point, which is of course decided by the relative humidity of the
air.

Second question, one layer of gauze, and wet it so it's fully wet, but
doesn't have to be dripping. The right way is to actually have a gauze
"sock" over the bulb with the tail in a pool of water. This way water will
wick up to the bulb as it dries out to help keep the reading consistent. I
just leave a short tail hanging past the bulb and soak that part
thoroughly, which leaves plenty of water in reserve.

Third question, No, I think you should really have two thermometers side
by side. It's important to read them simultaneously. I can watch the
readings on my psychrometers waver up and down (both bulbs in sync)
from 1/2 to 1 degree quite easily over the minute or two it takes to fully
cool the wet bulb. If someone opens a door, furnace kicks on, vacuum
blower stirs up the air, etc., the temp can change in a point when ever air
stirs around the room. 1 degree fluctuation can change your humidity
readings by quite a bit. Just make sure no air flows over the dry bulb after
flowing over the wet bulb.
Author:  grumpy [ Mon Dec 10, 2007 3:05 am ]
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What David said regarding air speed. Methinks it's 40mph or moe that's needed....

I'd play around with one bulb for now, and do a few dry runs, to see if it does fluctuate by more than a degree. If it seems to hold steady even with the shop vac running, go ahead and wet it. I suppose this would be a function of shop size; mine is big enough that the temperature won't easily fluctuate minute to minute, but in a small room, I suppose it could.
Author:  David Collins [ Mon Dec 10, 2007 3:19 am ]
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True, my shop is well sealed as a whole, but divided in to three separate
area loosely connected and the air flow between them is really weird. if the
furnace kicks on, a door is opened, or the air filter is running, air moves in
strange ways around my shop. Other shops may not have this problem.

As to air flow, I looked up in my Psychro-Dyne manual, and here's a quote
from the "Theory of Operation" section

- "It has been found by many years of experimentation that 10 feet-per-
second or more of air flow is essential for accurate results. Your PSYCHRO-
DYNE is designed to provide 15 feet-per-second air flow over the
thermometers throughout a reasonably long battery life" -
Author:  grumpy [ Mon Dec 10, 2007 4:11 am ]
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My reasoning for the 40mph number is that most wind chill temperature charts stop at 40mph and state that further wind speed increases "have little additional effect".  And what we're measuring here is, in its simplest form, wind chill effect of air moving over a wet object....

40mph works out to be a tad over 58.6 ft/sec. Sounds incredibly fast, but drive 40mph, and hold a wet finger out the window(don't do tis while drive past a trooper...). Now go to your dust collector, and lick your finger and place it at the nozzle. To me, that feels faster...<g> crude, but hey....

15ft/sec works out to be around 10mph. If they are happy with that, then we should be also.

But in the end...  What I did, was try everything I had that pulled air(even the simply circulating fan that's always running), ad found that even the shop vac pulled enough air unless it was clogged up from being near full. But  did find that a fan wasn't enough, a regular home vacuum wasn't enough, and that even 3 inches away from the open intake of the dust collector wasn't enough. How did I know it wasn't enough? Because i went around and tried them all. Find the fastest air mover you have, an check with that. then move to the next one, and if you get the same readings, then you know the air speed drop wasn't enouygh t skew the readings. Now move to the next slower object, until you find one that doesn't give the same readings, then move back up. Only have one shop vac? Then begin with the largest ope hose. Then place a smaller nozzle thingie on it, and check that. Then go smaller again. if all the readings are consistent, you know the open hose is plenty. If the readings from the first downsize changes, look for a bigger source.... or wait longer.


Hillbilly Engineering, but it works.
Author:  Todd Rose [ Mon Dec 10, 2007 7:20 am ]
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Thanks a lot, Cecil, David, Mario, and Todd. As always, your help is much appreciated. I'll give it a whirl (well, not literally - I'm not doing the sling method ) and see what happens.
Author:  J.R. Hunter [ Mon Dec 10, 2007 7:54 am ]
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Ditto!


J.R.
Author:  Pete Licis [ Tue Dec 11, 2007 4:30 am ]
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[QUOTE=grumpy] What David said regarding air speed. Methinks it's 40mph or moe that's needed.... [/QUOTE]

I use a digital thermometer with a "sock" over the end. It's attached to about 3' of string, which I then spin around until the temperature drop stabilizes. I've found that I have to spin the thing at a rate of about 2 or 3 revolutions a second. Slower than that may does yield a stable temperature, but to my surprise I found if you spin faster, it will go lower. I think going faster than 3 revolutions doesn't really help.

I did the math, and assuming I did it hastily but correctly, it came out to 38 mph. That's why I quoted the Grumpy One above.
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