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Glass Thermometers: A Degree of History
E
ver since the early 1600s, when Galileo invented the first
crude instrument, called a thermoscope, to show changes in
temperature, the thermometer has evolved to become a crucial
instrument for determining temperature in a multitude of laboratory
and industrial applications.
The earliest thermometers, considered a curiosity by many people,
were used for medical and meteorological purposes and shared basic
features. All were glass tubes open at one end, partially filled with air,
set in a basin of water and graduated with primitive notches or dials.
It wasn't until 50 years later that the first closed-glass liquid
thermometer was developed in Italy by Leopoldo, Cardinal dei Medici,
a co-founder of the Academia de Cimento. Known as Florentine
thermometers, they were graduated into 50, 100 or 300 degrees and
roughly standardized by the heat of the sun and the cold of ice water,
and filled with distilled colored wine. Mercury and water
thermometers were also tried by the Florentines but their expansion
was too small. This problem was later overcome by making
thermometers with finer bores, thereby increasing their sensitivity. By
the middle of the eighteenth century, mercury thermometers had
superseded all others because of their more uniform expansion.
Today the glass thermometer is available in a variety of calibrations,
immersion styles, versions, lengths, divisions and backings and in °F
and °C versions with columns filled with mercury or safety liquid.
Lodged as a staple in laboratories and industries worldwide, the glass
thermometer is an enduring symbol for the advancement of
knowledge.
MEGA introduces its new line of glass
thermometers for laboratory and industrial
applications. Accurate, economical instruments,
glass thermometers measure the temperature of many
fluid types–liquid or gas. OMEGA offers precision
ASTM-designated thermometers in accordance with
strict performance specifications.
Immersion Styles
O
Care and Use of Glass Thermometers
Glass thermometers must be handled with care. Avoid
tapping a glass thermometer against a hard surface to
prevent small fractures, which could affect accuracy. A
glass thermometer should never be subjected to
extreme temperature changes. For example, removing
a glass thermometer from a boiling water bath and
immediately immersing it in an ice bath may cause the
thermometer to break or the liquid filling to separate.
With proper care, a glass thermometer can be a useful
instrument for years.
Column Separation
Occasionally, a thermometer arrives with a separation
of the mercury column due to rough handling in transit.
A column will reunite if: 1) the bulb is cooled in
brine/ice solution, or 2) the bulb is cooled in an
alcohol/dry ice solution, or 3) the separation is slowly
heated into the expansion chamber.
A total immersion thermometer is designed to indicate
temperatures correctly when the bulb and the entire
liquid column are immersed in the substance being
measured. A minimal length of the thermometer must
be visible.
A partial immersion thermometer has a line around it
indicating maximum immersion depth.
For maximum accuracy, select the proper
thermometer. A total immersion style offers the best
accuracy when used properly. A partial immersion style
is appropriate for use when immersion depth is limited.
When used correctly, either type will yield reliable
results.
A total immersion thermometer can be used accurately
at partial immersion if the following correction is
applied:
(T-t) 0.000089°F x N
(T-t) 0.00016°C x N
T = temperature of the bath
t = average temperature of stem
N = number of degrees emergent
CAUTION
Mercury sealed in a thermometer is not dangerous.
However, if the thermometer is broken, the mercury metal
or vapor may be hazardous to health or materials.
Parts of a Glass Thermometer
Bulb – reservoir for the thermometric liquid.
Stem – glass capillary tube through which the mercury or organic liquid fluctuates with changes in temperature.
Scale – the scale is graduated in degrees, fractions or multiples of degrees.
Contraction Chamber– an enlarged capillary bore which serves to reduce a long length of capillary, or prevent contraction of
the entire liquid column into the bulb.
Expansion Chamber– an enlargement of the capillary bore at the top of the thermometer to prevent buildup of excessive
pressures in gas-filled thermometers.
Partial Immersion Style
Immersion Line
Bulb
Stem
Scale
Total Immersion Style
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Glass Thermometer Options
Any encapsulation will increase lag time, but OMEGA®
encapsulation does not affect the accuracy of even
precision thermometers. Price for Teflon encapsulation
ranges from $9.
NIST Calibration
Available for ASTM and Precision
Styles
An optional factory certificate is available that will show
the corrections of a thermometer stated to one-tenth of
one scale division as tested against our National
Institute of Standards and Technology (NIST) certified
thermometers, and will reference the NIST test number
involved. You may specify the test points to be taken.
The charge is $70 per test point. Thermometers may be
calibrated over the temperature range of -112 to 572°F
(-80 to 300°C).
To Order: Add suffix "-E" to model number and the
appropriate amount to price of thermometer:
to 6" length, $9; 6-12" length, $11; 13 to 18" length,
$13; 19 to 24" length $16.
Ordering Example: GT-6452W-E is a 6" total
immersion style, white back, mercury-filled laboratory
thermometer, 30 to 125°F range ($17) with optional
Teflon® encapsulation ($9). Total price = $17 + 9 = $26.
ASTM Thermometers
Page
E-5 to 12
Meets Accuracy Tolerance and Dimensional
Specifications for American Society of Testing
Materials (ASTM)
Ordering Example: ASTM-44F-NC (68°F, 70°F) is a 12"
total immersion style, kinematic viscosity thermometer,
66.5 to 71.5°F range ($90), calibrated at 68°F and 70°F
($70 per point, or $140).Total price = $90 + 70 +70 = $230.
CERTIFICATE
OF
CONFORMANCE
E
Contents
To Order: Add the suffix "-NC" to the model number and
$70 per calibration point to the price of the thermometer.
Indicate the calibration point(s) in parentheses following
the part number.
Certificate of Conformance
Available for ASTM, Laboratory and
Precision Styles
Laboratory Thermometers
A factory Certificate of Conformance will show accuracy
tolerance of a specific type of liquid-in-glass
thermometer according to NBS Monograph 150, state
conformance to it as tested against our NIST-certified
thermometers, and reference the NIST number
involved. The charge is $40 per thermometer.
To Order: Add suffix "-CC" to model number and $40
to the price of the thermometer.
E-13 to 18
White Back, Yellow Back, Red Magnified Mercury and
Red Safety Liquid-Filled
Precision °F & °C Thermometers E-19 to 22
Yellow Back and Red Magnified Mercury
Make for Superior Visibility
Ordering Example: GT-1544Y-CC is a total immersion
style, yellow back, precision thermometer, -2 to 52°C
range ($35) with optional factory Certificate of
Conformance ($40). Total price = $35 + 40 = $75.
Low Cost Thermometers
TEFLON®
E-23 to 24
Economical, General Purpose,
Ideal For Frequent Purchases
Teflon Encapsulation
Available for Laboratory and
Precision Styles
®
Laboratory and precision style OMEGA® thermometers
may be ordered encapsulated in transparent PFA
Teflon® for use wherever freedom from contamination
must be assured. The entire thermometer is sealed
permanently in a coating which permits use in corrosive
chemicals or acids up to 435°F (225°C) and prevents
contamination if breakage does occur.
Accessories
Clip-on Magnifier, Handheld Lens,
Protective Metal Sheath and Mercury Spill Kit
Expansion Chamber
E-4
E-25