Methods
For Reuniting Separated Columns In Thermometers
The largest single cause for
the failure of precision thermometers in the lab is the separation of mercury
columns. This can occur in transit or in the lab. The life of the thermometer
can be greatly extended if the following procedures are employed. Other methods
may damage the thermometer.
Cooling Method
With the thermometer in an
upright position, gradually immerse only 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 and the separated portion retreat into the
bulb. Remove and swing thermometer in a 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 the bulb is immersed in the
CO2 and alcohol solution.
Caution: 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 the bulb to fracture.
Heating Method
This method applies to thermometers
with a maximum range of 260°C (500°F) equipped with expansion chambers
sufficiently large to accommodate the separations 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 enter the chamber. Tap the thermometer in the palm of gloved hand
reuniting the column. Allow to cool slowly.
Caution:
1.
Never use an open flame on the bulb.
2.
Never fill the expansion chamber more
than two-thirds full.
3.
Make certain the flash point of the
liquid is well above the highest temperature indicated on the thermometer.
4.
Thermometers whose ranges exceed 260°C
(500°F) cannot be reunited using heat without damaging the
instrument.
Reuniting
Organic-Filled Columns
Separated
columns in organic-filled (spirit) thermometers require a somewhat different technique
in order to be reunited. The simplest and safest method is to force the liquid
down the capillary by using a centrifuge, if one is available, with a cup deep
enough to ensure that the centrifugal force is below the liquid column.
Carefully insert the thermometer, bulb down, in the centrifuge. Have some
cotton wadding at the bottom of the cup to prevent any damage to the bulb. Turn
on the centrifuge and in just a few seconds all the liquid will be forced past
the separation. If the cup is not deep enough and all the centrifugal force is
not below the column, the column will split, forcing part of the liquid down.
The remainder will be forced up, filling the expansion chamber.
If a
centrifuge is not available, the column can be reunited by getting the liquid
to run down. This can be accomplished by holding the thermometer in an upright
position and gingerly tapping the stem above the separation against the palm of
your hand. As you gently tap the thermometer, observe the liquid above the
separation until it breaks away from the wall of the capillary and runs down to
join the main column.
