Advantages of Hot wire instruments | disadvantages of Hot wire instruments

This page covers advantages and disadvantages of Hot wire instruments. It mentions Hot wire instruments advantages or benefits and Hot wire instruments disadvantages or drawbacks.

There are two types of thermal instruments as follows.
• Hot-wire type
• Thermocouple instrument
They both use heating effect of current flowing through resistance to cause meter deflection. But each uses this effect in different manners.

Hot wire instruments

The hot-wire meter movement deflection depends on the expansion of a high resistance wire caused by the heating effect of the wire itself as current flows through it. A resistance wire is stretched between the two meter terminals, with a thread attached at a right angles to the centre of the wire. A spring connected to the opposite end of the thread exerts a constant tension on the resistance wire. Current flow heats the wire, causing it to expand. This motion is transferred to the meter pointer through the thread and a pivot. The figure-1 depicts arrangement used in Hot-wire instrument.

Benefits or advantages of Hot wire instruments

Following are the benefits or advantages of Hot wire instruments:
➨The deflection depends upon only the rms value of the current flowing through the wire, irrespective if its waveform and frequency. Hence, the instrument can used for ac as well as dc system.
➨The calibration is same for ac as well as dc measurement. So it is a transfer-type instrument.
➨They are free from stray magnetic fields because no magnetic field is used to cause their operation.
➨It is cheap in cost and simple in construction.
➨With suitable adjustments, error due to temperature variation can be made negligible.
➨This type of instruments are quite suitable for very high frequency measurement.

Drawbacks or disadvantages of Hot wire instruments

Following are the disadvantages of Hot wire instruments:
➨Power consumption is relatively high.
➨Nonuniform scale.
➨These are very sluggish in action as time is taken in heating up the wire.
➨The deflection of the instrument is not the same for ascending and descending values.
➨The reading depends upon the atmospheric temperature.

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