Page 287 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Advantages
• As their deflection depend on the rms value of the alternating current, they can be used on direct current also.
• Their readings are independent of wave form and frequency.
• Their calibration is the same both for AC as well as DC measurements.

• They are unaffected by stray fields.
Disadvantages
• They are unable to withstand over load without burning out.
• They have a cramped scale
• They have high power consumption as compared to moving coil instruments.Current consumption is about
200 Ma AT full load.
• Their zero position need frequent adjustment.
Thermocouple

There may be cases when the transduction element performs the action of both transduction and sensing. The
best example of such a transducer is a thermocouple.
Thermoelectric Sensors
The junction of two dissimilar metals forms a thermocouple. As shown fig.

When the two junctions are at different temperatures, a voltage is developed across the junction.
By measuring the voltage difference between the two junctions, the difference in temperature between the two
can be calculated.

• The thermoelectric effect: when one junction has a different temperature than the other, an electromotive force
is produced in the circuit and current flows.
• The magnitude of the force or potential depends on the temperature difference between the two junctions.

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CITS : Power - Electrician & Wireman - Lesson 50-53