Measurement Technology - The Electronic Nose Artificial Olfaction Technology - page 12

• Active Type and Passive Type:
Instrument systems are divided into active or passive ones according to whether
the instrument system output is entirely produced by the quantity being mea-
sured or the quantity being measured simply modulates the magnitude of some
external power source.
In active instruments system, the external power source is usually in electrical
form, but in some cases, it can be other forms of energy such as a pneumatic air
supply, electronic supply, or hydraulic supply for their operation.
In self-generating (or passive) instruments, the energy requirements of the
instruments are met entirely from the input signal.
• Contacting Type and Non-Contacting Type:
A contacting type of instrument is one that is kept in the measuring medium
itself. A clinical thermometer is an example of such instruments. On the other
hand, there are instruments of noncontacting or proximity type. These instru-
ments measure the desired input, even though they are not in close contact with
the measuring medium.
For example, an optical pyrometer monitors the temperature of, say, a blast
furnace, but is kept out of contact with the blast furnace. Similarly, a variable
reluctance tachometer, which measures the rpm of a rotating body, is also a
proximity type of instrument.
• Smart and Non-Smart (dumb) Measurement Systems:
Measurement systems having on-board intelligence (usually containing micro-
processors) are called smart measurement systems and are capable of storing
data, processing it, and sending alarms. It can have its own serial bus to connect
to other microprocessors.
A dumb/nonsmart or conventional instrument is that in which the input variable
is measured and displayed, but the data are processed by the observer. For
example, a Bourdon pressure gage is termed as a dumb instrument because
though it can measure and display a car tyre pressure, but the observer has to
judge whether the car tyre air inflation pressure is sufficient or not.
An intelligent or smart instrument may include several or all like the output of
the transducer in electrical form, the output of the transducer should be in digital
form, interface with the digital computer, elements for noise reduction, error
estimation, self-calibration, gain adjustment, etc., and elements for the output
driver for suitable digital display.
1.8 Transducer
A transducer is a mechanism that transfers one form of energy or physical quantity
into another, in accordance with some defined relationship. Where a transducer is
the sensing element, which responds directly to the physical quantity to be mea-
sured and this forms part of an instrumentation or control system, then the
transducer is often referred to as a sensor.