Some of the instruments are used for dc measurements only and some are used for ac measurements only. Some are used for dc as well as ac measurements. In this article we will be discussing about types of measuring instruments.
DC MEASURING INSTRUMENTS
The instruments, whose deflections are proportional to the current or voltage under measurement are used for dc measurements only. If such an instrument is connected in an ac circuit. The the pointer will deflect up-scale for one half cycle of the input waveform and down-scale for the next half cycle.
At lower frequencies of 50 Hz, the pointer will not be able to follow the variations in direction. And the pointer will pulsate slightly around the zero mark, seeking the average value of ac, i.e., zero. The permanent magnet moving coil (PMMC) instruments fall under the category of dc instruments.
AC MEASURING INSTRUMENTS
These types of measuring instruments utilizes the electromagnetic induced currents for their operation are used for ac measurements only. These instruments cannot be used for dc measurements because the electromagnetic induced currents are not generally available in dc circuits.The induction instruments fall under the category of ac instruments.
DC/AC MEASURING INSTRUMENTS
These types of measuring instruments having deflection proportional to the square of the current or voltage under measurement can be used for dc as well as ac measurements. Moving iron, dynamo-meter type moving coil, hot-wire, electrostatic instruments fall under this category.
The various types of measuring instruments used for measurement of current, voltage, power, and energy along with suitability for the type of measurements, type of control, type of damping, special features and applications are given below in tabular form.
AMMETERS AND VOLTMETERS
Ammeters and voltmeters, except electrostatic voltmeter, operates on the same principle. The ammeter carries the current to be measured or a definite fraction of it and this current produces the deflecting torque. On the other hand voltmeter carries the current proportional to the voltage to be measured which produces the deflecting torque.
Essential requirements of a measuring instruments are:
- Its introduction into the circuit, where measurements are to be made, does not alter the circuit condition.
- The power consumed by them for their operation is small.
It is connected in series with the circuit carrying the current under measurement.
It must be very low resistance because the voltage drop across the ammeter and power absorbed from the circuit should be as low as possible.
It is connected in parallel with the circuit across which the voltage is to be measured and must be very high resistance.
It is because the current flowing through the voltmeter and the power absorbed from the circuit should be as minimum as possible.
These are used for measurements of resistance. They incorporate a source of emf and a current measuring device.
Power loss in measuring instruments
Let Rₐ be the resistance of the ammeter and I be the current being measured.
Power loss in ammeter Pₐ = I²Rₐ watt
If R be the resistance of voltmeter and V be the voltage being measured,
Power loss in voltmeter = V²∕R watt.
Thus, in order that the power loss in instruments is small, so resistance of ammeters should be small and that of voltmeter should be large.
Types of Measuring Instruments
The main type of instruments used as ammeters and voltmeter are:
- Moving iron
- Moving coil
- Hot wire
Of these the moving coil type can be used for direct current measurements only, and the induction type for ac measurements only. The other type can be used with either dc or ac. The above types of instruments are to be discussed in later articles.