[CAUT] Reading low humidity (was seasonal SB failure)

[Porritt, David]

Fred:

I have an Amprobe THWD-1 that cost $106.00 from a wholesale outlet
several years ago.  It is now pretty generous in the humidity readings.
I've compared it to the very high tech and expensive graphing gauges
they use at our Museum and while it used to be pretty accurate it is now
about 8 to 9% high.  I also have a USB-502 data logger that I use to
document problem areas and it too is pretty accurate but it is brand
new.  I think most of these uncheap units are pretty accurate for a
while but then they aren't.  I've checked with Amprobe about calibrating
my unit, but calibrating costs more than a replacement!  Generally they
think of things in the $100 price range as disposable.

dp

David M. Porritt
dporritt@smu.edu
-----Original Message-----
From: caut-bounces@ptg.org [mailto:caut-bounces@ptg.org] On Behalf Of
Fred Sturm
Sent: Thursday, March 02, 2006 8:35 AM
To: College and University Technicians
Subject: [CAUT] Reading low humidity (was seasonal SB failure)

This thread seems to be heading in all sorts of directions. Heading  
back to low humidity, and what that does to tonal production, I have  
been puzzling recently over how to measure low humidities accurately.  
I have a sling psychrometer, but I have found that the lowest  
readings I can get with it are in the 18 - 20% range. I am thinking  
this is the limit of the particular instrument: that there is only so  
much evaporation that will take place from the wick, it only holds so  
much water, there are limits to how many calories that will be  
extracted to to the change of phase from liquid to vapor, something  
along those lines.
	It is bone dry here, no rain since November (well, we had a
trace  
last night). My Air-guide hydro-thermo (similar to Radio Shack) has  
been reading 11 - 15% consistently as it sits in the music building.  
My Mannix (the grey one, about $80 from Pianotek, with a bulb on the  
top, that reads to tenths of percent/degree) has been down to 3% in  
one location, and under 10% consistently in many. Another indication  
is frost on the windshield. There hasn't been any, with overnight  
lows of 20F on average. Meaning dew point is below 20F. Pretty dry.  
Under those conditions, I thought the psychro would read lower, but  
no, 18%.
	So I'm wondering if anyone has thoughts about accurate
measurement  
of humidity below 15%. I guess a dew point devise would work, where  
you lower the temp of a reflective surface (eg, polished stainless  
steel), and at the point where it begins to cloud over, that temp is  
the dew point (by definition). Not something I can make, and probably  
quite expensive to purchase. How do these electronic devises work?  
How accurate are they really? How much do they change in calibration  
over time?
Regards,
Fred Sturm
University of New Mexico
fssturm@unm.edu



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