Page 317 - Instrumentation Reference Book 3E

Temperature measurement considerations 301
Flexible fiber
optic light

Figure 14.70 Radiation thermometer for gas turbine blades

obtained is displayed on a visual display unit.
This then builds up an image of the object as
“seen” by the infrared radiation from its surface.
As well as producing a “picture” of the object, the
temperature of the surface is indicated by the
color of the image, producing a temperature
map of the surface. Surface temperatures can be
so imaged to cover a wide range from sub-ambi-
ent to very high temperatures. The technique has
a very high resolution of temperature of the order
of a small fraction of a “C. Applications are to be
found in such diverse fields as medicine and geo-
logical survey from space.
The technique is dealt with in very much
greater detail in Part 3, Chapter 21.

14.7.3.5 Turbine blade temperatures
In the development and design of gas turbines there
is a requirement to measure the temperature and
temperature profile of the turbine rotor blades. This
presents some problems, ES the turbine may be
running at speeds of the order of 25,000 revolutions
per minute. The rotor may consist of, say, 50 blades
so that the time available to measure each blade
temperature profile as it passes a point will be about
a microsecond. A technique has been developed by
Land Infrared Ltd. to carry out this measurement
using fiber optic radiation thermometers. In this
arrangement a small optical probe is inserted
through the turbine wall and focused onto the rotor
Time blades. The probe is connected by a fiber optic
cable to a detector head amplifier unit nearby.
(b9 Figure 14.70 shows a schematic diagram of focus-
Figure 14.71 Measurement of the temperature profile of ing a measurement head. By designing the probe so
gas turbine blades: (a) geometry of focusing of that it focuses on a very small target area it is pos-
thermometer, (b) temperature profile as “seen” by radiation
thermometer and electrical output. sible to “read” a turbine blade temperature profile