As a general rule thermometers are graduated to read correctly for total
immersion, that is, with bulb and stem of the thermometer at the same
temperature, and they should be used in this way when compared with a
standard thermometer. If the stem emerges into space either hotter or
colder than that in which the bulb is placed, a "stem correction" must
be applied to the observed temperature in addition to any correction
that may be found in the comparison with the standard. For instance, for
a particular thermometer, comparison with the standard with both fully
immersed made necessary the following corrections:
_Temperature_ _Correction_
40°F 0.0
100 0.0
200 0.0
300 +2.5
400 -0.5
500 -2.5
When the sign of the correction is positive (+) it must be added to the
observed reading, and when the sign is a negative (-) the correction
must be subtracted. The formula for the stem correction is as follows: