Measuring temperatures of -10 to 150° C can be accomplished quite easily by means of an ordinary laboratory thermometer. However, measuring temperatures in the range of liquid nitrogen can prove to be very difficult with an ordinary thermometer. Thermocouple thermometers however, are fairly accurate over a wide range of temperatures. A thermocouple is an electrical junction between two dissimilar metals. This junction produces a small voltage at different temperatures.By calibrating the voltage with known temperatures, an accurate thermocouple thermometer can be made. Commercial thermocouples of various types are usually already calibrated and are readily available.
When working with very low temperatures it is inconvenient to work with the Celsius or Fahrenheit scales because of their inherent negative numbers. The K scale,with 0 K representing the temperature where a substance has zero heat energy, is a more appropriate temperature scale to use. This scale is very convenient for measuring the very low temperatures of liquid nitrogen. On this scale liquid nitrogen would have a temperature of 77 K.
The three main temperature scales used for measuring temperature are Fahrenheit, Celsius, and Kelvin. The Kelvin scale is used for most scientific work because it is proportional to the kinetic energy in a substance. The following formulas may be used to convert from one temperature scale to another.
Formulas:
Common Temperature Reference Points:
Fahrenheit Celsius Kelvin
Absolute Zero -460 -273 0
Liquid Helium (boiling) -452.1 -268.8 4.2
Liquid nitrogen (boiling -321 -196 77
Water (freezing) 32 0 273
Water (boiling) 212 100 373
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