MadSci Network: Physics Query:

### Re: How is the temperature reading affected by size of bulb of thermometer?

Date: Mon Aug 14 02:13:48 2006
Posted By: Dan Bolton, Undergraduate, Astronomy and Physics, Last: University of Hawaii at Hilo
Area of science: Physics
ID: 1152840526.Ph
Message:
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Thanks for the question.

After reading it carefully I believe there are a couple of ways to
interpret
the request.  I will try to explore a few of them.

There are a lot of different types of thermometers as I'm sure you know.
The most common type of thermometer is the clinical thermometer; it is
also
sometimes referred to as a bulb thermometer.  Your question asks how and
why
the temperature sample might differ by changing the bulb size. We should
first accept the fact that a clinical thermometer does not measure
temperature just because it has a bulb filled with mercury, or more
commonly, alcohol.  The device itself is carefully designed to measure
the
expansion of a certain amount of expandable fluid versus a linear
distance
through the thermometer cavity.  Simply adding a larger bulb without
changing the other properties of the thermometer will certainly give you
a
different result.   Of course, not all thermometers have the same size
bulb,
but each thermometer is designed to measure a certain amount of fluid
expanding over an expected distance so that each thermometer returns the
correct result.

Here are some possible scenarios:

Larger bulb (with more Mercury or Alcohol) added to a thermometer
designed
for a small bulb:  This will likely give a result of more heat, because
more
fluid will expand through the capillary tube.

Larger Bulb (with same amount of Mercury or Alcohol) added to a
thermometers
designed for a small bulb:  The fluid will have to expand into the empty
cavity of the partially filled bulb and then expand into the capillary
tube,
this may give varying results depending on how much of the bulb is empty
and
free for expansion.

Larger Bulb (Solid glass except for the original bulb cavity and fluid in
the center) for said thermometer design:  The glass around any
thermometer
fluid must expand first before any fluid inside can expand.  This is
often
the cause of some thermometers dropping in temperature readings at first
and
then rising to meet the conditions of the test.  More glass would take
longer to expand and may give you a slightly lower temperature reading.

Larger Bulb on a thermometer designed for a Larger Bulb:  This will
probably
give you the correct temperature.

Some things to be aware of:  When you test temperature through this type
of
heat transfer, there will always be losses.  Some of the heat you are
measuring is stolen by the thermometer to expand the fluid.  The
equilibrium
temperature is the result.  Larger bulb thermometers will require more
energy and change the conditions of the test more than small bulb
thermometers.  (Clinical thermometers are not extremely accurate - good
for
telling the temperature of a human or animal and designed exactly for
such.)
Also, Mercury and Alcohol have something in common, that being they both
expand to an expected linear distance for the thermometer type.

Mercury and alcohol differ dramatically in other ways.  For one, Mercury
has
an extremely high level of fluid conductivity whereas alcohol has a very
low
fluid conductivity.  *The conductivity of a material to electricity is
related to the conductivity of heat and mercury is metal.  That being
said,
putting mercury in a thermometer tube designed for alcohol will give very
different results.  Mercury also expands faster, in fact mercury
thermometers have been known to explode due to the force of the expansion
inside.

* The conductivity relationship does not work at extremely high
temperatures.

Here are some websites related to this discussion. Although there doesn't
seem to be a lot of direct answers on the web to your question, which
makes
it kind of unique!

Heat transfer:  http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatra.html#c2

Thermometers: http://www.coolquiz.com/trivia/explain/docs/thermometer.asp

Temperature: http://eo.ucar.edu/skymath/tmp2.html

I hope I helped you find your answer.

-Dan

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