Measuring instruments may be defined as device for determining the value or magnitude of quantity or variable.
Measurement generally involves using instruments as physical means of determining a quantity or variable. These instruments serve as an extension of human faculties and enables the man to determine the value of an unknown quantity, which his unaided human faculties cannot measure.
There are three basic types of Measuring instruments:
- Mechanical Instruments:- These instruments are very reliable for static and stable conditions. But they suffer from a very major disadvantage. They are unable to respond rapidly to measurements of transient and dynamic conditions. This is due to the fact that these instruments have moving parts that are rigid, heavy and bulky and consequently have a large mass. Mass presents inertia problems and hence these instruments cannot faithfully follow the rapid changes which are involved in dynamic measurements. Another disadvantage of mechanical instruments is that most of them are a potential source of noise and cause noise pollution.
- Electrical Instruments:- Electrical methods of indicating the output of detectors are more rapid than mechanical methods. It is unfortunate that an electrical system normally depends upon a mechanical meter movement as indicating device. This mechanical movement has some inertia and therefore these instruments have a limited time (and hence, frequency) response. For example some electrical recorders can give full-scale response in 0.2sec where as the majority of industrial recorders have responses of 0.5 to 24 sec. Some galvanometers can follow 50 Hz variations, but even these are too slow for present day requirements of fast measurement.
- Electronic Instruments:- These days most of the process and system measurements require very fast responses. The necessity to step up response time and also the detection of dynamic changes in certain parameters, which require the monitoring time of the order of ms and μs, have led to the design of today’s electronic instruments and their associated circuitry. These instruments require vacuum tubes or semi-conductor devices. Semi-conductor devices have many advantages over their vacuum tube counterparts. Since in electronic devices, the only movement involved is that of electrons, the response time is extremely fast on account of very small inertia of electrons. Just for example, a Cathode Ray Oscilloscope(CRO) is capable of following transient and dynamic changes of the order of a few nsec (10-9sec).
Another advantage of using electronic devices is that very weak signals can also be detected by using pre-amplifiers and amplifiers. The foremost importance of the electronic instruments is the power amplification provided by the electronic amplifiers. Additional power may be fed into the system to provide an increased power output beyond that of the input. This has been only possible through the use of electronic amplifiers, which have no important mechanical counterpart.