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 _______________________________________________ caut list info: https://www.moypiano.com/resources/#archives
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