From: Ronald Wong (ronwong@inreach.com)
Date: Mon Apr 22 2002 - 02:44:55 PDT
Message-Id: <l03102801b8e9768a6376@[209.209.19.76]> Date: Mon, 22 Apr 2002 02:44:55 -0700 From: Ronald Wong <ronwong@inreach.com> Subject: Re: I need flexible thermometers
Marc brought up the following:
>I have a kid with a science fair project that involves measuring thermal =
>conductivity of various substances. He needs a thermometer that is =
>flexible and that can be attached to an object, ie. a rock, metal.=20
>
>Any ideas??? The walgreens style flexible forehead ones are not very =
>sensitive.
Marc:
This may be a too late to help your student with his project by now, but I
noticed that no one seems to have mentioned thermocouple wire as a possible
solution to your student's problem.
Here's something you might want to keep in mind for a similar need at some
future date.
You'll need two things: a Voltmeter that reads to a hundredth of a
millivolt and thermocouple wire. Oops - three things - the Handbook of
Chemistry and Physics.
The thermocouple wire comes in a spool and looks like ordinary zip cord
except that in this case the two wires are made of different metals.
Typically, one will be iron and the other an alloy called constantan (a
copper-nickel alloy).
You probably can get thermocouple wire from Radio Shack. Many physics and
chem labs have this stuff since it is very useful for quick and dirty
temperature measurements where precision, accuracy, and sensitivity, with
extreme robustness, is necessary. It's also cheap (compared to a glass
thermometer) - so ask around. Engineers are VERY familiar with this stuff
so if you know one, ask if (s)he has some that (s)he would like to
contribute to the project.
1. Cut off a length of wire suitable to your particular needs.
2. Strip the insulation off each end of the cord.
3. Twist the two wires at each end of the cord together.
4. Solder them together at each end so that the two different metals are
electrically bonded to each other (these ends are referred to as
junctions).
5. At a convenient place along the cord, make a break in one of the two
wires. Strip the insulation from the exposed ends and connect them to the
millivolt meter.
6. Secure one junction to the object whose temperature you will want to
measure (epoxy if necessary).
7. Place the other junction in an ordinary ice bath.
8. If the millivolt meter is an old mechanical one and the needle goes the
"wrong" way reverse the electrical connections to the meter.
9. Go to the Electricity and Magnetism section of the Handbook of C&P and
look up Calibration Tables in the Electricity and Magnetism section.
You'll find a number of calibration tables for thermocouples of various
pairs of dissimilar metals. Find the one for the pair of metals you
happen to be using (Iron-Constantan in this example. Conveniently,
there is one for Celsius and one for Fahrenheit for this pair of metals.
You'll notice, for this pair of metals, the useful range of the
thermometer is from -190° C./-310° F to 760° C/1400° F. Pretty
impressive for just a pair of dissimilar wires).
10. By making a graph of the data in the temperature range you are trying to
measure, you can measure to the nearest tenth of a degree without any
difficulty.
If you need greater precision than this, you can achieve it by "simply"
soldering the thermocouples in series to make what is called a thermopile -
but this takes us away from the simplicity of using a thermocouple for
simple temperature measurements. If your student wishes to pursue this to
higher level, (s)he can do the research into how this can be done correctly.
Actually, the student should do a little research just to see how this
arrangement can produce the results it does - very educational.
Just a thought.
ron
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