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Rejoining Separated Thermometer Columns

Mike Isley
Product Developer

March 2017


One of the most common problems with laboratory thermometers is column separation due to improper storage, vibration, or accidental dropping. The good news is that you can often join a separated column using a few simple techniques. Included here are techniques for joining separated columns in spirit-filled and mercury-filled thermometers.

The common spirit-filled thermometer contains a petroleum-based liquid with a dark-colored dye for contrast when reading the scale. Mercury-filled thermometers contain mercury, which is toxic, and for that reason are no longer sold by many companies. Always handle mercury-filled thermometers with care and have a mercury collector close by when you are working with them.


  • Gloves
  • Goggles
  • Bunsen Burner (for heating method only)
  • Test Tube Rack or Thermometer Storage Rack
  • Dry Ice (for cooling method only)
  • Alcohol (for cooling method only)
  • Small Beaker (for cooling method only)


For mercury-filled thermometers

Cooling method

  1. Add enough dry ice and alcohol to a small beaker to cover the thermometer’s bulb.
  2. Immerse only the thermometer’s bulb into the dry ice and alcohol.
  3. Allow all of the mercury column to retreat into the bulb as it cools down.
  4. Remove the thermometer and carefully place it upright in a test tube rack. As the thermometer warms up to room temperature, the separated liquid should rejoin into a solid column.

For spirit-filled thermometers

Tapping method

Hold the thermometer upright and gently tap it against the palm of your gloved hand above the column separation. This should force the upper separated column to release itself from the glass walls and fall back down and join the main column.

Another tapping method is to hold the thermometer vertically and allow the bulb to drop onto a soft surface such as a computer mouse pad. If this does not work, then use the heating method below.

Heating method

  1. Wearing gloves and safety goggles, ignite a Bunsen burner and move the thermometer’s bulb in and out of the heated air directly above the flame. See Fig. 1. Note: Do not insert the bulb into the flame. Just move the bulb over the flame and then away repeatedly for slow heating.

    Figure 1  Heating the thermometer’s bulb.
  2. Make sure the separated columns move slowly up the capillary tube. By the time the first column segment enters the expansion chamber, all columns should be rejoined.
  3. Allow no more than 3/4 of the expansion chamber to be filled with liquid. See Fig. 2. Be very careful not to overheat the bulb. If the liquid column rises too quickly and fills the expansion chamber completely, it could burst the chamber and expel the liquid.

    Figure 2  The thermometer’s expansion chamber.
  4. After filling the chamber to no more than 3/4 full, hold the thermometer upright and away from the burner to allow the joined column to recede from the chamber as it cools.
  5. Place the thermometer upright in a test tube rack to cool to room temperature. Repeat this procedure if there is still a separation and allow the thermometer to cool again in an upright position.

How to avoid column separation

When storing thermometers, it is best to keep them upright in a storage rack, or if stored in a drawer, at 15º from horizontal (expansion chamber end up). This slight angle enables gravity to keep the liquid column together. Also avoid subjecting your thermometers to drops and vibration.