Page 311 - Introduction to Marine Engineering

Instrumentation and control 285

Temperature
indicator
Hot Ceramic _ "
junction insulator
/
\^
— 1 A \
r 11 y/£,
th - : ••. . • » : y ' r\
W Cold
J " Compensating junction
Metal leads
sheath
Figure 15.8 Thermocouple

Radiation pyrometer
A pyrometer is generally considered to be a high-temperature
measuring thermometer. In the optical, or disappearing filament, type
shown in Figure 15.9, radiation from the heat source is directed into the
unit. The current through a heated filament lamp is adjusted until, when
viewed through the telescope, it seems to disappear. The radiation from
the lamp and from the heat source are therefore the same. The current
through the lamp is a measure of the temperature of the heat source,

Absorption Filament
screen lamp Monochromatic
filter

Light from
hot body

Too high or Correct
too low
Filament matching
Figure 15.9 Optical pyrometer

and the ammeter is calibrated in units of temperature. The absorption
screen is used to absorb some of the radiant energy from the heat source
and thus extend the measuring range of the instrument. The
monochromatic filter produces single-colour, usually red, light to
simplify filament radiation matching.