Measurement and Control Basics Fourth Edition

Chapter 7 - Temperature Measurement: Radiation Pyrometers

A pyrometer is any temperature-measuring device that includes a sensor
and a readout. However, in this section we will discuss only radiationtype
pyrometers. A radiation pyrometer is a noncontact temperature
sensor that infers the temperature of an object by detecting its naturally
emitted thermal radiation. An optical system collects the visible and infrared
energy from an object and focuses it on a detector, as shown in
Figure 7-21. The detector converts the collected energy into an electrical
signal to drive a temperature display or control unit.

Figure 7-21. Block diagram of radiation pyrometer

The detector receives the photon energy from the optical system and converts
it into an electrical signal. Two types of detectors are used: thermal
(thermopile) and photon (photomultiplier tubes). Photon detectors are
much faster than the thermopile type. This enables you to use the photon
type for measuring the temperature of small objects moving at high speed.

Radiation pyrometers are used to measure the temperature of very hot
objects without being in contact with them. Molten glass and molten metals
during smelting and forming operations are typical of the objects they
measure. In selecting the correct radiation pyrometer for an application
you must consider several factors. In either narrow or wide fields of view,
the cross-sectional area can vary greatly. It can be rectangular, circular,
and slot shaped, depending on the kind of apertures used in the instrument.
In some instruments, a telescopic eye magnifies the radiant energy
so much smaller objects at longer distances can be measured. Hot objects
as small as 1/16 inch in diameter can be measured with some instruments.

The construction of the instrument components, such as the lens and
curved mirrors, control the sight path. The materials of construction determine
the optical characteristics of the device. For example, glass does not
transmit light well beyond 2.5 microns. It is therefore suitable only for
high-temperature applications where high-energy outputs are present.
Other common optical materials are quartz (transmitting well to
4 microns) and crystalline calcium fluoride (transmitting well up to about
10 microns). Band pass filters are used in some instruments to cut off
unwanted light at certain wavelengths.

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