MadSci Network: Physics

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

Thanks for the question.

After reading it carefully I believe there are a couple of ways to 
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 
sometimes referred to as a bulb thermometer.  Your question asks how and 
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 
expansion of a certain amount of expandable fluid versus a linear 
through the thermometer cavity.  Simply adding a larger bulb without 
changing the other properties of the thermometer will certainly give you 
different result.   Of course, not all thermometers have the same size 
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 
for a small bulb:  This will likely give a result of more heat, because 
fluid will expand through the capillary tube.

Larger Bulb (with same amount of Mercury or Alcohol) added to a 
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 
this may give varying results depending on how much of the bulb is empty 
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 
fluid must expand first before any fluid inside can expand.  This is 
the cause of some thermometers dropping in temperature readings at first 
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 
give you the correct temperature.

Some things to be aware of:  When you test temperature through this type 
heat transfer, there will always be losses.  Some of the heat you are 
measuring is stolen by the thermometer to expand the fluid.  The 
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 
telling the temperature of a human or animal and designed exactly for 
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 
an extremely high level of fluid conductivity whereas alcohol has a very 
fluid conductivity.  *The conductivity of a material to electricity is 
related to the conductivity of heat and mercury is metal.  That being 
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 

* The conductivity relationship does not work at extremely high 

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 
it kind of unique!

Heat transfer:



I hope I helped you find your answer.


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