Calibration Of Hygrometers And Hygrothermographs
The National Park Service uses thermometers, temperature conditions and making adjustments
hygrometers, hygrothermographs, and data- as necessary. A psychrometer (either sling or
loggers to monitor relative humidity (RH) and aspirated) or an electronic hygrometer/
temperature conditions in museum collection thermometer can be used to measure actual RH
storage and exhibit areas. This Conserve 0 and temperature conditions in the vicinity of the
Gram explains how hygrometers and hygrother- instrument being calibrated. The psychrometer
mographs can be calibrated. must be used properly to achieve accurate
readings (see Conserve 0 Gram 3/l). Once the
A hygrometer is a direct-read, humidity-sensing true relative humidity and temperature are
instrument incorporating an element of hygro- determined, the hygrometer or hygrothermo-
scopic (i.e., moisture absorbing and releasing) graph is adjusted to reflect them.
material that dimensionally changes according to
humidity conditions. The hygroscopic element Repeating the calibration process 10 to 20
moves a pointer on the dial face to indicate RH. minutes after the initial calibration will
A hygrothermograph is a recording instrument determine if, because of play in instrument
that continuously marks RH and temperature linkages, additional small adjustments are
conditions on a rotating chart for durations of necessary so that the instrument will more
either one day, one week, one month, or two accurately record actual conditions.
months. A hygrothermograph utilizes a hygro-
scopic RH sensor and a bimetal temperature The hygrothermographs and hygrometers used in
sensor. the National Park Service are manufactured by a
number of companies. Because adjustment
In order for humidity-sensing instruments to methods are peculiar to each instrument, the
accurately sense and record conditions, they table on page 2 is provided as a guide to indicate
require routine calibration. Instruments using the location of adjustment devices on the differ-
hygroscopic sensing elements have a tendency to ent brands of instruments in common use in the
drift from a set point. It is recommended that Service, how adjustments are made, and what
hygrometers and hygrothermographs be calibra- tools, if any, are needed for making adjustments.
ted at least quarterly, preferably monthly. When
environmental conditions change rapidly or dra- Donald R. Cumberland, Jr.
matically in an area being monitored, it may be
Curatorial Services Division
necessary to check calibration even more often. National Park Service
Harpers Ferry, West Virginia 25425
The calibration process involves checking
instrument readings against actual RH and/or Formerly issued as Conserve 0 Gram 3/16. Revised 1993.
The Conserve 0 Gram series is published as a reference on The series is distributed to all NPS units and is available to non-NPS
collections management and curatorial issues. Mention of a product, institutions and interested individuals by subscription through the
a manufacturer, or a supplier by name in this publication does not Superintendent of Documents, U.S. Government Printing Office,
constitute an endorsement of that product or supplier by the National Washington, D.C. 20402, FAX (202) 512-2233. For further
Park Service. Sources named are not all inclusive. It is suggested information and guidance concerning any of the topics or procedures
that readers also seek alternative product and vendor information in addressed in the series, contact the National Park Service, Curatorial
order to assess the full range of available supplies and equipment. Services Division, Harpers Ferry, WV 25425, (304) 535-6410.
Hygrothermograph Hygrometer 3
MANUFACTURER MODE1 ADJUSTMENT ADJUSTMENT ADJUSTMENT MANUFACTURER MODEL ADJUSTMENT ADJUSTMENT ADJUSTMENT
ILLUSTRATION DEVICE TOOL NEEDED ILLUSTRATION DEVICE TOOL NEEDED
BELFORT 5-594 AREA B ADJUSTMENT NONE AIRGUIDE 605 HYGROMETER SLIDING BALLPOINT PEN
KNOBS 1 METAL TAB OR STYLUS
WEATHERMEASURE H-31 1 AREA A ADJUSTMENT NONE AIRGUIDE 113 HYGROMETER SLIDING BALLPOINT PEN
KNOBS 2 METAL TAB OR STYLUS
QUALIMETRICS 5023 AREA A SQUARED ADJUSTMENT PASTORELLI & 2-INCH HYGROMETER ADJUSTMENT STRAIGHT SLOT
SHAFTS KEY RAPKIN 3 SCREW SCREWDRIVER
SIERRA-MISCO 2004 AREA A ADJUSTMENT PHILLIPS ABBEON-CAL MZAY HYGROMETER ADJUSTMENT STRAIGHT SLOT
SCREWS SCREWDRIVER 3 SCREW SCREWDRIVER
COLE-PARMER 37 250- AREA A ADJUSTMENT SCREWDRIVER
Consult the instrument manual for more in-depth information. If unavailable in the park, instructions may be requested from Curatorial Services Division, WASO.