Page 83 - Instrumentation Reference Book 3E

68 Measurement of length

Magneto-resistive sensing elements are those in
which their electrical resistance varies with the
level of ambient magnetic field. These can be used
as linear, or as proximity, sensors by moving the
sensor relative to a field which is usually provided
by a permanent magnet.
Thickness of a layer being deposited in a
deposition chamber can be measured by several
means. One way is to monitor the mass build-up
on a test sample placed in the chamber alongside
that being coated. Another method is to directly
monitor the change in optical transmission dur-
ing deposition.
Statistical calculation on the signal strength of
an ionizing radiation source can be used to deter-
mine distance from the source.
Pressure formed, or liquid displaced, can be
radiation
Reflected
used to drive a pressure- or volume-sensitive
- / device in order to measure movement of the
sea floor
driving element. This method has been used in
the measurement of volumetric earth strain as
it can provide very sensitive detection due to
the cube-law relationship existing between
volume change input and length output.
The following chapter deals specifically with
Figure 3.1 9 Holographyapplied to seafloor mapping.
Two radiations are used to convert the image size for reasons strain measurements. Chapter 6, on vibration, and
of convenience. Chapter 5, on level measurement, each include
descriptions of length sensors.
Figure 3.19, uses an acoustic interference holo-
gram which is then viewed by optical radiation 3.6 Automatic gauging systems
for reasons of convenience.
The most serious disadvantages of holography Tool-room and factory gauging has its roots in
are the cost of the apparatus, slowness to produce the use of manually read measuring machines and
an output, and difficulties in obtaining numerical tools such as those shown in Figures 3.7 and 3.8.
measurements from the recorded information. These required, in their non-electronic forms,
Optical ways of measuring length are also dis- high levels of skill in their use and are very time-
cussed in Chapter 12. consuming. In general, however, they can yield
the best results given that the best machines are
3.5.5 Miscellaneous methods used.
The advent of electronic sensing of length and
The above descriptions have shown that even for angle has gradually introduced a transformation
a few restricted classes of sensor there are many in the measurement practices in the tool-room
principles that can be used to produce transduced and on the factory floor. The cost of providing,
length signals. A comprehensive coverage would using manual procedures, the very many inspec-
require several volumes on this parameter alone. tion measurements needed in many modern pro-
This short subsection is included to emphasize the duction processes has often proven to be
availability of many more methods that may be uneconomic and far too slow to suit automatic
appropriate in given circumstances. Many of the production plants. As an example, piston manu-
unusual methods are less likely to be marketed facture for a car engine can require over twenty
for potential sales would not justify quantity length parameters to be measured for each piston
manufacture. In applications of the aerospace produced on the final grinding machine. The time
industry, in original equipment needs of science available to make the measurements is of the
and in industrial testing, in development, and in order of fractions of minutes. It has, in such
isolated applications they may be the most viable instances, become cost-effective to design and
methods to adopt. It should not be construed that install automatic measuring machines that gener-
lack of commercial interest implies that a method ate the extensive data needed.
is necessarily unworkable. Here are a few of these Automatic measuring systems are character-
less commonly used methods. ized by their ability to deliver electronic signals