Instrument Classification and Characteristics
About the instrument classification and
characteristics
Instruments can be subdivided into separate
classes according to several criteria. These sub classifications are
useful in broadly establishing several attributes of particular
instruments such as accuracy, cost and general applicability to different
applications.
Instruments are either active or passive
according to whether the instrument output is entirely produced by the
quantity being measured or whether the quantity being measured simply
modulates the magnitude of some external power source. This can be
illustrated by examples.
An example of a passive instrument is the pressure measuring device. The
pressure of the fluid is translated into movement of a pointer against a
scale. The energy expended in moving the pointer is derived entirely from
the change in pressure measured: there are no other energy inputs to the
system.
An example of an active instrument is a float-type petrol-tank level
indicator. Here, the change in petrol level moves a potentiometer arm, and
the output signal consists of a proportion of the external voltage source
applied across the two ends of the potentiometer. The energy in the output
signal comes from the external power source; the primary transducer float
system is merely modulating the value of the voltage from this external
power source.
In active instruments, the external power source is usually electrical in
form, but in some cases it can be in other forms of energy such as
pneumatic or hydraulic.
One very important difference between active and passive instruments is
the level of measurement resolution which can be obtained. With the simple
pressure gauge, the amount of movement made by the pointer for a
particular pressure change is closely defined by the nature of the
instrument. Whilst it is possible to increase the measurement resolution
by making the pointer longer, so that the pointer tip moves through a
longer arc, the scope for such improvement is clearly restricted by the
practical limit of how long the pointer can conveniently be. In an active
instrument, however, adjustment of the magnitude of the external energy
input allows much greater control over measurement resolution. Whilst the
scope for improving this resolution is much greater incidentally, it is
not infinite because of the limitations placed on the magnitude of the
external energy input, in considering heating effects and for safety
reasons.
In terms of cost, passive instruments are normally of a simpler
construction than active ones and are therefore cheaper to manufacture.
The choice between active and passive instruments for a particular
application therefore involves carefully balancing the measurement
resolution requirements against cost.
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Null or Deflection Type Instruments
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Monitoring and Control Instruments
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Analog and Digital Instruments
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Static Characteristics of Measuring Instruments
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Accuracy and Precision in Measuring Instruments
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Sensitivity of Measurement to Disturbance
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Instruments Calibration
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Choice of
Instruments

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