U of S | Mailing List Archive | alt-photo-process-l | Re: Measuring humidity

Re: Measuring humidity

I want to comment the following points concerning relative humidity measurements
a) Most of the sensors I have seen are accurate + - 4-5% or worst. Usually are calibrated at 55% at 25ºC. I've checked my Oregon that has two measuring points and I got readings of 55% and 64% for the same point of measure.
b) The most accurate system for measuring humidity and related parameters is with 2 thermometers One for measurement of air temperature and the second with a wet bulb for measuring the dew point. With these two temperatures and a table you can get relative humidity and absolute water content per cubic meter of air. I think this last parameter is really the most important concerning gum printing. Using relative humidity alone can be confusing and meaningless except if you always work at the same temperature. In order to put it more clear consider the following data
Air ºC RH % Water per m3 of air g/m3
15 100 12.815
20 100 17.274
25 100 23.015
30 100 30.32
35 100 39.541
Imagine your workroom is at 30ºC and your hygrometer is reading 57% This means that every cubic meter of air contains .57x30.32=17.28 g/m3 of water. At 20ºC humidity should be at 100% for the same content of water.
c) For those interested in calibration their hygrometer
d) For those interested in computing relative humidity as a function of dew point and water contents in air
I hope this helps

-----Original Message-----
From: kthayer@pacifier.com
To: alt-photo-process-l@usask.ca
Sent: Wed, 27 Sep 2006 11:44 PM
Subject: Re: Measuring humidity

I've spent the morning searching for a hygrometer, and finally came home with one that seems to work okay.

I found another Radio Shack I hadn't known about, and a very helpful person inside (the other Radio Shacks couldn't tell me anything: couldn't tell me if the item existed, if it was on order, if it had come in, if they were ever likely to get it, if I could special order it, didn't seem the least interested in helping me find a way to obtain it) who looked it up online and said there's no such item in Radio Shack's catalog. So maybe it's been dropped from their inventory since all you guys bought yours.

She suggested I go to a large marine supply place, so I went there, but they didn't have anything either. Again, there was a very helpful person who was interested enough in my problem that she called someone at their head office in Seattle, and after talking to him a while called me over and said "I think he's got the answer for you" and handed me the phone. The guy in Seattle said, "Here's what you do. You take two thermometers, and you get the bulb of one of them wet, and you put them in a sling thing and sling them around, and then you take the difference in the temperatures." I said, well, yes, I could do that I suppose, but I was hoping to find an instrument that would do it for me. He said sorry, he'd never heard of such a thing. (BTW, Camden, THAT's funny).

At any rate, I finally found a temperature-humidity thing at a garden store for $34.99 (I had already exhausted all the hardware stores and building supply places a couple of weeks ago) and have been having fun taking it around to various places in the house and outside. Unlike the cheaper one I got, this one is very responsive, as Judy said hers is. So far I have ascertained that the temperature and humidity in the workroom are the same as the temperature and humidity outside, which is what I would have expected with the room wide open to the outdoors, and also within a couple of points of the temperature and humidity at the airport. We're in our legendary fall weather which is drier and warmer than the rest of the year; this afternoon it's about 70 degrees and 64% humidity. So I still don't know if it will measure accurately at more normal higher humidity ranges, but so far so good.

On Sep 16, 2006, at 5:45 AM, Ender100@aol.com wrote:


I got the digital hygrometer with the remote sensor at Radio Shack.

In a message dated 9/15/06 10:41:45 PM, dsbryant@bellsouth.net

The nicest hygrometers and most usefull ones are those that have a remote
unit...you can put the remote unit in a humidity chamber and set >>
the other
unit elsewhere and monitor the humidity...they also have cumulutive
readouts, so you can get the average RH too.

Where did you get that?

I use a digital hydrometer that I purchased at Radio Shack but for more
accuracy I still use my wet/dry bulb thermometer.

Don Bryant

Mark Nelson
Precision Digital Negatives
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