Choice of Instruments
Choosing the different types of
instruments
The starting point in choosing the most
suitable instrument to use for measurement of a particular quantity in
a manufacturing plant or other system is the specification of the
instrument characteristics required, especially parameters such as the
desired measurement accuracy, resolution and sensitivity. It is also
essential to know the environmental conditions that the instrument
will be subjected to, as certain conditions will immediately eliminate
the possibility of using some types of instrument. Provision of this
type of information usually requires the expert knowledge of personnel
who are intimately acquainted with the operation of the manufacturing
plant or system in question. Then, a skilled instrument engineer,
having knowledge of all the instruments that are available for
measuring the quantity in question, will be able to evaluate the
possible list of instruments in terms of their accuracy, cost and
suitability for the environmental conditions and thus to choose the
most appropriate instrument. As far as possible, measurement systems
and instruments should be chosen which are as insensitive as possible
to the operating environment, although this requirement is often
difficult to meet because of cost and other performance
considerations.
Published literature is of considerable help in the choice of a
suitable instrument for a particular measurement situation. Many books
are available which give valuable assistance in the necessary
evaluation by providing lists and data about all the instruments
available for measuring a range of physical quantities. However, new
techniques and instruments are being developed all the time, and
therefore a good instrumentation engineer must keep abreast of the
latest developments by reading the appropriate technical journals
regularly.
Generally, the better the characteristics, the higher the cost.
However, in comparing the cost and relative suitability of different
instruments for a particular measurement situation, considerations of
durability, maintainability and constancy of performance are also very
important because the instrument chosen will often have to be capable
of operating for long periods without performance degradation and a
requirement for costly maintenance. As a consequence, the initial cost
of an instrument often has a low weighting in the evaluation exercise.
Cost is very strongly correlated with the performance of an
instrument, as measured by its static characteristics. Increasing the
accuracy or resolution of an instrument, for example, can only be done
at a penalty of increasing its manufacturing cost. Instrument choice
therefore proceeds by specifying the minimum characteristics required
by a measurement situation and then searching manufacturers'
catalogues to find an instrument whose characteristics match those
required. To select an instrument with characteristics superior to
those required would only mean paying more than necessary for a level
of performance greater than that needed.
As well as purchase cost, other important factors in the assessment
exercise are instrument durability and the maintenance requirements.
Assuming that one had $200,000 to spend, one would not spend $10,000
on a new motor car whose projected life was 5 years if a car of
equivalent specification with a projected life of 10 years was
available for 200 000. Likewise, durability is an important
consideration in the choice of instruments. The projected life of an
instrument often depends on the conditions in which the instrument
will have to operate. Maintenance requirements must also be taken into
account, as they also have cost implications.
As a general rule, a good assessment criterion is obtained if the
total purchase cost and estimated maintenance costs of an instrument
over its life are divided by the period of its expected life. The
figure obtained is thus a cost per year. However, this rule is
modified where instruments are being installed on a process whose life
is expected to be limited, perhaps in the manufacture of a particular
model of car. Then, the total costs can only be divided by the period
of time an instrument is expected to be used, unless an alternative
use for the instrument is envisaged at the end of this period.
To summarize, instrument choice is therefore a compromise between
performance characteristics, ruggedness and durability, maintenance
requirements and purchase cost. To carry out such an evaluation
properly, the instrument engineer must have a wide knowledge of the
range of instruments available for measuring particular physical
quantities, and he/she must also have a deep understanding of how
instrument characteristics are affected by particular measurement
situations and operating conditions.
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