Hygro-Meter

WLMD ID: ameu
Hygrometers are instruments used to determine the relative humidity of the air. Relative humidity (RH) is the amount of evaporated water in the air compared to the maximum amount that the air could hold at a particular temperature. The type pictured here is a psychrometer, or a wet and dry bulb hygrometer.

This psychrometer may have been manufactured by the Philadelphia Thermometer Company for The Ohio Chemical & Manufacturing Company sometime between 1924 and 1935. The bulb of the dry thermometer (on the left) was open to the air. The bulb of the wet thermometer (on the right) was covered by a piece of wet cloth. Due to the evaporation of the water around it, the wet bulb readings tended to be cooler than the dry bulb readings. The drier the air, the greater the temperature difference between the two thermometers. This difference was used to determine the RH in a room. So that the user would not need to perform calculations, this hygrometer is accompanied by a table that allowed the user to quickly obtain the humidity using the temperature readings. The table is located in the central rotating cylinder.

Hygrometers were commonly used in operating rooms from the late 1920s through the 1970s. During that time period, maintaining operating room humidity above 50% or so was one of many safeguards employed in the prevention of static sparks, which could ignite combustible anesthetic mixtures then in use.

Catalog Record: Hygro-Meter

Access Key: ameu

Accession No.: 296

Title: Hygro-meter / Phila Thermo Co ; made expressly for The Ohio Chemical &
Manufacturing Company by Wm. A. Baker, owner of patent and copyrights.

Author: Schwartz, John Leonard.

Corporate Author: Philadelphia Thermometer Company.
Corporate Author: Ohio Chemical & Manufacturing Company.

Title variation: Alt Title
Title: Hygrometer.

Title variation: Alt Title
Title: Psychrometer.

Title variation: Alt Title
Title: Wet and dry bulb thermometer.

Title variation: Alt Title
Title: Wet and dry bulb hygrometer.

Publisher: Philadelphia : Phila Thermo Co, and Wm. A. Baker ; Cleveland : The Ohio
Chemical & Manufacturing Company, [between 1924 and 1935?].

Physical Descript: 1 humidity gauge : wood, metals, glass, paper ; 22 x 11 x 4 cm.

Subject: Humidity – instrumentation.
Subject: Explosions – prevention and control.
Subject: Fires – prevention & control.
Subject: Health Facilities – fires and fire prevention.
Subject: Operating Rooms – safety measures.

Note Type: General
Notes: The early year in the date range for the possible year of manufacture is
based on a year that appears on an accession card for this object made by Dr.
Paul Wood. Written on the card is, “Hygrometer from Ohio chemical co. at
Seton Hospital. 1924 (Sisters thought it was means against explosion. Hung
on wall when 1935 explosion happened.) Received from Dr. Paul M. Wood”. The
1924 date may be too early as a patent for this device was applied for in
1928. The end year in the date range for the possible year of manufacture is
based on the year of the explosion provided by Dr. Wood. The date range could
change if reliable information indicates that it should be corrected.

Note Type: Citation
Notes: Collins VJ. Hazards in anesthesia practice. In: Principles of Anesthesiology.
2nd ed. Philadelphia: Lea & Febiger; 1976:817.

Note Type: Citation
Notes: In depth hygrometer. The Museo Galileo website. https://catalogue.museogalileo
it/indepth/Hygrometer.html. Accessed September 9, 2015.

Note Type: Citation
Notes: Middleton WEK. Catalog of Meteorological Instruments in the Museum of History
and Technology. Washington, D.C.: Smithsonian Institution Press; 1969:66-69.
https://www.sil.si.
edu/smithsoniancontributions/HistoryTechnology/pdf_hi/SSHT-0002.pdf. Accessed
September 9, 2015.

Note Type: Citation
Notes: Schwartz JL, inventor; Philadelphia Thermometer Co. Hygrometer. Patent 1,933,
283. October 31, 1933. https://www.google.com/patents/US1933283. Accessed
September 9, 2015.

Note Type: Citation
Notes: Wardell CH. Minimizing the fire and explosion hazard of anesthetic agents.
Anesth Analg. 1929:8(2):98. https://journals.lww.
com/anesthesia-analgesia/Citation/1929/03000/Minimizing_the_Fire_and_Explosio
_Hazard_of.9.aspx. Accessed September 9, 2015.

