Page 579 - Instrumentation Reference Book 3E

Mechanical measurements 561

radiation falls to a finite area; and if the thickness is
constant and known the reading will be a measure
of the density.
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23.3.3.2 Measurement of coating thickness ...........
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In industry a wide variety of processes occur
where it is necessary to measure and sometimes
automatically control the thickness of a coating
appkd to a base material produced in strip form.
Examples of such processes are the deposition of
tin, zinc: or lacquers on steel, or adhesives, wax, Figure 23.15 Differential beta-transmission gauge.
clay bitumen, or plastics to paper, and many Courtesy Nuclear Enterprises Ltd. SI :first source; D1 : first
other processes. detector; S2: second source; Dz: second detector; B: base
By nucleonic methods measurement to an material; C: coating; M: differential measurement indicator.
accuracy of 2~1 percent of coating thickness can
be made in a wide variety of circumstances by coating is applied, followed by the total thickness
rugged equipment capable of a high reliability. of the material with its coating, and then subtract-
Nucleonic coating-thickness gauges are based on ing the former from the latter. The difference
the interaction of the radiation emitted from a provides the coating thickness. The readings are
radioisotope source with the material to be meas- obtained by passing the uncoated material
ured. They consist basically of the radioisotope through one measuring head and the coated
source in a radiation shield and a radiation detec- material through the other, the coating being
tor contained in a measuring head, and an electric applied between the two positions. The intensity
console. of radiation transmitted through the material is a
When the radiation emitted from the source is measure of total thickness. Separate meters
incidlent on the subject material, part of this record the measurement determined by each
radiation is scattered, part is absorbed, and the head, and a third meter displays the difference
rest lmsses through the material. A part of the between the two readings, which corresponds to
absorbed radiation excites characteristic fluores- the coating thickness.
cent X-rays in the coating andlor backing. Typical applications of this gauge are the meas-
Depending on the measurement required, a sys- urements of wax and plastics coatings applied to
tem is used in which the detector measures the paper and aluminum sheet or foil, or abrasives to
intensity of scattered, transmitted, or fluorescent paper or cloth.
radiation. The intensity of radiation monitored by
the detector is the measure of the thickness (mass per Betu-backscatter gauge (Figure 23.16) The beta-
unit area) of the coating. The electric console con-
tains units which prolcess the detector signal and backscatter gauge is used to measure coating
indicate total coating thickness andlor deviation thickness when the process is such that the mater-
ial is only accessible from one side and when the
from the target thickness. The measuring head may coating and backing material are of substantially
be stationary or programed to scan across the mate- different atomic number. The radioisotope
rial. Depending on the type and thickness of coating source and the detector are housed in the same
and base materials, and machine details, one of four
gauge types is selected: differential beta transmis- enclosure. Where radiation is directed, for exam-
sion, beta backscatter, X-ray fluorescence, and pre- ple, on to an uncoated calender roll it will be
ferential absorption.

Differential beta-transmission gauge (Figure
23.15) The differential beta-transmission gauge
is used to measure coating applied to base mater-
ials in sheet form when the coating has a total Source
weight of not less than about one-tenth of the
weiglht of the base material. when both sides of
the base and coated material are accessible, and
when the composition of coating and base is
fairly similar. Here the thickness (mass per unit
area) of the coating is monitored by measuring Figure 23.1 6 Beta-backscatter gauge. Courtesy
first the thickness of the base material before the Nuclear Enterprises Ltd.