|
| F = 9 ⁄ 5 C+32° = 9 ⁄ 4 R+32° | C = 5 ⁄ 9 (F-32°) = 5 ⁄ 4 R | R = 4 ⁄ 9 (F-32°) = 4 ⁄ 5 C |
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 | Temperature | Correction |
|---|---|---|---|
| 40°F | 0.0 | 300°F | +2.5 |
| 100°F | 0.0 | 400°F | -0.5 |
| 200°F | 0.0 | 500°F | -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:
Stem correction = 0.000085 × n (T- t )
[Pg 81] in which T is the observed temperature, t is the mean temperature of the emergent column, n is the number of degrees of mercury column emergent, and 0.000085 is the difference between the coefficient of expansion of the mercury and that in the glass in the stem.
Suppose the observed temperature is 400 degrees and the thermometer is immersed to the 200 degrees mark, so that 200 degrees of the mercury column project into the air. The mean temperature of the emergent column may be found by tying another thermometer on the stem with the bulb at the middle of the emergent mercury column as in Fig. 12. Suppose this mean temperature is 85 degrees, then
