Training Guide in Surface Weather Observations - Multiple Choices

6.8 Correcting Thermometer Errors
Sometimes there may be breaks in the mercury or alcohol column, the thermometer may be too difficult
to reset, or it may reset itself between readings. The following instructions tell how to correct some of
these problems. If correction is impossible, replace the thermometer.
6.8.1 Correcting Maximum Thermometers
The constriction may not be small enough in some thermometers to prevent the mercury from
withdrawing into the bulb when the temperature falls after reaching its maximum value. Sometimes
rough handling will cause this problem. To test for this defect, place the thermometer in a vertical
position. If the mercury withdraws into the bulb without spinning the thermometer, the thermoneter
must be replaced.
If the constriction is too small, it may require many spins of the thermometer to get the mercury to return
to the bulb, especially at low temperatures.
Sometimes a small amount of mercury will lodge in the upper end of the bore. If so, hold the
thermometer vertically with the bulb upward. Tap the metal back of the thermometer gently with a
finger until the column joins the mercury at the bottom. Lower the bulb, allowing the column to slide
slowly down the bore to the constriction.
When the mercury cannot be united as above, remove the thermometer from the support and whirl it as
in Method II of section 6.8.2. If all fails, replace the thermometer.
6.8.2 Correcting Minimum Thermometers
Sometimes the alcohol column of a minimum thermometer separates into small parts, causing incorrect
readings. Separations may appear as small bubbles making the column too long and readings too high,
and trapping the index. Some alcohol may separate completely and remain in the upper portion of the
bore, resulting in readings too low. The thermometer should be inspected regularly for these problems.
The methods described below may have to be repeated several times, taking 15 to 20 minutes, before
the column can be joined. The thermometer should be kept in a vertical position for several hours after
parts of the column have been joined, in order that alcohol clinging to the sides of the bore will drain
down. When repeated attempts fail to join the alcohol column, replace the thermometer.
Method I: Tapping
Grasp the thermometer slightly below the middle
with the bulb end down. Strike the edge of the
metal back sharply against the palm of your hand
as shown in Figure 6-11. Repeat this procedure
several times. The thermometer must not be held
so that your fingers or any part of your hand
presses against the stem.
The bulb end may also be tapped on an open
book.
Figure 6-11. Joining the Alcohol Column - Tapping Method
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