Relative Humidity/Temperature Meter

I am not familiar with the unit that Susan Hughes asks about, but in
general, paper element hygrometers are probably least accurate, acetate
or nylon film types are next, hair bundle types are next, and solid
state sensors can be anywhere. Many people swear that the solid state
units are most accurate, but they do require frequent checking and/or
calibration to be sure of their accuracy.  The recalibration kits for
the solid state sensors tend to be fairly expensive, especially when you
consider that you have to use one at least every three months.

ALL hygrometers should be checked and calibrated regularly.  Absolute
accuracy is of much less importance, however, than repeatability and
consistency.  If you have a unit that that reads consistently, you can
always adjust for inaccurate calibration.  The old standbys of a hair
bundle hygrometer (or hygrothermograph as is most common) and a battery
aspirated wet/dry bulb hygrometer as a calibration device have proved to
be quite reliable, at least for checking consistency of conditions.  Be
aware that below about 35-40% RH the hair bundle types become somewhat
less accurate than in the middle range.

Most people have become very dependent on digital devices, assuming that
the numbers they read represent real information.  Too often, these
numbers are at least as arbitrary as the indications of an analog
device.  The solid state sensors used in these electronic devices are of
various types, each of which has its own error characteristics.  The
sensors can be damaged, or can become contaminated or "poisoned", or
they can essentially wear out, but the device does not then tell you
that the numerical readings are spurious.  Hence, frequent calibration
is necessary to assure reliability of the readings.

Although you pay for sophistication, your dollars don't necessarily buy
equal reliability or accuracy.  Decide what level of absolute accuracy,
consistency, long term reliability, long term maintenance costs
(components + labor resources) you wish to buy.  The higher the "tech"
the higher the initial and ongoing costs tend to be.  Avoid the $39.99
wonders sold by Radio Shack and other vendors as they usually cannot be
recalibrated and are usually over 10% in error, even in the middle
range.  Dial hygrometers are typically as incorrect, whether or not they
are "factory certified" as the trials of shipping are enough to stretch
the sensors.  Most dial types can be recalibrated, however, although
some require a a great deal of ingenuity.  Sorry for the wordiness, but
this is not a simple subject.  Good Luck

Geoffrey Brown
Kelsey Museum
University of Michigan

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                  Conservation DistList Instance 7:74
                  Distributed: Friday, April 15, 1994
                        Message Id: cdl-7-74-001
                                  ***