Rejoining Separated Columns in Thermometers: 6 Dos and Don’ts

thermometer column separation

If you work in a materials testing lab, most likely you’ve experienced failure of a liquid-in-glass thermometer due to a separated column of mercury or indicating fluid. Separation can happen during shipping, or from improper storage or use. If the fluid in the column is separated, all subsequent readings will be inaccurate. This is a common occurrence and not necessarily a reason to return or discard the thermometer. In many cases, the separated fluid column can be rejoined by cautiously following certain steps and the thermometer can be returned to use.

This blog post will explore some common practices developed by others to reunite a separated column in both mercury-filled thermometers and spirit-filled thermometers, and some dos and don’ts to keep in mind when doing so.


Dos and Don’ts #1 … Attempting to rejoin a separated column on your own.

  • Do be experienced and careful if you attempt this process. Rejoining a separated mercury column is not an “I’ll give it a try” endeavor. Mercury is a hazardous material and breakage of a glass thermometer is considered a mercury spill requiring strict procedures to prevent human exposure. It requires expertise and skill, keeping safety a priority.
  • Don’t hesitate to ask for assistance. If you have not done this procedure before, get in touch with the manufacturer or a calibration lab experienced in the process for guidance and help.

Dos and Don’ts #2 … Remember that mercury is a hazardous material!

  • Do exercise extreme care and caution when rejoining a separated column in a mercury thermometer.
  • Don’t forget to wear safety goggles and gloves, or other protective gear or items that may be required.

Dos and Don’ts #3 … Remember the regulations and safety protocols.

  • Do follow regulatory agency and internal laboratory policies and safety procedures fully to prevent breakage and avoid a hazardous mercury spill. If a spill occurs, notify your safety officer immediately.
  • Don’t attempt these procedures on a mercury thermometer without having a complete mercury spill kit on hand.

Dos and Don’ts #4 … Cooling is the preferred method.

  • Do try the cooling method first. It is considered the safest way to rejoin the separated mercury, and can be performed using the following easy-to-find items:
- Solid Carbon Dioxide (co2), also known as dry ice
- Alcohol or Acetone
- Gloves and Safety Goggles
  • Don’t touch the bulb when the cooling method has been completed until it has warmed enough to allow the mercury to flow from the bulb into the capillary.

The purpose of this procedure is to rejoin the separations and ensure that the gas remains above the mercury. Holding the thermometer upright, slowly and gradually immerse only the bulb of the instrument into a solution of dry ice and alcohol contained in a small beaker or other such vertical container. During this gradual insertion, the mercury should begin to solidify. Be careful to not immerse the stem or column of the thermometer into the cooling solution or allow the mercury to freeze as that can cause the bulb to fracture. If necessary, moving the bulb in and out of the solution many times can slow down and better control the cooling action. When all the separated portions have been brought into the bulb, take the thermometer out of the container and swing it into a short arc to force the mercury into the bulb.

Keeping the bulb positioned upright in a thermometer storage rack or similar vessel as it returns to room temperature also keeps the nitrogen gas on top of the mercury. Except for deep immersion thermometers, this method can be used with most mercury thermometers to reunite a separated column. Storing thermometers in a vertical storage rack when not in use helps reduce the likelihood of mercury separating again.

Dos and Don’ts #5 … Using the Heating Method for rejoining.

  • Do use this method only for thermometers with 260°C (500°F) maximum range, and that have expansion chambers able to accommodate the separation and a portion of the main column. Thermometers with higher ranges than indicated above would be damaged by using heat to rejoin the instrument column.
  • Don’t ever heat the bulb with an open flame or fill the expansion chamber more than two-thirds full, or use the heat method to rejoin columns in thermometers with a range in excess of 260°C (500°F). Any of these could damage the thermometer or cause the bulb to burst.

In addition to protective gloves, goggles and other necessary safety gear, a large beaker is used when performing the heating method. The bulb and stem are immersed as far as possible into the beaker containing a liquid with flash point well above the highest indication of the thermometer. The beaker is heated and the liquid stirred using the inserted thermometer until the separation and part of the main column enter the expansion chamber. Once the fluid is in the chamber, remove the thermometer from the beaker and hold it upright vertically. Gently tap it into the palm of your gloved hand to reunite the column. Allow the thermometer to cool slowly. Again, keeping unused thermometers in a storage rack will help prevent future separation.

Note that an alternative to the beaker is to use a well-stirred, temperature-regulated liquid bath.

Dos and Don’ts #6 … Gas bubbles in thermometers with a contraction chamber.

It’s worth noting that mercury thermometers with a contraction chamber can be prone to separations, as well as bubbles of gas. Rough handling during shipping or frequent jarring of the instrument in the lab can cause gas bubbles in the bulb.

  • Do examine the tip of the bulb regularly for any gas bubbles.
  • Don’t tap the thermometer bulb on a hard surface. Doing so can cause the gas to break up into many smaller bubbles and worsen the situation.

To resolve the issue, use the cooling method described above to cool a portion of the bulb until a large bubble of gas enters it. Remove the thermometer from the cooling container, and hold it upright with the bulb side up, and begin to gently tap it against the palm of your hand. As gas is lighter than mercury, the gas bubble will rise up the length of the bulb. Invert and tilt the thermometer so it is now stem up, and roll it around to allow the gas bubble to come into contact with the bulb surface. The large bubble will pick up the other smaller gas particles as they come in contact. Continue rolling the thermometer in this manner until all smaller gas particles have been picked up. Once all the bubbles are absorbed, continue to follow the Cooling Method described above to get the gas positioned above the mercury in the thermometer. Be careful to let the bulb warm to the point that the mercury is no longer solid before touching it.

Finally, keep in mind that there are federal, state and local ordinances that control mercury instruments. Regulations are constantly changing and companies may not be allowed to ship mercury thermometers to some areas.


As with Liquid-in-Glass mercury thermometers, column separations can occur in non-mercury or spirit-filled thermometers. Rejoining separations in these instruments doesn’t pose as great a hazardous material risk; nonetheless, the process still requires experience and expertise. One simple and safe method is using a centrifuge to force the liquid down the capillary. The centrifuge cup should be deep enough so that centrifugal force is below the liquid column. If the centrifugal force is not below the column, the column will split and force some of the liquid down, but the rest will go up into the expansion chamber. Cotton padding should be placed in the bottom of the cup to prevent damage to the bulb. Place the thermometer bulb down in the cup and turn on the centrifuge to quickly force all the liquid past the separation – this should take just a few seconds.

If no centrifuge is available, another option is to hold the thermometer upright and gently tap the stem of the thermometer against the palm of your hand. As you are tapping, keep watching the liquid above the separation – it should break away from the capillary wall and run down to join the main column. Continue the tapping action and observation until all the liquid has rejoined the main column. If liquid did make its way into the expansion chamber, continue this process until all the liquid is reunited.

You can find additional information on these temperature devices at AASHTO re:source, through its Anatomy of a Liquid-in-Glass Thermometer or in NIST’s Special Publication 1088: Maintenance, Validation and Recalibration of Liquid-in-Glass Thermometers.