The instruments used to measure the physical and electrical quantities are known as measuring instruments. The measuring instruments may be classified as follows:
Absolute instrument & Secondary instruments:
Absolute instrument measures the process variable directly from the process without the use of conversion. Such instruments do not require comparison with any other standard. The tangent galvanometer is an example for the absolute instrument. These instruments are used as standards in labs and institution.
Secondary instrument: These instruments are so constructed that the deflection of such instruments gives the magnitude of the electrical quantity to be measured directly. These instruments required to calibrated with respect to the standard instrument. These instruments are usually used in practice.
Secondary instruments are classified as:
- Indicating instrument: Those instruments that measure and indicates the magnitude of the electricity. The indications are given by a pointer moving over a calibrated scale. Ordinary ammeters, voltmeters, wattmeters, frequency meters, power factor meters, etc., fall into this category.
- Integrating instrument: Integrating instruments are those which measure the total amount of either quantity of
electricity (ampere-hours) or electrical energy supplied over a period of time. The ampere-hour meters and energy meters fall in this class.
- Recording instrument: These instruments record continuously the variation of the magnitude of the electric quantity for a definite period of time. Such instruments are generally used in powerhouses where the current, voltage, power, etc., are to be maintained within a certain acceptable limit.
Analog and Digital instrument:
Analog instrument: The signals of an analog unit vary in a continuous fashion and can take on an infinite number
of values in a given range. Fuel gauge, ammeter and voltmeters, wristwatch, speedometer fall in this category.
Digital Instruments: Signals that vary in discrete steps and that take a finite number of different values in a given range are digital signals and the corresponding instruments are of digital type. Digital instruments have some advantages over analog meters, in that they have high accuracy and high speed of operation. Digital multimeter is an example for the digital instrument.
Mechanical, Electrical and Electronics Instruments:
Mechanical instrument: Mechanical instruments are very reliable for static and stable conditions. As they use mechanical parts these instruments cannot faithfully follow the rapid changes which are involved in dynamic instruments. But they are cheaper in cost and durable.
Electrical Instruments: When the instrument pointer deflection is caused by the action of some electrical methods
then it is called an electrical instrument. The time of operation of an electrical instrument is more rapid than that of a mechanical instrument. This mechanical movement has some inertia due to which the frequency response of these
instruments is poor.
Electronic Instruments: Electronic instruments use semiconductor devices. They are very fast in response. In electronic devices, since the only movement involved is that of electrons, the response time is extremely small owing to very small inertia of the electrons. With the use of electronic devices, a very weak signal can be detected by using pre-amplifiers and amplifiers.
Deflection and Null Output Instruments:
In a deflection-type instrument, the deflection of the instrument indicates the measurement of the unknown quantity. The measurand quantity produces some physical effect which deflects or produces a mechanical displacement in the moving system of the instrument.
An opposite effect is built in the instrument which opposes the deflection or the mechanical displacement of the moving system. The balance is achieved when opposing effect equals the actuating cause producing the deflection or the mechanical displacement. Permanent Magnet Moving Coil (PMMC), Moving Iron (MI), etc., type instruments are examples of this category.
In Null type instruments, a zero or null indication leads to the determination of the magnitude of the measurand quantity. The null condition depends upon some other known conditions. These are more accurate and highly sensitive as compared to deflection-type instruments. A dc potentiometer is a null- type instrument.
Self-operated and Power-operated Instruments:
Self-operated instruments don’t need any outside power for its working. The output energy is supplied wholly or almost wholly by the input measurand. Dial indicating type instruments belong to this category.
Power operated instrument need external power for its working. External power can electric current, hydraulic or pneumatic energy. In such cases, the input signal supplies only an insignificant portion of the output power.