Instruments- Basics

The essential elements of an instrument are;

1. A detector
2. An intermediate transfer device
3. An indicator, recorder or a storage device

 The history of development of instruments shows three phases of instruments.

a)      Mechanical , b) Electrical,  and c) Electronic instruments

a)      Mechanical instruments

  These instruments are very reliable for static and stable conditions. But, these instruments have moving parts that are rigid, heavy, and bulky and hence have a large mass. Mass presents inertia problems and hence, these instruments cannot faithfully follow the rapid changes, which are involved in dynamic measurements.      Thus, the disadvantages of mechanical instruments are;

·         They are unable to respond rapidly to measurements of dynamic and transient conditions.

·         Most of them are a potential source of noise and cause noise pollution.

b)     Electrical instruments

   Electrical method of indicating the output of detectors is more rapid than mechanical methods. But, electrical instruments depends on the mechanical movement of an indicating device, having some inertia and thus have a limited time response (0.5 – 24 s).

c)      Electronic instruments

  These instruments are used for fast responses required for scientific and industrial measurements. They are used for the detection of electromagnetically produced signals such as radio, video, and micro waves, space applications, and computers.     These instruments make use of semiconductor devices. In electronic devices, the only movement involved is that of electrons. Thus, the response time is extremely small on account of very small inertia of electrons.

eg:- A Cathode Ray Oscilloscope (CRO) is capable of following dynamic and transition changes of the order of a few nano seconds (10^-9s).

            Advantage:-

                                i.            Can detect very weak signals. ( eg:- In the area of Bio Instrumentation, the bio electric potentials are very weak, ie lower than 1 mV)

                              ii.            They can monitor inaccessible or dangerous locations.

                            iii.            They can be used to measure non electrical quantities.

                            iv.            They have

·          higher sensitivity,

·         Lower weight

·         Lower power consumption

·         Higher degree of reliability compared to electrical or mechanical instruments

………………………………………………………………………………………………….. Reference:- A.K Sawhney, Electrical and Electronic Instrumentation and Measurements, page no:- 2-3