Study the Temperature Sensors Questions and Answers for your interview preparation. This article focus on Thermocouple and Pyrometer Questions.
When a pair of dissimilar metals are joined together at both ends as shown in the below figure, then emf is induced at the junction of the metals as per the difference in temperature at both junctions, this effect is called the Seebeck effect.
Detailed Explanation:
On heating around a meter long say Pt Rh 13% alloy wire by a small burner millie volts potential difference exists between the ends. The higher the temp difference between the ends the higher the mV. This is called the Seebeck effect.
By measuring the potential difference between the Pt Rh 13% wire ends one can get the hot end temp values by referring to a table that tabulates the hot end developed mV at various temp differences between the ends.
However a wire capable of withstanding the hot end temp say 1200 degrees C or even much more to connect an mV meter across the PtRh 13% wire ends; in addition, it shall develop very low millivolts at different temp differences for max sensitivity.
Scientists by experiment and theoretical analyses have found pure Pt wire meets these requirements. Hence, PtRh 13% Vs. Pt is a thermocouple wire pair.
The other commonly used TC wire pairs are Chromel Vs Alumel, Iron Vs Constantan, Copper Vs Constantan, etc.
When a pair of dissimilar metals are joined together, emf is induced at the junction of the metals.
If the metal is connected to an external circuit some amount of current is drawn, due to which the induced emf will be slightly altered.
When a pair of dissimilar metals are joined together, emf is induced at the junction of the metals.
If a temperature gradient exists along either/ both of the metals, the junction emf may undergo an additional slight alteration called as Thomson effect.
A temperature measuring device formed by connecting two dissimilar metals and is based on the principle of the Seebeck effect, Peltier effect, and Thomson effect.
A series of identical thermocouples connected together in a casing is called a thermopile.
It is also used for measuring average temperature.
An emf will not be induced in a circuit with a system consisting of homogeneous metal.
The net emf in the circuit remains unaltered if a third metal is introduced, provided the junctions formed by the third metal are at the same temperature.
The emf generated at the junction temperature T1& T3 is equal to the algebraic sum of emf generated at the junction in T1-T2 and T2-T3, where T2 lies between T1 &T2
A strip or a block where the uniform temperature is maintained is called an isothermal block.
Merits:
Demerits:
Technique for measuring temperature without contact. It depends on the relation between the temperature of the heating body and the electromagnetic radiation emitted by the black body.
Used for very high-temperature measurement 700 deg c -3500 deg c without the actual contact with the radiating body, Very high response
A body that absorbs all radiation without transmitting any radiation is called a black body.
A black body radiates energy at all spectral wavelengths at a maximum rate corresponding to its temperature.
An object having emittance less than unity and constant at all wavelengths is called a grey body
Used for very high-temperature measurements 700 deg C to 3500 deg C.
Size or thickness of the thermocouple wires, Sheath material, protection tube, and thermowell, Position /location of the thermocouple with respect to process
Within the visible region, a given wavelength has a fixed color and the energy of radiation is interpreted as intensity or brightness.
Hence the brightness of the light of given color emitted by the hot source indicates the value of temperature.
Protecting the thermocouple metals from contamination and rusting. Preventing against oxidizing, sulphating and in molten baths to withstand thermal and mechanical shocks.
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Dear sir,
I know the calculation of rtd from formula.
But what is the formula for thermocouple
For calculation?