Reuniting Separated Mercury Columns

The largest single cause for the failure of precision thermometers in the lab is due to separated mercury columns. This can occur in transit or in the lab. The life of the instrument can be greatly extended if the following procedures are rigidly employed. Other methods may cause damage to the thermometer.

Cooling Method:
With the thermometer in an up right position gradually immerse ONLY half of the bulb in a solution of solid CO²(Dry-Ice) and alcohol so that the mercury column retreats slowly into the bulb. Do not cool the stem or mercury column. Keep the bulb in the solution until the main column as well as the separated portion retreats into the bulb. Remove and swing thermometer in short arc forcing all the mercury into the bulb.

Most mercury thermometers can be reunited using this method regardless of range (with the exception of deep immersion thermometers) provided ONLY HALF THE BULB is immersed in the CO²


  • Do not touch the bulb until it has warmed sufficiently for the mercury to emerge from the bulb into the capillary.
  • Never subject the stem or mercury column to the CO² solution as it will freeze the mercury column in the capillary and may cause bulb to fracture.

Heating Method:
This method applies to thermometers with a maximum range of 260° C or 500° F, equipped with expansion chambers sufficiently large enough to accommodate the separation plus a portion of the main column. Immerse as much of the bulb and stem as possible in a large beaker containing a liquid whose flash point is well above the highest indication of the thermometer being reunited. Heat the beaker stirring the liquid with the thermometer until the separation and a portion of the main column enters the chamber. Tap the thermometer in the palm of a gloved hand to aid in reuniting the column. Allow the thermometer to cool slowly.


  • Never use open flame on bulb.
  • Never fill expansion chamber more than 2/3 full.
  • Make certain Flash Point of liquid is well above the highest temperature indicated on the thermometer.
  • Thermometers whose range exceed 260° C or 500° F CAN NOT be reunited using heat without damaging the instrument.
Did you know?
The first sealed thermometer was produced by the German physicist Daniel Gabriel Fahrenheit in 1714.

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