Measurement Technology - The Electronic Nose Artificial Olfaction Technology - page 11

accuracy. The extra effort required to use such an instrument is perfectly
acceptable in this case because of the infrequent nature of calibration operations.
• Manually Operated Type and Automatic Type:
Any instrument which requires the services of human operator is a manual type
of instrument. The instrument becomes automatic if the manual operation is
replaced by an auxiliary device incorporated in the instrument. An automatic
instrument is usually preferred, because the dynamic response of such an
instrument is fast and also its operational cost is considerably lower than that of
the corresponding manually operated instrument.
Manually operated measurement systems require the intervention—a form of
human intervention for their operation. This can be costly, and will affect the
speed of response of the system. If such systems can be automated, then human
intervention can be dispensed with, resulting in cost reduction and enhancement
of speed. Most null-type devices are manually operated, and most deflection
devices are automatic.
• Analog Type and Digital Type:
Analog instruments are those that present the physical variables of interest in the
form of continuous or step-less variations with respect to time. These instru-
ments usually consist of simple functional elements. Therefore, the majority of
present-day instruments are of analog type as they generally cost less and are
easy to maintain and repair.
An analog instrument gives an output that varies continuously as the quantity
being measured changes. The output can have an infinite number of values
within the range that the instrument is designed to measure.
On the other hand, digital instruments are those in which the physical variables
are represented by digital quantities which are discrete and vary in steps. Fur-
thermore, each digital number is a fixed sum of equal steps which is defined by
that number. The relationship of the digital outputs with respect to time gives
the information about the magnitude and the nature of the input data.
• Indicating Instruments and Instruments with a Signal Output:
The measurement system could just provide an output indication, or the output
could be fed it into a control system (e.g., for negative feedback). The signal
output could be a voltage or current or an optical signal or a pneumatic signal.
The problem with indicating type measurement system is the need for human
intervention. Humans make mistakes when reading scales.
The class of indicating instruments normally includes all null-type instruments
and most passive ones. Indicators can also be further divided into those that have
an analog output and those that have a digital display. A common analog
indicator is the liquid-in-glass thermometer.
Instruments that have a signal-type output are commonly used as part of auto-
matic control systems. In other circumstances, they can also be found in mea-
surement systems where the output measurement signal is recorded in some way
for later use. Usually, the measurement signal involved is an electrical voltage,
but it can take other forms in some systems such as an electrical current, an
optical signal, or a pneumatic signal.