thermo/hygrometers

[Dennis Johnson]

        I think that it needs to be emphasized that while these digital
hygrometers are very convenient and can be reasonably accurate, they must
be periodically calibrated. The problem though with this self evident
realization is that, at least on mine, this is not easy. I use the $150.00
DH100 model from Pacer Industries in Chippewa Falls WI. (which also is sold
by Pianotek). The company strongly recommends sending it in once a year for
service and recalibration which costs $40.00. I had the company send me the
calibration instructions, because as new it was reading about 7-8% to dry,
and the instructions are rather involved. The unit is to be set on the
shelf of a closed glass container filled with saturated solutions
simulating the high and low limits. The low limit is set by a solution of
distilled water and Potassium-Acetate, the high limit uses a similar
procedure with a solution of distilled water and Ammonium Sulphate. It is
recommended to use a potentiometer (whatever that is) to set these
indications for high and low settings. I just sent it in.
        Another way around this is have a good sling available to monitor
its accuracy. In this way it can still serve as a reliable gauge for a
somewhat longer period. However, also note that slings (our reliable
standard) also vary. The most likely explanation for this is that the
inexpensive slings, the type most of us have, have no reservoir for the
wick to remain moist while slinging. Consequently, as we sling the wick is
drying out and will on average read about 5-7% dryer than a sling with a
water reservoir for the wick.
        Another problem with the digital units is that the wider the high
and low limits are set during calibration, the less accurate that
particular unit will be throughout its range. In some lab settings where
accuracy is critical these units are set to read only a 5-10% range. I
choose the DH100 because I felt it was more accurate, even over its wider
range, than the less expensive alternative. Also, the school offered to pay
for it. I compare them in some ways to the SAT. They are convenient and can
save time, reliable when used with verification, and are certainly here to
stay, but do not and cannot replace the traditional manual methods.
        Just a few things to keep in mind.


Dennis Johnson
St.Olaf College
johnsond@stoalf.edu