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Subject: Equipment for collections care

Equipment for collections care

From: Terry Conners <conners>
Date: Thursday, October 7, 1999
Barry Knight <barry.knight [at] english-heritage__org__uk> writes

>In Instance 13:22 Vicki Cassmann asks for suggestions for equipment
>for a collections care kit.
>Personally, I would omit the psychrometer and thermohygrograph:
>psychrometers are unreliable unless you take great care with them,
>and can give misleading results in mountainous areas (because the
>psychrometer equation depends on the atmospheric pressure, which
>depends on the altitude).

Regarding the comments about psychrometric equations being sensitive
to atmospheric pressure (hence the stated preference for hand-held
electronic meters calibrated with saturated salt solutions) I am in
general agreement--but the discrepancy caused by altitude is perhaps
not going to be terrifically significant. I did some quick
calculations based on a dry bulb of 20 deg. C and a wet bulb of 10
deg. C for both 760 mm Hg (1 atm) and 700 mm Hg and found that the
calculated relative humidities are 24.5 and 26.7% respectively
(using Carrier's equation). Other problems with wet bulb-type
psychrometers are likely to be more significant sources of problems,
especially dirty wicks, improperly wet wicks or insufficient air
flow over the wick (a minimum of 4.6 meters per second is
recommended in one of my reference books!--probably not very
practical in many installations anyway.) Wet bulb errors of only
0.1% can lead to 1% error in the calculated relative humidities at
room temperature, especially at higher humidities. Direct sunlight
can also cause misleading readings.

Of course, there are problems with the electronic gadgets too--they
are nearly always more accurate near the mid-range and calibration
*is* very important as previously noted! Note that every meter has a
stated accuracy and that this might be +/-  2% or even +/-  5% ---
the worst accuracy and precision have almost always at the extremes
of relative humidity in my experience.

Terry Conners
Assoc. Professor of Forest Products
Mississippi State University, USA

                  Conservation DistList Instance 13:24
                  Distributed: Friday, October 8, 1999
                       Message Id: cdl-13-24-002
Received on Thursday, 7 October, 1999

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