Thermometers are one of the most important instruments used in the laboratory. They are a relatively inexpensive means of reliably measuring temperature, provided they are properly used and not abused. It is essential that the instruments be used in the manner prescribed if true temperatures are to be ascertained. The following are some terms that should be understood and adhered to when true temperatures are to be obtained.
Total Immersion: Thermometer is immersed into medium up to the point of reading.
Partial Immersion: Thermometer is immersed up to immersion line or to depth inscribed on the back of the thermometer.
IMPORTANT:
Thermometers are not always used to indicate true temperatures, particularly where they are used in methods specified by many technical societies such as ASTM. It is most important to position the thermometer exactly as outlined in the method. Any deviation can cause erroneous indications.
The largest single cause for the failure of precision thermometers in the lab is due to separated mercury columns. This can occur in transit or in the lab.
The life of the instrument can be greatly extended if the following procedures are rigidly employed. Other methods may cause damage to the thermometer.
A. 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 as well as the separated portion retreats 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-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.
B. Heating Method:
This method applies to thermometers with a maximum range of 260°C or 500°F, equipped with expansion chambers sufficiently large to accommodate the separation plus a portion of the main column. Immerse as much of
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:
- Never use an open flame on the bulb.
- Never fill the expansion chamber more than 2/3 full.
- Make certain the flash point of the liquid is well above highest temperature indicated on the thermometer.
- Thermometers whose ranges exceed 260°C or 500°F CAN NOT be reunited using heat without damaging the instrument.
