Thermal Instruments.
In thermal instruments the action depends upon the heating effect of the current under mainly of two types namely
(i) hot wire instruments and
(ii) thermocouple instructions measurement.
Thermal instruments are ments. The hot wire instrument operates on the fact that when a current is passed through a wire the wire gets heated and so expands while thermocouple instrument operates on there are fact that when the junction of 2 dissimilar metals is heated by passing current through it, an emf is developed.
These instructions. ments are free from errors due to frequency, waveform and external magnetic fields when used on ac and so can be used for measurement of current at frequencies above the range of moving iron and dynamometer type instruments.
Hot wire ammeters were very popular in the last decade of the nineteenth century and in the first two decades of the twentieth century but because of their inherent drawbacks they have been largely superseded by thermocouple instruments and are not used extensively nowadays.
However, owing to their good transfer characteristics at high frequencies, they are employed for determination of the differences of indications of other types of instruments on ac and de and in special measurement applications.
Hot wire instruments have the advantages of (i) no stray magnetic field effect (ii) same cable thermal instruments the action depends upon the heating effect of the current under mainly of two types namely (i) hot wire instruments and (ii) thermocouple instructions measurement. Thermal instruments are ments.
The hot wire instrument operates on the fact that when a current is passed through a wire the wire gets heated and so expands while thermocouple instrument operates on the fact that when the junction of two dissimilar metals is heated by passing current through it, an emf is developed.
These instructions. ments are free from errors due to frequency, waveform and exter nal magnetic fields when used on ac and so can be used for measurement of current at frequencies above the range of moy. ing iron and dynamometer type instruments.
Hot wire ammeters were very popular in the last decade of the nineteenth century and in the first two decades of the twentieth century but because of their inherent drawbacks they have been largely superseded by thermocouple instruments and are not used extensively nowadays.
However, owing to their good transfer characteristics at high frequencies, they are employed for determination of the differences of indications of other types of instruments on ac and de and in special measurement applications.
Hot wire instruments have the advantages of
- no stray magnetic field effect
- same calibration for dc as well as for ac
- fair accuracy
- simple construction
- low cost
- negligible temperature error if suitably adjusted and
suitability for measurement of currents at very high frequencies and disadvantages of
- delicate construction
- relatively higher power consumption
- uneven scale
- incapability of taking overload
- sluggish in action
- need of frequent adjustment of zero position due to temperature variations and
- different deflections for ascending and descending values.
Thermocouple instruments have the advantages of
(i) instrument indications almost independent of frequency and waveform
(ii) no stray magnetic field effect
(iii) high sensitivity and
(iv) utility as transfer instruments and
disadvantages of
(i) considerable power losses and
(ii) delicate construction Applications. These instruments are used for measurement of currents from power frequencies up to 100 MHz, the upper limit is determined by the skin effect and stray capacitance depends on whether the instrument is an ammeter or a voltmeter and its current rating ratio for dc as well as for ac
(iii) fair accuracy
(iv) simple construction
(v) low cost
(vi) negligible temperature error if suitably adjusted and
(vii) suitability for measurement of currents at very high frequencies and
disadvantages of
(i) delicate construction
(ii) relatively higher power consumption
(iii) uneven scale
(iv) incapability of taking overload
(v) sluggish in action
(vi) need of frequent adjustment of zero position due to temperature variations and
(vii) different deflections for ascending and descending values.
Thermocouple instruments have the advantages of
(i) instrument indications almost independent of frequency and waveform
(ii) no stray magnetic field effect
(iii) high sensitivity and
(iv) utility as transfer instruments and
disadvantages of
(i) considerable power losses and
(ii) delicate construction Applications. These instruments are used for measurement of currents from power frequencies up to 100 MHz, the upper limit is determined by the skin effect and stray capacitance depends on whether the instrument is an ammeter or a voltmeter and its current rating.
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