Example:—Temperature observed 72°.5
Barometer reading observed, 29.415 inches,
from which C = 0.1165, and this number, according to the conditions of the formula, is to be subtracted from the observed reading. The true reading in the case given is, therefore,
| 29.298 inches or | 744.2 | millimeters. |
| The observed reading | 747.1 | “ |
| And the correction | 2.9 | “ |
Unless extremely accurate work be required the correction for temperature is of very little importance in nitrogen determinations in fertilizers. Each instrument sent out by the Weather Bureau is accompanied by a special card of corrections therefor, but these are of small importance in fertilizer work. In order then to get the correct weight of the gas from its volume the reading of the thermometer and barometer at the time of measurement must be carefully noted. However, after the end of the combustion, the azotometer should be carried into another room which has not been affected by the combustion and allowed to stand until it has reached the room temperature.
Every true gas changes its volume under varying temperatures at the same rate and this rate is the coefficient of gaseous expansion. For one degree of temperature it amounts to 0.003665 of its volume. Representing the coefficient of expansion by K the volume of the gas as read by V, the volume desired at any temperature by V′, the temperature at which the volume is read by t and the desired temperature by t′, the change in volume may be calculated by the following formula:
V′ = V[1 + K(t′ - t)].
Example.—Let the volume of nitrogen obtained by combustion be thirty-five cubic centimeters, and the temperature of observation 22°. What would be the volume of the gas at 0°?
Making the proper substitutions in the formula the equation is reduced to the form below:
V′ = 35[1 + 0.003665(0°-22°)]
or V′ = 35(1 - 0.08063) = 32.18.