Page 15 - Mechanical Engineers' Handbook (Volume 2)

4 Instrument Statics

is disturbed to some extent, making a perfect measurement unrealistic. Therefore, pressure
cannot be measured without an accompanying change in volume, force cannot be measured
without an accompanying change in length, and voltage cannot be measured without an
accompanying flow of charge. Instead measures must be taken to minimize the energy trans-
fer from the source to be measured if the measurement is to be accurate.
There are several categorical characteristics for a transducer (or measurement system).
When the measurand maintains a steady value or varies very slowly with time, transducer
performance can be described in terms of static characteristics. Instruments associated with
rapidly varying measurands require additional qualifications termed dynamic characteristics.
Other performance descriptors include environmental characteristics (for situations involving
varying environmental operating conditions), reliability characteristics (related to the life
expectancy of the instrument under various operating conditions), theoretical characteristics
(describing the ideal behavior of the instrument in terms of mathematical or graphical rela-
tionships), and noise characteristics (external factors that can contribute to the measurement
process such as electromagnetic surroundings, humidity, acoustic and thermal vibrations,
etc.). In this chapter, we will describe the considerations associated with evaluating numerical
values for the static characteristics of an instrument.

1.2 Definitions
The description of a transducer and its role in a measuring system is based on most of the
definitions that follow. Further details of these definitions can be found in other works. 2–4

Static calibration is the process of measuring the static characteristics of an instrument.
This involves applying a range of known values of static input to the instrument and
recording the corresponding outputs. The data obtained are presented in a tabular or
graphical form.
Range is defined by the upper and lower limits of the measured values that an instrument
can measure. Instruments are designed to provide predictable performance and, often,
enhanced linearity over the range specified.
Sensitivity is defined as the change in the output signal relative to the change in the
input signal at an operating point. Sensitivity may be constant over the range of the
input signal to a transducer or it can vary. Instruments that have a constant sensitivity
are called ‘‘linear.’’
Resolution is defined as the smallest change in the input signal that will yield a readable
change in the output of the measuring system at its operating point.
Threshold of an instrument is the minimum input for which there will be an output.
Below this minimum input the instrument will read zero.
Zero of an instrument refers to a selected datum. The output of an instrument is adjusted
to read zero at a predefined point in the measured range. For example, the output of
a Celsius thermometer is zero at the freezing point of water; the output of a pressure
gage may be zero at atmospheric pressure.
Zero drift is the change in output from its set zero value over a specified period of time.
Zero drift occurs due to changes in ambient conditions, changes in electrical condi-
tions, aging of components, or mechanical damage. The error introduced may be
significant when a transducer is used for long-term measurement.
Creep is a change in output occurring over a specific time period while the measurand
is held constant at a value other than zero and all environmental conditions are held
constant.