Page 179 - Mechanical Engineers' Handbook (Volume 2)
P. 179

168   Temperature and Flow Transducers

                          Table 9 Resistance and Relative Resistance of Typical Probes
                                                            Probe 1                  Probe 2
                                           1/T                    Relative                 Relative
                          Temperature ( C)  (K)      Resistance  Resistance   Resistance  Resistance
                               80         .00518    1660000       225.6968   2211000       225.7044
                               70         .00493     702300        95.4861    935600        95.5084
                               60         .00469     316500        43.0320    421500        43.0278
                               50         .00448     151050        20.5370    201100        20.5288
                               40         .00429      75790        10.3046    101000        10.3103
                               30         .00412      39860         5.4194     53100         5.4206
                               20         .00395      21870         2.9735     29130         2.9737
                               10         .00380      12460         1.6941     16600         1.6946
                                0         .00366       7355         1.0000      9796         1.0000
                               10         .00353       4482          .6094      5971          .6095
                               20         .00341       2814          .3826      3748          .3826
                               30         .00330       1815          .2468      2417          .2467
                               40         .00319       1200          .1632      1598          .1631
                               50         .00310       811.30        .1103      1081          .1104
                               60         .00300       560.30        .0762       746.30       .0762
                               70         .00292       394.50        .0536       525.40       .0536
                               80         .00283       282.70        .0384       376.90       .0385
                               90         .00275       206.10        .0280       274.90       .0281
                               100        .00268       152.80        .0208       203.80       .0208
                               110        .00261       115           .0156       153.20       .0156
                               120        .00254        87.70        .0119       116.80       .0119
                               130        .00248        67.80        .0092        90.20       .0092
                               140        .00242        53           .0072        70.40       .0072
                               150        .00236        41.90        .0057        55.60       .0057



                          reading depending on the temperature level. The accuracy is better at high temperatures.
                          These devices are vulnerable to errors from several sources: calibration, size of source,
                          emissivity uncertainty, background radiation (especially in the lower temperature ranges),
                          and gas path absorption of the infrared by water vapor or CO .
                                                                          2
                             One way to avoid gas path absorption and source emissivity uncertainty is to use a
                          blackbody cavity source and transmit the energy via a closed optical path. This is the prin-
                          ciple behind the fiber-optic blackbody-sensing system described by Dils. 49  A fiber-optic
                          blackbody radiation sensor measures the intensity of the radiation emitted from a blackbody
                          cavity at the end of an optical fiber and deduces temperature from the intensity, using well-
                          established laws of radiant emission.
                             The system has four main elements: the cavity, the high-temperature fiber, the low-
                          temperature fiber, and the processor. Two of these elements are exposed to the high-
                          temperature environment. A typical high-temperature fiber is a single-crystal sapphire fiber
                          about 1.25 mm in diameter and usually between 100 and 250 mm long. The cavity can be
                          formed by sputtering a thin metallic film onto the end of the sapphire fiber, typically about
                          5  m. The metal layer can be covered by a film of alumina (also about 5  m) to protect it
                          from oxidation or erosion.
                             The general arrangement and the principal features of the data interpretation are shown
                          in Fig. 25.
                             The cavity radiates as a ‘‘dark-gray body’’ (almost a blackbody) and the high-
                          temperature fiber conducts the radiant energy from the cavity to the low-temperature fiber,
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