Gas Thermometers: Calibration & Explanation

[danago]

Hi. Recently in my physics class we attempted to create gas thermometers. here is a picture i drew on the setup of the thermometers:

http://img93.imageshack.us/img93/8338/setup2rj.gif

We calibrated them using already calibrated thermometers.

I think i have a basic understanding of how it works. As the liquid and air inside the test tube are heated, the air pressure increases, forcing the liquid up the glass tube.

Could someone please explain other factors that may affect how it works? And if possible, maybe give me a link that might further explain this type of thermometer.

Thanks,
Dan.

 

[Astronuc]

Gas or liquid thermometers simply work on the principle of volumetric thermal expansion of a gas or liquid. The fluid (gas or liquid) is constrained in 2 dimensions, and therefore all the expansion occurs in one dimension - e.g. the glass tube in your thermometer. The thermal expansion hopefully is linear over the temperature range of application, and one has to calibrate at a minimum of two temperatures (but preferably more) to obtain a valid range. A linear thermal expansion coefficient implies that the gradations are equally spaced.

Now in addition to temperature, the gas is subject of effects of pressure. Gases are compressible, liquid are essentially incompressible. In your thermometer, it appears that the tob of the glass tube is open to the atmosphere, so air pressure would affect its performance (reading).

In a liquid thermometer, the liquid is in a sealed container and the gas in the volume into which the liquid expands is evacuated to preclude any gas pressure effect. Liquids however do evaporate, so one must select a liquid with a low vapor pressure in the temperature of interest.

At temperatures in which liquids would boil, metals must be used. The useful range of thermal expansion of a solid is much greater than for liquid. Still at higher temperatures, thermocouples must be used. Here different metals or alloys establish a voltage difference as a function of temperature.

Some links on thermal expansion -
http://hyperphysics.phy-astr.gsu.edu/Hbase/thermo/thexp.html

Temperature
http://hyperphysics.phy-astr.gsu.edu/Hbase/thermo/temper.html
http://hyperphysics.phy-astr.gsu.edu/Hbase/thermo/temper2.html

Temperature and thermometry -
http://eo.ucar.edu/skymath/tmp2.html

 

[danago]

Thanks very much for that :)