Re: Measuring humidity
On Sep 28, 2006, at 1:57 PM, sitgesn1@netscape.net wrote:
Katharine,
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
http://www.smartec.nl/pdf/apphs1002.pdf
d) For those interested in computing relative humidity as a
function of dew point and water contents in air
http://www.cactus2000.de/uk/unit/masshum.shtml
I hope this helps
Yes it does, thanks; that's very interesting. Things are never as
simple as they seem, are they.
I guess I had always assumed, showing my ignorance, that relative
humidity took temperature into account, and as such it was an
indicator of water content in and of itself. Thanks for setting me
straight. I've always said it's not the humidity, it's the heat, and
this just tends to confirm that.
But apparently I'm not the only one making this mistake, because
we've all been using RH as if it were a meaningful variable, and
obviously it's not, unless, as you say, temperature is held
constant. The graphs in the literature showing the relationship
between humidity and dark reaction, and humidity and sensitivity,
were no doubt constructed holding temperature constant.
Unfortunately they don't say that, or specify the temperature,
although I suppose we could assume 25C, isn't that the usual standard
temperature?
Katharine