In the course of 20 years of visiting museums and observing their
conservation efforts I have been to one (!) where a hygrothermograph was
actually being callibrated.

I have been to many where a one-week chart was changed every 5-6 weeks,
which makes them a little bit difficult to interpret.

Sling psychrometers are not an adequate check on accuracy unless one
person is in charge of callibration.  The sock on the wet bulb must be
clean and the water to soak it must be clean; but it is also the case
that the rate of rotation must be constant, and it will not be accurate
if the swinging is performed in a quiet, still corner of a room.

For these reasons (and others) I have not relied on my recording
hygrothermographs in recent years.  I have one each one-hair bundle and
eight-hair bundle hygrothermograph.

For the past 5 years I have depended on electronic data loggers.  I have
purchased two DataBears (Mac/DOS) because they do a good job of graphing
the data, and they are relatively easy to calibrate.  I use a series of
saturated salt solutions which are far more accurate than sling
psychrometers.

Over the past year I have been working with a couple of electronic
engineers to develop a data logger to record temp/%RH/light because I
knew that it could be done less expensively than those units currently on
the market.

The first engineer bowed out because there were fewer than half a dozen
inquiries after our first trade show (the AIC conference in Nashville).

The second engineer understands that this market (the museum market)
while very important, is also very poor.

We will see where it goes.

Jack C. Thompson
Thompson Conservation Lab
Portland, OR

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