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Subject: Relative humidity and partial pressure of water

Relative humidity and partial pressure of water

From: Simon Hogg <s.hogg>
Date: Thursday, August 8, 1996
Conservators in general look to control the relative humidity as a
preventative conservation measure.  However, should we / they be
looking at the partial pressure of water p(H2O) or indeed the vapour
pressure of water?

The same RH at different temperatures corresponds to a difference of
total water content of the atmosphere and a different partial vapour
pressure. Thus, the conservator restricts the RH to a constant value
and assumes that the water diffusion/counter-diffusion at the
surface of an object will remain in equilibrium, and thus the water
content of the object will remain constant.  However, although the
RH remains constant, any fluctuations in temperature (daily or
seasonal) will result in a change of vapour pressure of water and
will thus shift the water content of the object away from
equilibrium.

What this means is that RH control is not the 'holy grail.'  This
does not take any account of the difference in reaction rates with
different temperatures, merely the ingress/egress of moisture,
leading to swelling/shrinking of objects over a period of time.

Is this, I wonder, symptomatic of the need of a conservator (or curator) to
be able to read a figure off a dial, to be able to put a meter in a case and
measure the rh, rather than being able to derive further information from
the rh and temperature measurements, plus air pressure, and other things.  Is
this seen as too difficult, or too much trouble to do.  Or is it purely a
misunderstanding of the meaning of relative humidity?


Simon Hogg
Imperial College, London, UK

                                  ***
                  Conservation DistList Instance 10:17
                 Distributed: Thursday, August 8, 1996
                       Message Id: cdl-10-17-002
                                  ***
Received on Thursday, 8 August, 1996

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