Page 181 - Mechanical Engineers' Handbook (Volume 2)

170 Temperature and Flow Transducers

radiation constant: 1.4387 10 , m K
4
C 2
C (
, T ) calibration constant for photodiode
d
i
F(
) narrow-bandpass filter function
BS(
, T ) beamsplitter function
i
LTF(T ) low-temperature fiber transmission function
i
The dependence on T reflects changes in the optical properties of the mirrors and prisms
i
used in steering the beams to their respective sensors.
The temperature range is 600–2000 C. Accuracy begins to fail at lower temperatures,
while the sapphire fiber cannot tolerate higher temperatures.
The calibration precision is limited only by the precision with which the electrical
signals can be read, assuming the equation governing the radiant emission to be absolutely
accurate. The measurement accuracy depends on how well the sensor is brought into equi-
librium with the specimen temperature.
The fiber-optic probe is sensitive to all the usual installation errors, which could be
calculated from its physical and thermal properties. But it introduces one new concern:
radiation loss along the transparent fiber. This is not an error mode that has been recorded
in the literature, but it amounts to only a small fraction of the sensor’s total radiation error
under usual conditions. In addition, there is a small (3–10-K) error introduced because of
radiation lost through the sides of the hot fiber near the cavity, the Brewster loss. These new
error modes are not viewed as deterrents to the use of the probe as a reference system for
high temperatures; rather they are part of the correction which must be applied at final
calibration.
The sensing capsule is 1.25 mm in diameter and can be from 1 to 15 mm long. The
optimum length is calculated on the basis of minimizing the radiation error of the cavity.
Standard thin-film units will respond to 10-kHz fluctuations in temperature. Response
is flatter than that of a thermocouple, rolling off at only 3 dB per octave instead of 6.
Wickersham 50 developed a temperature-measuring system based upon the temperature
sensitivity of the response of certain phosphors: fluor-optic temperature sensing. He found a
family of phosphors whose fluorescent emission characteristics were strongly temperature
dependent. Pulsing these phosphors with ultraviolet light resulted in an emission spectrum
with two properties that depended on temperature: the ratios of amplitudes of certain bands
and the decay times of the principal energy-containing bands. Two classes of instruments
were then developed: ones that used the ratios of wavelengths and others that used the decay
time. Experience favored the decay time instrument, and those are now the dominant type
in the marketplace. Phosphors exist whose emissions are useful at temperatures of only a
few kelvins, while others emit best in the range of 2000 C. Fluor-optic temperature-
measuring systems are under development for high-temperature measurements on aircraft
engine components and also for low-temperature measurements in systems with high mag-
51
netic fields or electric fields. Considerable development has proceeded on phosphors. Since
the sensor is an optical fiber, the system is not vulnerable to electromagnetic interference
(EMI) and does not present a shock hazard to the operator. 52 Recent developments have
focused on the illumination scheme, searching for more efficient coupling of the source and
the phosphor by using blue light-emitting diodes (LEDs) as the illuminant source. 53
Thermo-chromic liquid crystals have been used for mapping temperature fields in low-
temperature systems (room temperature up to 100 C). These coatings have a reversible re-
action to temperature change. Colorless below their lowest transition temperature, they
display red when heated to the bottom of their range, with their color moving through the
spectrum to blue at their highest active temperature. The full range of colors can be com-
pressed into a temperature band of 1 C (narrow-band material) or stretched out over 10 C