Radiation Pyrometers--These are similar to thermo-electric pyrometers in that a thermo-couple is employed. The heat rays given out by the hot body fall on a concave mirror and are brought to a focus at a point at which is placed the junction of a thermo-couple. The temperature readings are obtained from an indicator similar to that used with thermo-electric pyrometers.
Optical Pyrometers--Of the optical pyrometers the Wanner is perhaps the most reliable. The principle on which this instrument is constructed is that of comparing the quantity of light emanating from the heated body with a constant source of light, in this case a two-volt osmium lamp. The lamp is placed at one end of an optical tube, while at the other an eyepiece is provided and a scale. A battery of cells furnishes the current for the lamp. On looking through the pyrometer, a circle of red light appears, divided into distinct halves of different intensities. Adjustment may be made so that the two halves appear alike and a reading is then taken from the scale. The temperatures are obtained from a table of temperatures corresponding to scale readings. For standardizing the osmium lamp, an amylacetate lamp, is provided with a stand for holding the optical tube.
+-----------------+------------------+ | Metal | Temperature | | |Degrees Fahrenheit| +-----------------+------------------+ |Wrought Iron | 2737 | |Pig Iron (gray) | 2190-2327 | |Cast Iron (white)| 2075 | |Steel | 2460-2550 | |Steel (cast) | 2500 | |Copper | 1981 | |Zinc | 786 | |Antimony | 1166 | |Lead | 621 | |Bismuth | 498 | |Tin | 449 | |Platinum | 3191 | |Gold | 1946 | |Silver | 1762 | |Aluminum | 1216 | +-----------------+------------------+
Determination of Temperature from Character of Emitted Light--As a further means of determining approximately the temperature of a furnace, Table 9, compiled by Messrs. White & Taylor, may be of service. The color at a given temperature is approximately the same for all kinds of combustibles under similar conditions.
CHARACTER OF EMITTED LIGHT AND CORRESPONDING APPROXIMATE TEMPERATURE[9]
+--------------------------------------+-----------+ | Character of Emitted Light |Temperature| | | Degrees | | | Fahrenheit| +--------------------------------------+-----------+ |Dark red, blood red, low red | 1050 | |Dark cherry red | 1175 | |Cherry, full red | 1375 | |Light cherry, bright cherry, light red| 1550 | |Orange | 1650 | |Light orange | 1725 | |Yellow | 1825 | |Light yellow | 1975 | |White | 2200 | +--------------------------------------+-----------+