[NatureNS] Relative Humidity

Date: Sat, 16 Aug 2008 15:12:13 -0400
From: "Paul S. Boyer" <psboyer@eastlink.ca>
To: naturens@chebucto.ns.ca
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An easier way to calibrate relative humidity would be against a sling  
psychrometer.  This device consists of two calibrated, identical  
thermometers, one of which has a small piece of porous cloth wrapped  
around its bulb, saturated with distilled water.  The two thermometers  
are fixed to a board and are slung around through the air for several  
minutes until the wet-bulb thermometer stops dropping in temperature,  
and the two thermometer-readings are then compared.  The dry-bulb  
thermometer will read the normal, ambient air temperature, and the wet- 
bulb will read a usually lower temperature caused by the evaporative  
cooling of some of its water.  The difference between the two  
temperatures (the wet-bulb depression) is almost directly proportional  
to the vapor-pressure deficit in the air, and can easily be converted  
to various other measures of humidity, such as the commonly used  
relative humidity.  Tables are most often used for this conversion,  
because the formulas are messy, but one can work them out on a  
spreadsheet quite easily.

It is also possible to draw air over the wet- and dry-bulb with an  
electric fan.  The home instruments which have stationary,  
unventilated wet- and dry-bulb thermometers are very unreliable  
(actually next to worthless).  Most home instruments with dial gauges  
measure humidity by its influence on some material such a hair or  
plastic.  They can be off by 10% or even more, for several technical  
reasons which I won't get into.

See: http://en.wikipedia.org/wiki/Hygrometer

I have never measured a relative humidity over 100%.  I strongly  
suspect that your data loggers are very inaccurate.  It is notably  
tricky accurately to measure humidity with such simple devices, and I  
doubt very much that they use a reliable method.

—Paul


On Aug 15, 2008, at 9:24 PM, David & Alison Webster wrote:

> Hi Randy & All,            Aug 15, 2008
>   It depends upon how clean the air is. Condensation is usually  
> initiated around hydroscopic nuclei. My Meteorology book (Taylor)  
> says that "With perfectly pure air from which all nuclei have been  
> removed by repeated washing, condensation will not start until a  
> relative humidity of about 420% is reached...".
>
>   But in all but ultra pure air, supersaturation so far as I know is  
> transient as caused e.g. by sudden cooling as in a cloud chamber  
> where as I recall the detection of charged particle movement relies  
> on charge-induced condensation of supersaturated air.
>
>   Frankly I would suspect probe calibration [I tried recording RH  
> using Omega (?) probes about 1992 and none of three (?) new probes  
> worked well enough to be of use due to output drift] to be faulty if  
> checks of the headspace of constant humidity solutions have not been  
> run. Any Chemistry and Physics Handbook edition should have this  
> information indexed as 'constant humidity'. For example, the RH of  
> air over a saturated solution of ZnSO4.7H2O is 90% at 20oC. One  
> could get a rough idea of calibration just using a covered jar with  
> excess crystals in a dark closed closet but a covered wide-mouth  
> Dewar flask in a constant temperature chamber would give more  
> reproducible readings.
> Yt, DW
>
> Randy Lauff wrote:
>
>> Every so often, a student will preamble a question with, "You're  
>> going to think I'm stupid but..."
>>
>>
>> So this sounds like one of those questions. I bought some data  
>> loggers which record relative humidity. They are routinely  
>> recording over 100%. Can relative humidity be over 100% (i.e. is  
>> the air supersaturated with water)? And if so, what is the maximum  
>> Relative Humidity possible?
>>
>> Thank you,
>> Randy
>> _________________________________
>> RF Lauff
>> Way in the boonies of
>> Antigonish County, NS.
>>
>
>

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