Measurement Technology - The Electronic Nose Artificial Olfaction Technology - page 9

Indirect method:
Measurement by direct method is not always possible, feasible, and practicable.
Here the meaning of the measurement and the purpose of the processing operation
are not identical but are related to each other. This method is used where direct
method is not applicable, and the measuring parameter has got some correction
with the desired parameter that could not be measured directly. These methods, in
most of the cases, are inaccurate because they involve human factors. They are
also less sensitive. In simple words ''it's obtained by measuring other quantities''.
In engineering applications measurement systems are used. These measurement
systems use indirect methods for measurement purposes.
1.6 Instrument and Measurement Systems
Measurement involves the use of instruments as a physical means of determining
quantities or variables. The instrument serves as an extension of human ability and
enables the man to determine the value of unknown quantity or variable which his
unaided human abilities cannot measure, a measuring instrument exists to provide
information about the physical values of some variable being measured. Because
of modular nature of the elements within it, it is common to refer the measuring
instrument as a measurement system.
1.7 Classification of Instruments
• Deflection Type and Null Type:
A deflection type instrument is that in which the physical effect generated by the
measuring quantity produces an equivalent opposing effect in some part of the
instrument which in turn is closely related to some variable like mechanical
displacement or deflection in the instrument.
For example, the unknown weight of an object can be easily acquired by the
deflection of a spring sourced by it on the spring balance as shown in Fig. 1.3 .
Similarly, in a common Bourdon gage, the pressure to be measured acts on the
C-type spring of the gage, which deflects and produces an internal spring force
to counter balance the force generated by the applied pressure. Deflection
instruments are simple in construction and operation. In the deflection type
measurement systems, the value of the variable to be measured is shown
immediately on a scale or reading.
An alternative type of pressure gage is the deadweight gage shown in Fig. 1.4 ,
which is a null-type instrument. Here, weights are put on top of the piston until
the downward force balances the fluid pressure. Weights are added until the
piston reaches a datum level, known as the null point. Pressure measurement is
made in terms of the value of the weights needed to reach this null position.