Vernier Tech Info Library TIL #664

Question

How do you use the Thermocouple to measure very low temperatures?

Answer

The Thermocouple does not measure absolute temperatures like a Temperature Probe does. It works by measuring the difference in temperature between the two terminals of the probe. Commonly, the red terminal is placed in the higher temperature environment, while the other terminal is placed into the lower temperature environment. For measuring very low temperatures, place the red terminal in an ice bath and the black terminal in whatever you are measuring. The temperatures measured with the other probe junction will be the difference between the temperature reading and 0C, or just the temperature in C below zero.

When working with very low temperatures, it is best to calibrate the probe first. Calibration should be done with known, very cold temperatures, such as liquid nitrogen. Follow these general directions:

1. For the first calibration point, place both the reference junction (black terminal) and the probe junction (red terminal) of the thermocouple into liquid nitrogen. When the displayed voltage stabilizes, enter a value of
"-196" (deg C).
2. For the second calibration point, leave the reference junction in the liquid nitrogen, and place the probe junction into an ice-water bath (be sure to continuously stir the probe junction in the ice cubes). When the displayed voltage stabilizes, enter a value of "0".

When making measurements using this calibration, you always need to place the reference junction in liquid nitrogen. Place the probe junction in the higher temperature source.

Created by: dholmquist on May 01 2000
Last updated by: dholmquist on September 05 2001