Utulity & FAQ


Joining of Seprated Mercury

Mercury Separation? Relax! This is not a Defect!

Please understand - A separation of mercury in your Thermometer is not a defect! It is a condition, normally caused by shock in transit, which of course must be rectified before using the thermometer, or you will experience significant errors in your reading.

Please resist the impulse to put the thermometer into dry ice or heat it! (Yet) More often than not you will make the separation more difficult to rejoin, you may also damage the thermometer. Please read these instructions before attempting to rejoin the separations!

Most thermometers are filled above the mercury column with pressurized Nitrogen Gas. The Nitrogen serves many purposes; it is an inert gas, which minimizes the possibility of oxidation occurring in side the thermometer, the pressure is what makes the column retreat when the thermometer is removed from heat. The Nitrogen is of course invisible. When we have mercury separation in the capillary of the thermometer, the spaces between the pieces of mercury are actually the quantities of gas. In most cases it is virtually impossible to ‘tap‘ the column back together. We cannot force the mercury through the gas in such a confined space, so don’t bother trying; you may well break the thermometer. To be able to ’tap’ the mercury back together, we must move separations in to a larger chamber.

The trick to remember is that while the mercury separations are in capillary, they cannot be joined, when we move them into a larger space, they can be easily manipulated.

Firstly we have to determine the type of separation we have:

  1. Separations in the thermometer with a contraction chamber: If your thermometer has range that starts significantly above the room temperature, for example Range 98 to 152 ºC, the thermometer is constructed with an enlargement in the capillary between the bulb and the main scale. This enlargement or contraction chamber is where the mercury normally resides in the room temperature. This type of thermometer is extremely prone to mercury separations, especially during transit. Fortunately the separations are usually very easy to rejoin.

Firstly, determine how the separations appear. If all the mercury appears to be within the chamber, the thermometer may be tapped gently (vertically) on to a padded surface or on the palm of your hand until

the separated portion falls and rejoins with the mercury in the lower portion of the chamber. If the separated mercury is lodged in the upper portion of the chamber, and or is located in the column above the chamber, it will be necessary to bring the separation down in to the chamber so that it may be tapped as described above. Cool the thermometer bulb a little at a time (dip it into a mixture of Ice and Water) until the mercury retreats into the chamber. While it is lying in the chamber, tap as described above to rejoin the separation.

If the separation is located in the lower portion of the chamber, or in the capillary below the chamber, we must do the Reverse of the above. Warm the bulb a bit at a time until the separation is located in the chamber. When the separation is in the chamber tap as described above to bring the separated mercury down (actually we are forcing the gas up).

Sometimes, with severely separated mercury in thermometers it becomes necessary to rejoin the separations in stages. Just remember that it is necessary to move the separation into the chamber to be able to rejoin it

  1. Separation in the thermometer without a contraction chamber (separation in the column): This type of separation is less common, and a little trickier to rejoin. There are basically two methods.
  2. Cooling Method (Preferred) - Obtain a small quantity of Dry Ice, immerse the thermometer bulb only (take care not to immerse the entire bulb or any portion of the stem) halfway into the Dry Ice and observe the descending mercury column carefully. The main column will disappear into the bulb, followed by the separated pieces of mercury. Wait for a few more seconds and then withdraw the thermometer from the Dry Ice gently and carefully, then tap it onto a padded surface. The tapping will permit the separated pieces of mercury to fall and rejoin the main mass of mercury now within the bulb. Allow the thermometer to warm naturally (do not heat it) in a vertical position, and observe the mercury column as it ascends into the capillary to be certain it is intact.


  1. Heating Method, Caution: Do not attempt this method with the thermometers whose Range extends beyond 200 ºC or damage may result. Most thermometers have a small chamber at the top of the capillary near the ring, called safety chamber. The purpose of this safety chamber is to provide over range protection in case the thermometer is heated beyond its scale range. This chamber may be used to rejoin separations provided,
    1. The amount of mercury is very small (not more than a few scale divisions in length) and,
    2. The thermometers range dose not exceeds 200 ºC.

The thermometer may be heated in hot water or hot oil with the temperatures to be attained so that the separations enter the safety chamber followed by a small portion of main (intact) column. Great care must be exercised not to fill the safety chamber more than halfway, or breakage of the bulb and spillage of the mercury may occur. The separations will normally fall to the side of the safety chamber, and the main column will come into contact with them. Remove the thermometer from the heat, maintain it in a vertical position, and observe the mercury column as it retreats to be sure it is intact and rejoined.

  1. Separations in the expansion chamber: Some thermometers are designed for very low range example – A. S. T. M. 62 C that has a range of –38 to +2 ºC, the mercury at room temperature resides in an expansion chamber, which is located at the top of the thermometer but slightly below the safety chamber. Again often in transit this mercury can become separated. Normally the separation is visually apparent and can be rejoined quickly and easily by simply tapping. After rejoining we must carefully see that there must not be any gas bubble in the mercury lying in the expansion chamber. If it is there, it must be removed by tapping. If we see the separation in the column the thermometer must be heated (warm water) until the separation (gas) enters the expansion chamber where it can be joined by tapping.


Again the trick to remember is that while the mercury separations are in the capillary they cannot be easily rejoined. When we move them into a larger space they can be easily manipulated. After rejoining mercury separations it is highly recommended that thermometer be verified in a known temperature prior to being placed into use. If the thermometer reads correctly at this known temperature it may be safely assumed that the separation has been correctly rejoined.

If the thermometer’s indication at a known temperature is high, there is gas (a separation) either in the bulb / contraction or in the column which is displacing mercury and causing a false reading. We have to go back, find the separation and remove it.

If the thermometer’s indication at a known temperature is low, it may be assumed that there is a mercury separation somewhere above the column, hence, we have to look in the upper reaches of the column or in the expansion chamber.

If all your efforts fail, call JRM and we will be pleased to help you out and take care of the problem. If required, the thermometer will be replaced free of charge.