High Temperature Measurements--The temperatures to be dealt with in steam-boiler practice range from those of ordinary air and steam to the temperatures of burning fuel. The gases of combustion, originally at the temperature of the furnace, cool as they pass through each successive bank of tubes in the boiler, to nearly the temperature of the steam, resulting in a wide range of temperatures through which definite measurements are sometimes required.
Of the different methods devised for ascertaining these temperatures, some of the most important are as follows:
1st. Mercurial pyrometers for temperatures up to 1000 degrees Fahrenheit.
2nd. Expansion pyrometers for temperatures up to 1500 degrees Fahrenheit.
3rd. Calorimetry for temperatures up to 2000 degrees Fahrenheit.
4th. Thermo-electric pyrometers for temperatures up to 2900 degrees Fahrenheit.
5th. Melting points of metal which flow at various temperatures up to the melting point of platinum 3227 degrees Fahrenheit.
6th. Radiation pyrometers for temperatures up to 3600 degrees Fahrenheit.
7th. Optical pyrometers capable of measuring temperatures up to 12,600 degrees Fahrenheit.[6] For ordinary boiler practice however, their range is 1600 to 3600 degrees Fahrenheit.
[Illustration: 228 Horse-power Babcock & Wilcox Boiler, Installed at the Wentworth Institute, Boston, Mass.]
Table 7 gives the degree of accuracy of high temperature measurements.
ACCURACY OF HIGH TEMPERATURE MEASUREMENTS[7]
+------------------------+------------------------+ | Centigrade | Fahrenheit | +-------------+----------+-------------+----------+ | | Accuracy | | Accuracy | | Temperature | Plus or | Temperature | Plus or | | Range | Minus | Range | Minus | | | Degrees | | Degrees | +-------------+----------+-------------+----------+ | 200- 500 | 0.5 | 392- 932 | 0.9 | | 500- 800 | 2 | 932-1472 | 3.6 | | 800-1100 | 3 | 1472-2012 | 5.4 | | 1100-1600 | 15 | 2012-2912 | 27 | | 1600-2000 | 25 | 2912-3632 | 45 | +-------------+----------+-------------+----------+
Mercurial Pyrometers--At atmospheric pressure mercury boils at 676 degrees Fahrenheit and even at lower temperatures the mercury in thermometers will be distilled and will collect in the upper part of the stem. Therefore, for temperatures much above 400 degrees Fahrenheit, some inert gas, such as nitrogen or carbon dioxide, must be forced under pressure into the upper part of the thermometer stem. The pressure at 600 degrees Fahrenheit is about 15 pounds, or slightly above that of the atmosphere, at 850 degrees about 70 pounds, and at 1000 degrees about 300 pounds.