Evaulation_Engineering_March_2020 - 22

THERMAL TEST

FAHRENHEIT'S MERCURY
THERMOMETER CEDES FIELD
TO TCS, RTDS, THERMISTORS
Data recorders, sensor transmitters, and I/O modules bring temperature
measurement and control into the digital world
By Rick Nelson, Contributing Technical Editor
	 Mention temperature measurement
to average persons, and they will
likely visualize a mercury-in-glass thermometer. They might not know that it was
invented in the early 18th century in
Amsterdam by Daniel Gabriel Fahrenheit,
but they will be familiar with Fahrenheit's
temperature scale, which, after some finetuning, places water's freezing point at
32°F and its boiling point at 212°F, or
180°F higher than the freezing point.
Fahrenheit's scale continues to hold
sway in the United States, although the
Celsius scale predominates elsewhere. But
alas, the mercury-in-glass thermometer,
which had represented the state-of-theart since its invention in 1714, is being
phased out in the 21st century because
of mercury's toxicity. As a sign of the mercury thermometer's demise, NIST reports
that it began an active mercury-reduction
campaign in 2007 and stopped calibrating mercury thermometers entirely on
March 1, 2011.1
NIST emphasizes that it continues
to offer thermometric calibration services for nonmercury devices, including
organic-liquid-in-glass thermometers,
which you can still purchase. Curiously,
Fahrenheit is credited with having helped
to perfect an alcohol-in-glass thermometer before turning his attention to the
mercury-based instrument.

22

EVALUATION ENGINEERING MARCH 2020

Of course, liquid-in-glass thermometers-organic or mercury-have never
been ideal for the types of experiments
conducted in an electronics test lab or environmental test chamber. Liquid-in-glass
thermometers are fragile, they have slow
equilibrium and response times, and they
don't offer electrical outputs that could
serve as inputs for old analog chart recorders or that could be digitized at fast
update rates for modern data loggers and
computer analysis. The attention has
turned to devices such as thermocouples
(TCs), resistance temperature detectors
(RTDs), and thermistors.

Thermocouples and Seebeck
Although not as old as liquid-in-glass
thermometers, thermocouples are not
new. Their existence extends back to
observations in 1821 by the German
physicist Thomas Johann Seebeck that a
temperature gradient across a conductor
induces an electromotive force. In a thermocouple, the bare ends of two insulated
wires of dissimilar metals are welded together to form a tip, which can be affixed
to a device under test whose temperature
is to be measured. The same temperature
gradient across the different wires will
induce a different voltage. Consequently,
the voltage differential measured at the
other end of the equal-length wires at a

controlled reference temperature (the
cold junction) can then be used to calculate the DUT temperature.
Thermocouples can operate over extreme temperature ranges-for example,
NIST reports it calibrates these devices
from -196°C to 2,100°C. 2 One drawback
is the need for a controlled reference
temperature, which could be provided
by a 0°C ice bath. If a fluctuating ambient
temperature is used as a cold junction,
an additional temperature sensor will
be needed to report the ambient temperature at the time of the measurement.
Thermocouples come in a variety types
(each designated by a letter) with different pairs of metals. Type J, for example,
combines iron and constantan, the latter
being a copper-nickel alloy. Commonly
used types in addition to J include B, E,
K, N, R, S, T, and C, whose makeup and
characteristics are in accordance with
standards set by ASTM International. 3

RTDs and thermistors
RTDs and thermistors are conceptually easier to understand. RTDs, such as
platinum resistance thermometers (PRT),
take advantage of the fact that the resistance of a metal (platinum, for example)
increases with temperature. They lack the
extreme temperature ranges of thermocouples. For example, NIST reports that
Evaulation_Engineering_March_2020

Table of Contents for the Digital Edition of Evaulation_Engineering_March_2020