Note Type: Physical Description
Notes: One device for obtaining the relative humidity of the surrounding atmosphere;
It consists of a wooden backboard, two glass thermometers, Fahrenheit scales
engraved on a metal plate, a chart, and fabric wick; The metal plate is
raised almost a cm off of the backboard; The thermometer on the left appears
to be an alcohol thermometer with red dye; It is affixed with clamps to the
metal plate so that graduation marks in Fahrenheit are to the left and right
of the thermometer; The numbers are on the left side of the scale, marked
every ten degrees from 20 to 120; In the center is a cylinder positioned so
that it rotates about a vertical axis; The cylinder is made of clear glass
and has a plastic end-caps; Inside the cylinder is a chart; The cylinder
rotates so that a vertical line with possible dry thermometer temperatures, a
horizontal line with numerical values for the possible temperature
differences between the two thermometers, and numerous humidity readings are
visible; To the right of the rotating chart (cylinder) is a vertical rod on
which a pointer can be moved up and down along; To the right of the rod is
the wet thermometer which appears to be an alcohol thermometer with a blue
dye; The dye in both of the thermometers reaches the top of each thermometer
and does not seem to change any longer; The wet thermometer is also affixed
with clamps to the metal plate so that graduation marks in Fahrenheit are to
the left and right of the thermometer; The numbers are on the right side of
the scale, marked every ten degrees from 30 to 120; A tubular fabric wick is
attached to the bulb of the wet thermometer; Engraved or stamped into the
metal plate, above the rotating chart is, “HYGRO-METER”; Below the rotating
chart the following is stamped or engraved into the metal plate in a circular
design, “PHILA THERMO CO PHILA”; Inside the circle formed by these words is,
“PAT PEND”; Nailed to the backboard, below the metal plate, is a paper label
marked with the following, “Humidity should be 54% or greater in order to
dissipate static electricity in operating rooms. [new line] Made Expressly
for [new line] THE OHIO CHEMICAL & MANUFACTURING COMPANY [new line] 1177
Marquette Street, Cleveland [new line] by Wm. A. Baker, owner of patent and
copyrights.”; Below this label is a metal clip, the bottom end of which is
bent; The water reservoir the clamp could have held is not with this item;
The accession number is painted on the right lower front corner of the
backboard, “296”; A metal loop is affixed to the upper back of the board so
that the device may be hung on a wall.

Note Type: Reproduction
Notes: Photographed by Mr. Steve Donisch, June 4, 2015.

Note Type: Acquisition
Notes: The accession card for this object made by Dr. Paul Wood indicates that he
added it to the museum collection.

Note Type: Not Applicable
Notes: A hygrometer is an instrument used to determine the relative humidity of the
air surrounding it. Relative humidity (RH) is the amount of evaporated water
in the air compared to the maximum amount that the air could hold at a
particular temperature. There are a number of different kinds of hygrometers.
The type pictured here is a psychrometer, or a wet and dry bulb hygrometer.
‘Psychro’ comes from the Greek word for ‘cold’. Scientist Ernst F. August
(1797-1870) coined the word pychrometer in the early 1800s while refining the
process for measuring RH using a wet and a dry thermometer.

This psychrometer may have been manufactured by the Philadelphia Thermometer
Company for The Ohio Chemical & Manufacturing Company sometime between 1924
and 1935. The bulb of the dry thermometer (on the left) was open to the air.
The bulb of the wet thermometer (on the right) was covered by a wet piece of
cloth. (A metal water reservoir was once held in the now bent clamp located
on the lower end of the device. One end of the wet cloth would rest inside
the water reservoir.) Due to the evaporation of water around it, the wet bulb
readings tended to be cooler than the dry bulb readings. The drier the air,
the greater the temperature difference between the two thermometers. The
temperature difference between the two thermometers was used to determine the
RH in a room.

So that the user did not need to perform calculations, hygrometers were often
accompanied by tables that could be used to determine the humidity. The table
in this hygrometer was placed in a space-saving rotating cylinder. The
left-most vertical column of the table contained possible values for the dry
thermometer, and the upper-most horizontal row contained possible values for
the temperature difference. The intersection of the corresponding row and
column reflected the RH.

Hygrometers were commonly used in operating rooms from the late 1920s through
the 1970s. During that time period, maintaining operating room humidity above
50% or so was one of many safeguards employed in the prevention of static
sparks. A static discharge could ignite combustible anesthetic mixtures then
in use. Consequently, all sources of ignition, including discharge of static
electricity, had to be carefully controlled. Higher humidity makes the air
more conductive, so it can absorb and more evenly distribute excess static
charges.

Note Type: Exhibition
Notes: Selected for the WLM website (noted September 4, 2015).