Good advice, Helen, but I want to quibble just a bit: sling
psychrometers are less useful (the obtained data less
reliable) than "aspirated psychrometers" - battery operated -
because they are too subject to operator variation. Any
instrument using thermometers, such as either type of
psychrometer, must be placed in a location of stable temp to
allow equilibration for a time. What that time is, is hard to
say. I was taught to not rely on the thermometers in a
psychrometer for the purpose of calibrating the temp side (the
[usually] bimetallic strip), but to instead use a very accurate
separate themometer. Themometers come in different gradation of
distance from NIST standard, ranging down from actual
NIST-tested ones, through "NIST-derived," etc. We rely on a
Kodak Process Thermometer, Type 3. This is a color photography
processing thermometer, and color processing is notoriously
inflexible in all variables, including temp. This is not an
endorsement, but Kodak claims that "The Fahrenheit scale is
accurate to +-0.25 degree at 15, 25, 77, 86, and 122 deg F and
+- 0.50 degree over the rest of the scale." That should be
close enough.
Then, to achieve the equilibration stability, we stand the
thermometer in a tall vessel of water - a beaker, as it happens
- for 24 hours, to allow the water to water to equilibrate with
ambient air temp. This method eliminates the effects of vagrant
drafts.
Some of our older instruments were made by Bendix. I believe
that their corporate successor with these is the Belfort
Instrument Co. in Baltimore. This, of course, raises the issue
of pens using oli-base inks, inked by the owner, vs. the
proprietary ballpoint types. There are pros & cons to each.
Henry Grunder
Conservation/Preservation Coordinator
The Library of Virginia
>According to WCG1962:
>
> In response to your query about hygrothermographs:
>
> (1) A data logger does not accurately calibrate this instrument.  Both data
> loggers and hygrothermographs should be calibrated with a psychrometer,
> either swing or electric.  Psychrometers are the only equipment that provide
> accurate RH levels.
>
> (2)  Where are your hygrothermographs from?  The brand name and supplier
> location should be listed on the machine.  Companies that sell this
> equipment have extra hair and pens.  Here are the most common suppliers:
> Cole-Parmer Instrument Company, 800-323-4340
> Conservation Materials (sell an English recording hygrothermograph),
> 702-331-0582
> NovaLynx, 916-477-5226
>
> (3) A little more life can be given to a recording hygrothermograph by doing
> the following:
>       a. brush dust off the hairs with a fine sable brush
>       b. take the machine into a very humid area (such as the bathroom while
> you are showering) for an hour or two, and then back into the regular museum
> environment. This regenerates the flexibility of the hair for a time.
>
> (4) If you calibrate the machine with a psychrometer, it should work well
> for awhile.  Note the rate of drift (ie. does it need re-calibration in a
> month, a week, or a year?)  Usually I check machines every month and only
> have to re-calibrate every 6-12 months.  If I have to re-calibrate less than
> every 4 weeks, then the machine should be returned to the manufacturer for work.
>
> Good luck!
>
> Helen Alten
> Field Services Director
> The Upper Midwest Conservation Association
> 2400 3rd Ave. S.
> Minneapolis, MN 55404
> ______________________________________________
> I have six 10 year old recording hygrothermographs - the kind with hair
> bundles and a wind-up clock mechanism - forgot to bring the brandname
> home.  Although I've recalibrated them using a datalogger running in
> "real-time,"  I'm looking for a company that will do it properly,
> replacing the hair bundles if necessary.  My predecessor sent one or two
> to California at great cost; I'm hoping for someplace on the east coast of
> USA.  The guilt of not using them is getting overwhelming, but in their
> present state it would be a waste of time.
> Thanks.
> Will Garrison
> Collections Manager
> Historic Deerfield, Inc.
> Massachusetts
>