Ask A Scientist© Chemistry Archive

name        ashley
status      student
age         16

Question -  What physical property of alcohol, or mercury, makes one
thermometer more accurate than the other and why?
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Since mercury freezes at -40 degrees (interesting coincidence!), alcohol
thermometers are much more useful at lower temperatures.  Similarly, there
comes a temperature where "alcohol" boils, above which mercury is much more
useful.  Accuracy of a mercury or spirit thermometer is more a function of
the column cross-section being appropriate to the marked lengths indicating
temperatures.  While the cost, durability, precision, portability, safety
and response times of these liquid-column thermometers make them well-suited
for laboratory use, other applications call for thermometers that measure
temperature using thermocouples, thermistors, linear metallic resistors,
bimetals, or electromagnetic radiation.  (A few examples given, others
exist.)  In short, (unless there are special circumstances), it comes down
to preferences. My lab assistant likes spirit thermometers with purple
columns.
Tim Spry
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To put thermometers into perspective, any measurable property of a material
that changes with temperature can be used as a thermometer. So, electrical
voltage, electrical resistance, magnetism, electromagnetic radiation of many
wavelengths from the ultraviolet to radio waves, the expansion of gases, and
last but more familiar, the expansion of a liquid with temperature are all
used to measure temperature.

The liquid / capillary thermometer, which includes the alcohol and mercury
thermometer, uses the expansion of the volume of a liquid to measure
temperature. This type thermometer has a bulb at one end to maximize the
amount of liquid that is changing in volume, and a narrow capillary to
"amplify" the change in volume. Since the volume of a cylindrical capillary
is V= pi*r^2*L, within certain constraints it pays to minimize the radius,
r, so that a given temperature change results in a large change in the
length of liquid in the capillary,L. For sensitivity, it is desirable that
the temperature coefficient of expansion of the liquid be as large and as
linear as possible.

Traditionally, mercury has been used because: it has a large liquid range
[m.p. = -38C; b.p. = 35 C] as well as a linear, and fairly large coefficient
of expansion. However, if measurement at lower temperatures is needed
another medium, usually ethanol, is used. However, its liquid range [m.p.
= -114 C; b.p. = 79 C] limits its utility at higher temperatures.

The use of mercury thermometers has declined in recent years because of the
realization of its toxicity, and the difficulty in properly disposing of the
mercury if a thermometer breaks. In addition, various thermoelectric devices
have become available at low cost.
Vince Calder
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