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Name: Amy
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: Around 1999 


Question:
Hello, I am an educator at the Science Museum of Minnesota. I have been asked to find out what temperature mercury freezes at and is no longer accurate as a thermometer. I know that the freezing point of mercury is -38F, but are thermometers specially insulated or treated so that they are accurate to a lower temp?

Please help me find this information! I have been looking for days! :)

Thank you


Replies:
I don't know what kind of thermometer you used, but you could try something which resembles a thermocouple. The wires are generally available in a physics laboratory. Once you have the wire, two strands-of different metal. Then calibrate the thermocouple with a good Voltemeter, you will need to read millivolts. Calibrate at room temp and a very low temp (liquid nitrogen). Room temp should provide zero voltage and liquid nitrogen some millivolts.

If you are desperate, there are some really good physicists at the University of Minnesota, Dahlberg or Goldman. Have one of their graduate students give you a hand. It will take 5 minutes.

Dr. Myron


Thermometers to measure temperatures below the freezing point of mnercury use some other measuring technique. Thermocouples are popular for lower temperatures, as are spirit thermometers that use alcohol instead of mercury. The thermometers I use in lab for measuring low (around dry ice temperatures) use pentane as the liquid instead of mercury. These thermometers are good only below room temperature.

Richard Barrans Jr., Ph.D.
Chemical Separations Group
Argonne National Laboratory


A mercury thermometer will probably lose accuracy somewhat above the freezing point, because as one approaches the phase transition from liquid to solid, one loses the linear dependence of the column length on temperature. This is because the expansion coefficient of materials is generally temperature- dependent. I have no data handy on the specific temperature dependence of mercury, unfortunately. You might try consulting tables of thermodynamic data of the elements, which might give the density as a function of temperature.

From this you could plot the molar volume as a function of temperature, and see whether the plot is linear or nonlinear over the temperature range you are interested in.

Thermometers are advertised by their manufacturer to work only within specified temperature ranges, and outside of that range they may no longer work with the same (linear) calibration.

I hope this is helpful. All the best,

Prof. Robert Topper
Dept of Chemistry
The Cooper Union



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