Sling psychometers are generally said to be too dependent upon
operator style to be relied upon. For that reason "aspirated
psychometers" are preferred. The general principle is the same
(i.e., "wet bulb/dry bulb) and the same caveats regarding
deionized water on the socks, clean socks, area of smampling,
etc. pertain; but the air is driven by a little battery- operated
fan. The obtained results are still read against a chart.
As I have remarked earlier, I am inclining more and more toward
agreement with Jack Thompson . . . even though calibration of a
hygrothermograph is no big deal, and can be done easily with a
psychometer and a darkroom-grade (NIST-derived accuracy)
thermometer stabilized in a beaker of water.
According to Jack Thompson:
>
> 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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>