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digital hygrometer
http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10102&t=12803		 Page 1 of 1
Author:  Heath Blair [ Wed Jul 11, 2007 9:59 am ]
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ok, so i bought a digital hygrometer about a week ago. its made by honeywell, but i dont recall the model. i bought it knowing full well that digital hygrometers are notoriously inaccurate, but wanted to jump in and get some kind of idea about humidity levels in my work space. so i left it out in my garage over night. high reading was 90 some odd degrees and around 40% humidity (i dont recall exactly now, but thats pretty close). and low reading was around 80 degrees and 30% humidity. inside my comfortably air conditioned house, tempurature stays around 78 degrees and between 39% and 46% humidity. then i tried the salt water in a bag trick as per the instructions here on this forum. high reading over night was 79% and low was 73%. when i came home from work it had leveled out at 75%. so what does all of this mean. is my hygro right on? to what level should i trust it? any and all help would be appreciated. humidity talk always seems so mystical to me. i need some straight answers. thanks.Big smile
Author:  WaddyThomson [ Wed Jul 11, 2007 10:24 am ]
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Heath, the only way to really know is to either make or buy a sling Psychrometer.  There have been lots of discussions in the archives, but here is a plan for making one, and it has a chart with it to make the readings. HERE

I have a couple in my shop, and the digital one is consistently about 4 to 5 percentage points low.  Once you know how it reads, you can pretty much know when you are right.
Author:  Bill Bergman [ Wed Jul 11, 2007 11:32 am ]
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You can get by with one thermometer with the gauze cover. Sling it dry first, record the temperature, then wet it and repeat. Do not use a mercury bulb, becasue eventually you may break the thermometer, and the mercury is toxic.
Author:  Heath Blair [ Wed Jul 11, 2007 11:42 am ]
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wow, that is so interesting waddy! i always wondered what that crazy sling psychrometer was. now i know. and knowing is half the battle. thanks for the heads up bill. do they even make mercury thermometers anymore? i think i might just buy one instead of making it. my list of things to make is getting pretty long . so the salt test... were my efforts in vain? thanks guys.
Author:  JohnAbercrombie [ Wed Jul 11, 2007 11:52 am ]
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Heath-
I'd be a bit cautious with the salt calibration method.

I have an inexpensive (Caliber III) hygrometer, and the instructions specifically warn against using the salt method.

I wouldn't be surprised if the electronics in those units are not protected in any way against corrosion effects.

My feeling is that a few points on either side of the actual relative humidity is not going to be a big deal, but some folks like to be verwy, verwy accurate about everything!

Cheers

John
Author:  Heath Blair [ Wed Jul 11, 2007 12:22 pm ]
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yeah john, i think i lean more towards your sentiments. there are places to be gnat's rear end accurate and then there are places where "it'll do". my hygrometer, and this may be the case with others as well, has a remote sensor unit and a main unit that displays the readings. the main unit is indoor only and the remote sensor can be used outdoors. the remote sensor sends a signal to the main unit. hopefuuly i didnt mess anything up with the salt calibration. i just thought it was curious that the reading during the calibration was actually 75%. i was expecting it to be off by some margin. anyways...
Author:  WaddyThomson [ Wed Jul 11, 2007 1:03 pm ]
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They definately still use mercury thermometers.  They are way more accurate than the spirit filled thermometers.  You can still buy mercury or spirit filled Sling Psychrometers.  Best price I have seen for a SP is about $45 from one of the internet places.  Sometimes you can pick them up on ebay for a little less, but usually someone who knows what he wants is bidding, and they get up there pretty fast in the last hour or so.
Author:  old man [ Wed Jul 11, 2007 2:56 pm ]
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Go in the bathroom and close the door. Turn the shower on hot. When the mirror begins to fog the humidity is 100%. Check your hygrometer.

Ron
Author:  LuthierSupplier [ Wed Jul 11, 2007 4:33 pm ]
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HERE is a link to the one we are talking about. It is very accurate. How do I know?...well, I just bought one of these Boveda Humidity One-Step Calibration pak for $5, and followed the directions. It was 1% off. Hope this helps!
Tracy
Author:  WaddyThomson [ Thu Jul 12, 2007 1:28 am ]
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I noticed in the info on the Caliber III that it is guaranteed to within +/- 1%.  Almost none of the other digitals I have seen have guaranteed accuracies at all.  Mine is off about 5 humidity % points at 45% - that's 11%.
Author:  Mattia Valente [ Thu Jul 12, 2007 7:30 am ]
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Note on the 'slingy' thingy; Mario Proulx mentioned a neat trick over on the MIMF; basically, all you need is a wet and dry bulb reading and a chart to get the relative humidity (see PDF link), but the quick, easy, repeatable, no crazy swinging things around way to get the measures is 'simply' to suck (ie, shopvac/dust collection) across a dry thermometer, and one with a bit of gauze dipped in a water receptacle (ie, wicking water in). Air's ambient, airflow's constant and consistent, and when the number equalize, consult chart, voila! Result!

I found my mercury thermometers at my local army suprlus place; new-ish, made in china, work a treat.
Author:  burbank [ Thu Jul 12, 2007 7:53 am ]
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No to hijack, but as long as Heath's question seems to have been answered, anyone know how a digital sling psychrometer might work? Doesn't need water or salt. I got it like new for silly cheap on ebay, seems to be of the type used by hvac folks.
Author:  WaddyThomson [ Thu Jul 12, 2007 8:10 am ]
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If it is the wet/dry bulb type, put water on the gauze cover over the wet bulb, and sling it around or suck(not blow) air across it for about 90 seconds.  Record the two temps, and look up on a chart.  It will tell you the actual humidity.
Author:  Heath Blair [ Thu Jul 12, 2007 10:02 am ]
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wow guys, thanks. alot of good information here.
Author:  Brock Poling [ Thu Jul 12, 2007 12:07 pm ]
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You know.. you can get abbeon's on ebay pretty much all day for $25 - $40. They can be calibrated as well.

After calibrating mine I checked it with a sling psychometer and it was about <1% off. I bought 2 just so I would have a point of comparision, but usually they are both within .5% of the same reading.

Author:  Heath Blair [ Thu Jul 12, 2007 4:42 pm ]
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thanks for the heads up brock. ive heard both good things and bad about the abbeons as well. the fact that they can be calibrated is great though. since ive already bought one hygrometer, i dont think i could explain to my wife how i need another one to make sure the one i already have is accurate. i would like to try the sling psychrometer though. either way, i think im just going to have to live with what ive got now.
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