Calibration of Hygrometers and Hygrothermographs

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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.
                                                                          Museum Specialist
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.

                                                                                                                         Hygrometer        1
                                                                                                                             (Back View)

                                                                                                                                                             Hygrometer         2
                                                                                                                                                              (Back View)

                                                                                                                      Screw Adjustmen

                                       Hygrothermograph                                                                                 Hygrometer       3
                                                                                                                                         (Side View)

                               HYGROTHERMOGRAPH                                                                                         HYGROMETER
     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
                                 00                             SCREWS

    Consult   the instrument    manual    for more in-depth    information.   If unavailable   in the park,   instructions    may be requested   from   Curatorial   Services   Division,   WASO.