Air Bubbles in Liquid Columns

Sometimes, because of shipping, general use, or sloppy handling, the liquid column of a thermometer will separate, leaving trapped air bubbles. Because of the capillary size, it is difficult for the liquid to pass by the air space and rejoin. Sometimes such separations are glaringly obvious. Other times, the amount of liquid separation is small and is difficult to see. 

One technique for monitoring the quality of a thermometer is to use a second thermometer for all measurements. Any disagreement between the temperatures from the two thermometers may be caused by a problem as air breaks in the liquid 

There are two techniques for rejoining separated liquid ends heating and cooling. The option chosen will depend on the location of the break, as well as on the existence and location of contraction or expansion chambers. 

Generally it is better (and safer) to cool the liquid in thermometer than it is to heat it. First, try to cool the thermometer with a (table) salt-and-ice slush bath. This method should bring the liquid into the contraction chamber or bulb. Once the liquid is in the chamber or bulb, it should rejoin, leaving the air bubble on top. If there is not a clean separation of the air bubble, it may be necessary to softly tap the end of the thermometer. This tapping should be done on a soft surface such as a rubber mat, stopper, or even a pad of paper. Alternatively, you may try swinging the thermometer in an arc (such as a nurse does before placing it in your mouth). Once joined, the liquid in the thermometer can slowly be reheated. 

An alternate technique to rejoin broken liquid columns is to expand the liquid into the contraction or expansion chamber by heat. Be careful to avoid filling the expansion chamber more than two-thirds full, as extra pressure may cause the top of the thermometer to burst. Never use an open flame to intentionally heat any part of a thermometer as the temperature from such a source is too great and generally uncontrollable. 

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